Transimpedance amplifier for high-speed optical communications based on linear modulation

Abstract

This invention relates to a optical receiver circuit (200) comprising: at least one photo detector (207) configured to convert a received light signal to an input current signal, a transimpedance amplifier circuit (201) with an input to receive the input current signal from the at least one photo detector (207) and being configured to convert the received input current signal to an output voltage signal to generate an output signal of the transimpedance amplifier circuit (201), wherein the transimpedance amplifier circuit comprises a plurality of gain amplifier stages (209, 210, 211), a DC restoration component (205), wherein the DC restoration component (205) is configured to receive the output voltage signal of the transimpedance amplifier circuit (201) for restoring the DC component of the received current signal and configured for outputting a corresponding current signal, and an automatic gain control component (204) configured for controlling via at least one programmable feedback resistor (226, 227) the equivalent transimpedance of the transimpedance amplifier circuit based on the signal output by the DC restoration component (205) to provide a constant output voltage amplitude for different current ranges of the input current signal.

Claims

1. An optical receiver circuit (200) comprising: at least one photo detector (207) configured to convert a received light signal to an input current signal, a transimpedance amplifier circuit (201) with an input to receive the input current signal from the at least one photo detector (207) and being configured to convert the received input current signal to an output voltage signal to generate an output signal of the transimpedance amplifier circuit (201), wherein the transimpedance amplifier circuit comprises a plurality of gain amplifier stages (209, 210, 211), a DC restoration component (205), wherein the DC restoration component (205) is configured to receive the output voltage signal of the transimpedance amplifier circuit (201) for restoring the DC component of the received current signal and configured for outputting a corresponding current signal, an automatic gain control component (204) configured for controlling via at least one programmable feedback resistor (226, 227) the equivalent transimpedance of the transimpedance amplifier circuit based on the signal output by the DC restoration component (205) to provide a constant output voltage amplitude for different current ranges of the input current signal; wherein at least some of the gain amplifier stages (209, 210, 211) comprise a gain amplifier (212, 213, 214) and at least one local programmable feedback resistor (215, 216, 217, 218, 219, 220, 224) for controlling the gain of the respective gain amplifier stage (209, 210, 211, 214), wherein some of the local programmable feedback resistors are arranged to shorten the outputs of some of the gain amplifier stages (224), and wherein the automatic gain control component (204) is further configured to control at least some of the local programmable feedback resistors (215, 216, 217, 218, 219, 220) of the gain amplifier stages based on the signal output by the DC restoration component (205), and wherein the at least one programmable feedback resistor (227, 227) for controlling the equivalent transimpedance of the transimpedance amplifier circuit (201) is arranged between the input of transimpedance amplifier circuit and the output signal of the transimpedance amplifier circuit.

2. Optical receiver circuit (200) according to claim 1, comprising a plurality of programmable feedback resistors (226, 227, 215, 216, 105, 109, 113, 114, 115, 116, 117, 118) for controlling the equivalent transimpedance of the transimpedance amplifier circuit (201,101), and wherein at least some of the programmable feedback resistors (215, 216, 105, 109, 114, 115, 116, 117) for controlling the equivalent transimpedance of the transimpedance amplifier circuit are connected between the input of the transimpedance amplifier circuit and outputs of the different gain amplifier stages (123, 124, 125, 126).

3. Optical receiver circuit (200) according to claim 1, further comprising a fixed resistor (221, 225) connected between the input and the output signal of the transimpedance amplifier circuit for limiting the maximum equivalent transimpedance of the transimpedance amplifier circuit, and/or wherein the DC restoration component (205) is configured to subtract the DC component of the received current signal and wherein the automatic gain control component (204) is configured for controlling the equivalent transimpedance of the transimpedance amplifier circuit (201) based on a copy of the subtracted DC component.

4. Optical receiver circuit (200) according to claim 1, wherein the optical receiver circuit comprises two photo detectors (206, 207), wherein one photo detector (207) is configured to receive the light signal and the other photo detector (206) is shielded from the light signal and wherein the transimpedance amplifier circuit (201) has a differential topology with one branch, e.g. the positive branch (228), of the transimpedance amplifier circuit being connected to the photo detector (207) that is configured to receive the light signal and with the other branch, e.g. the negative branch (229), of the transimpedance amplifier circuit being connected to the photo detector (206) that is shielded from the light signal, or wherein the optical receiver circuit comprises a transimpedance amplifier circuit with a differential topology with one branch, e.g. the positive branch, of the transimpedance amplifier circuit being connected to the photo detector that is configured to receive the light signal and with the other branch, e.g. the negative branch, of the transimpedance amplifier circuit being connected to an equivalent electrical model of the photodiode to a circuit comprising a resistor and/or capacitor.

5. Optical receiver circuit (200) according to claim 1, wherein at least some of the local programmable feedback resistors for controlling the gain of the gain amplifier stages (217, 218, 219, 220) are connected to local inputs and outputs of some of the gain amplifier stages and/or wherein at least some of the local programmable feedback resistors for controlling the gain of the gain amplifier stages are arranged to shorten the outputs of some of the gain amplifier stages (224).

6. Optical receiver circuit (200) according to claim 1, wherein the at least one gain amplifier stage comprises a differential pair, e.g. cascoded transistor, with a resistive load (501), or with an active load with a p-channel metal-oxide-semiconductor field-effect transistor, PMOS, load, and/or wherein at least one, some, or each gain amplifier stage comprise a common-mode control circuit (502) for providing a signal suitable for controlling the reverse bias voltage of the photo detector.

7. Optical receiver circuit (200) according to claim 1, wherein the optical receiver circuit is configured to carry out one, some or all of the following steps: calculate an average current of the current signal generated by the at least one photo detector based on a copy of the current outputted by the DC restoration component (205), use the calculated average current to calculate a required equivalent transimpedance of the transimpedance amplifier circuit and for controlling the equivalent transimpedance of the transimpedance amplifier circuit (201) to provide a constant output voltage (203) amplitude for different current ranges of the input current signal reduce the gain of the gain amplifier stages (209, 210, 211).

8. Optical receiver circuit (200) according to claim 1, wherein the automatic gain control component (204) is configured to carry out one, some or all of the following steps (702, 703, 704) for controlling the equivalent transimpedance of the transimpedance amplifier circuit (201): use the current outputted by the DC restoration component (205) to calculate a required equivalent transimpedance of the transimpedance amplifier circuit (201), start reducing the equivalent transimpedance of the transimpedance amplifier circuit (201) by reducing the impedance of the at least one programmable feedback resistor (225, 226) connected between the input and output of the transimpedance amplifier circuit, once the impedance of the at least one programmable feedback resistor (226, 227) connected between the input and output of the transimpedance amplifier circuit is set to a given minimum value, sequentially reducing the impedance of possible further programmable feedback resistors connected between the input of the transimpedance amplifier circuit and outputs of different gain amplifier stages (215, 216) by starting with reducing the impedance of a programmable feedback resistor connected between the input of the transimpedance amplifier circuit and the output of the last gain amplifier stage.

9. Optical receiver circuit (200) according to claim 1, wherein the automatic gain control component (204) is further configured to carry out the step of: controlling the gain of the gain amplifier stages (209, 210, 211) by controlling, e.g. reducing, in sequence the resistive feedback or the shunt resistors of the local programmable feedback resistors of the gain amplifier stages (217, 218, 219, 220, 224) by starting controlling, e.g. reducing, the resistive feedback of the local programmable feedback resistor of the last gain amplifier stage (219, 220).

10. Optical receiver circuit (200) according to claim 1, wherein some or each of the programmable feedback resistors (215, 216, 217, 218, 219, 220, 224, 225, 226) comprise a plurality of transistors (802, 803, 804, 805) connected in parallel and wherein the resistance of the local programmable feedback resistors is controlled via the gate voltage (806, 807, 808, 809) of their transistors, and wherein some or all of the transistors of a programmable feedback resistor have different characteristics differ in scale or size, e.g. differ in their gate-width-to-gate-length ratio, e.g. increasing in their gate-width-to-gate-length ratio from the first to the last transistor of the respective programmable feedback resistor.

11. Optical receiver circuit (200) according to claim 10, wherein the transistors of a programmable feedback resistor are configured to be activated in sequence (900) configured to be activated in sequence from the first to the last transistor of the respective programmable feedback resistor.

12. Optical receiver circuit (200) according to claim 1, wherein the DC restoration component (205) comprises a sequential voltage generator (1002), wherein the sequential voltage generator output drives a current source that comprises a plurality of scaled transistors (1003), and wherein the scaled transistors are configured to be activated in sequence for increasing the DC current output of the DC restoration component, and wherein, for example, the DC restoration component comprises a low-pass filter (1001) connected to the output of the transimpedance amplifier circuit (203), wherein the output of the low-pass filter (1103) is used as input for the sequential voltage generator (1002), and wherein the DC current output of the DC restoration component is connected to the output of the at least one photo detector (207) that is connected to the input (228) of the transimpedance amplifier circuit (201), and/or wherein the automatic gain control component (204) comprises a dummy transimpedance amplifier circuit (1208) that is a scaled version of the transimpedance amplifier circuit (201) according to one of the preceding claims, wherein the dummy transimpedance amplifier circuit (1208) comprises a plurality of dummy gain amplifier stages (1205), and wherein the dummy transimpedance amplifier circuit (1208) is configured to receive as input a copy of the DC current outputted by the DC restoration component (1209) and wherein the dummy transimpedance amplifier circuit is further configured to convert the received input current signal to a voltage signal to generate an output signal (1202) for controlling the equivalent transimpedance of the transimpedance amplifier circuit, and wherein each dummy gain amplifier stage comprises a dummy gain amplifier and at least one dummy local programmable feedback resistor.

13. Optical receiver circuit (200) according to claim 12, wherein the dummy transimpedance amplifier circuit (1208) has the same equivalent transimpedance for a given programmed state of the dummy programmable feedback resistors (1206, 1207) than the equivalent transimpedance of the transimpedance amplifier configured with the same programmed state of the programmable feedback resistors, and wherein the automatic gain control component is configured to carry out one, some or all of the following steps: use a copy of the DC current outputted by the DC restoration component (1209) as an input of the dummy transimpedance amplifier circuit to transform this current into a voltage proportional to the required transimpedance, compare the output of the dummy transimpedance amplifier circuit to a given reference voltage (1201), and based on this comparison, generate a set of gate control voltages (1202) to program the transimpedance of the dummy transimpedance amplifier circuit by means of the dummy programmable feedback resistors, use the generated gate control voltages (1202) to set the equivalent transimpedance of the transimpedance amplifier circuit (201) to a value that sets the output voltage amplitude (203) the transimpedance amplifier circuit to a desired value.

14. Optical receiver (1303) for use in an optical communication system (1300) comprising at least one optical receiver circuit (1304) according to claim 1.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The following figures illustrate exemplary:

(2) FIG. 1: Exemplary schematic architecture of parts of an optical receiver circuit, e.g. an exemplary transimpedance amplifier circuit and an exemplary photo diode

(3) FIG. 2: Exemplary schematic architecture of an optical receiver circuit

(4) FIG. 3: Further exemplary schematic architecture of an optical receiver circuit

(5) FIG. 4: Exemplary time series of an exemplary input current signal

(6) FIG. 5: Example of a gain amplifier stage

(7) FIG. 6: Exemplary common-mode control circuitry

(8) FIG. 7: Exemplary impedance control sequence

(9) FIG. 8: Exemplary architecture of a programmable feedback resistor

(10) FIG. 9: Exemplary activation sequence of transistors in an exemplary programmable feedback resistor

(11) FIG. 10a: Exemplary DC restoration component architecture

(12) FIG. 10b: Exemplary voltage generation sequence in dependence of DC input current

(13) FIG. 10c: Exemplary closed loop response of exemplary transimpedance amplifier circuit

(14) FIG. 11: Exemplary sequential voltage control architecture

(15) FIG. 12: Exemplary architecture of an automatic gain control component

(16) FIG. 13: Example of an optical communication system

DETAILED DESCRIPTION

(17) FIG. 1 exemplary shows a possible architecture of parts 100 of an optical receiver circuit, in particular comprising an exemplary transimpedance amplifier circuit 101 and an exemplary photo diode 104.

(18) In particular, an exemplary transimpedance amplifier circuit 101 is shown with an input 102 to receive an input current signal from the at least one photo detector 104 and being configured to convert the received input, e.g. an input current signal, to an output voltage signal 103 to generate an output signal 103 of the transimpedance amplifier circuit.

(19) The DC restoration component and the automatic gain control component are not shown in this example.

(20) Furthermore, it is exemplary shown that the transimpedance amplifier circuit 101 comprises an exemplary plurality of gain amplifier stages, e.g. exemplary gain amplifier stages 123, 124, 125, 126. Any other number of gain amplifier stages can be implemented as well.

(21) Said exemplary gain amplifier stages 123, 124, 125, 126 comprise exemplary gain amplifiers 119, 120, 121, 122 and exemplary local programmable feedback resistors (Rlfs) 105, 109, 106, 110, 107, 111, 108 and 112.

(22) Furthermore, an exemplary plurality of further programmable feedback resistors 113, 118, 114, 117, 115 and 116 for controlling the equivalent transimpedance of the transimpedance amplifier circuit 101 are shown.

(23) For completeness it is noted, that programmable feedback resistors for controlling the equivalent transimpedance of the transimpedance amplifier circuit 101 that are connected between the input 102 of the transimpedance amplifier circuit 101 and the output, i.e. output signal 103, or that are connected between the input 102 of the transimpedance amplifier circuit 101 and outputs of different gain amplifier stages, may also be referred to as global programmable feedback resistors.

(24) It is further noted that the local programmable feedback resistors 105 and 109 can be under-stood also as a (global) programmable feedback resistors for controlling the (equivalent) transimpedance of the transimpedance amplifier circuit 101 in case of using/having only a single/the first gain amplifier stage 123.

(25) An exemplary sequence for controlling the equivalent transimpedance of the transimpedance amplifier circuit 101 may comprise: reducing resistance of programmable feedback resistor(s) 113, 118, Rgf4 reducing resistance of programmable feedback resistor(s) 114, 117, Rgf3 reducing resistance of programmable feedback resistor(s) 115, 116, Rgf2 reducing resistance of programmable feedback resistor(s) 105, 109, Rf1

(26) An exemplary sequence for controlling the gain of the transimpedance amplifier circuit 101 may comprise: reducing resistance of programmable feedback resistor(s) 108, 112, Rlf4 reducing resistance of programmable feedback resistor(s) 107, 111, Rlf3 reducing resistance of programmable feedback resistor(s) 106, 110, Rlf2 reducing resistance of programmable feedback resistor(s) 105, 109, Rf1

(27) FIG. 2 exemplary shows a further possible architecture of an optical receiver circuit 200. Said exemplary optical receiver circuit 200 can comprise an automatic gain control component 204 and DC restoration component 205, wherein said DC restoration component 205 can inter alia comprise a low-pass filter 208.

(28) Furthermore, the optical receiver circuit 200 comprises an exemplary transimpedance amplifier circuit 201 with input 202 and output 203 and with an exemplary plurality of gain amplifier stages, from which only an exemplary subset, namely the gain amplifier stages 209, 210, 211 are explicitly shown and denoted.

(29) Said gain amplifier stages can, analogous to the transimpedance amplifier circuit depicted in FIG. 1, comprise exemplary gain amplifiers 212, 213, 214 and exemplary local programmable feedback resistors 215, 216, 217, 218, 219 and 220.

(30) Furthermore, an exemplary shunt programmable resistor 224 is shown that can short the output of the last gain amplifier stage 211.

(31) Also the transimpedance amplifier circuit 201 may comprise a plurality of programmable feed-back resistors, e.g. global programmable feedback resistors, 226, 227 for controlling the equivalent transimpedance of the transimpedance amplifier circuit, wherein said programmable feed-back resistors, e.g. global programmable feedback resistors, 226, 227 for controlling the equivalent transimpedance of the transimpedance amplifier circuit can be connected between the input 202 of the transimpedance amplifier circuit 201 and the output 203 of the transimpedance amplifier circuit 201 or between the input 202 of the transimpedance amplifier circuit 201 and the outputs of different gain amplifier stages.

(32) In addition a fixed resistor 221, 225 connected in parallel between input and output of transimpedance amplifier circuit 201 can be used to limit the maximum value of the equivalent transimpedance of the transimpedance amplifier circuit 201. Furthermore, exemplary buffers 222, 223 are shown that can isolate the output of the last gain amplifier stage 211.

(33) Moreover, the exemplary optical receiver circuit 200 can comprise two photo detectors, e.g. photo diodes, 206, 207, wherein one photo detector 207 is configured to receive the input light signal and the other photo detector 206 is shielded from the input light signal and the transimpedance amplifier circuit 201 can have a differential topology with one branch 228, e.g. the positive branch, of the transimpedance amplifier circuit 201 being connected to the photo detector 207 that is configured to receive the light signal and with the other branch 229, e.g. the negative branch, of the transimpedance amplifier circuit 201 being connected to the photo detector 206 that is shielded from the light signal. However, it is also conceivable, that, for example, said possible negative branch of the transimpedance amplifier circuit could be connected to an equivalent electrical model (not shown) of a photo detector, e.g. an equivalent electrical model of a photodiode, for example, to a circuit comprising a resistor and/or capacitor.

(34) Such a possible differential architecture can inter alia improve the Power Supply Ratio (PSRR) and Common Mode Rejection Ratio (CMRR) as well as the common noise immunity.

(35) As described above, the automatic gain control component 204 can be in communication with some or all programmable feedback resistors of the optical receiver circuit 200, i.e., for example, with both the local programmable feedback resistors 215, 216, 217, 218, 219, 220 for controlling the gain of the respective gain amplifier stage and with some or all of the programmable feedback resistors 226, 227 for controlling the equivalent transimpedance of the transimpedance amplifier circuit based on the signal output by the DC restoration component 205.

(36) For completeness, it is noted that the terms R.sub.ctrl <M+1:N> and R.sub.ctrl <M+1:N> shown in FIG. 2 can be understood as referring to the possible plurality of transistors comprised in the programmable feedback resistors. As mentioned before, said programmable feedback resistors can follow an activation sequence, for example, resistors 226, 227, 219 and 220 can be activated first, and after, resistors 215, 216, 217, 218.

(37) FIG. 3 exemplary shows a possible architecture of an optical receiver circuit 300 for the purpose of better understanding some aspects of the present invention.

(38) Said exemplary optical receiver circuit 300 can comprise an automatic gain control component 307 and DC restoration component 308, wherein said DC restoration component 308 can inter alia comprise a low-pass filter (not shown).

(39) Furthermore, the optical receiver circuit 200 comprises an exemplary transimpedance amplifier circuit 303 with input 301 and output 302 and with an exemplary gain amplifier stage 312.

(40) Said gain amplifier stage 312 can, analogous to the transimpedance amplifier circuits depicted before, comprise an exemplary gain amplifier 304 and exemplary local programmable feedback resistors 305 and 306 that in the case shown also can act as global programmable feedback resistors and for controlling the (equivalent) transimpedance of the transimpedance amplifier circuit 303 based on the signal output by the DC restoration component 308 to provide a constant output voltage amplitude for different current ranges of the input current signal.

(41) The gain of the gain amplifier stage(s) should preferably be sufficiently high in order to get the maximum bandwidth of the current to voltage transimpedance amplifier response, in particular for high equivalent transimpedance values.

(42) Preferably, and as described above, the transimpedance amplifier circuit can therefore comprise more than one gain amplifier stage.

(43) In particular and in general it holds that a higher bandwidth for a higher (equivalent) transimpedance of the transimpedance amplifier circuit requires a higher gain of the gain amplifier(s) of the gain amplifier stage(s).

(44) For example, the bandwidth 3 dB of the transimpedance amplifier circuit with respect to the 3-dB point can be approximated by

(45) ${}_{-3dB}\frac{\sqrt{2}{A}_0}{R_F{C}_T}$
wherein, for example, A.sub.G is the open-loop gain of the gain amplifier of a gain amplifier stage, R.sub.F is the equivalent resistance of a feedback resistor and C.sub.F is the total equivalent input capacitance of the gain amplifier.

(46) For example, for a bandwidth of about 150 MHz and exemplary values of R.sub.F200 k and C.sub.F4 pF a DC gain of at least about 60 dB would be required.

(47) Furthermore, the transimpedance amplifier circuit 303 can comprise/can be followed by an output buffer 311, e.g. a unity gain amplifier or voltage follower, to isolate an/the output node of the transimpedance amplifier circuit.

(48) Analogous to FIG. 2, the exemplary optical receiver circuit 300 can comprise two photo detectors, e.g. photo diodes, 310, 309, wherein one photo detector 309 is configured to receive the input light signal and the other photo detector 310 is shielded from the input light signal and the transimpedance amplifier circuit 303 can have a differential topology with one branch, e.g. the positive branch, of the transimpedance amplifier circuit 303 being connected to the photo detector 309 that is configured to receive the light signal and with the other branch, e.g. the negative branch, of the transimpedance amplifier circuit 303 being connected to the photo detector 310 that is shielded from the light signal. However, it is also conceivable, that, for example said possible negative branch of the transimpedance amplifier circuit could be connected to an equivalent electrical model (not shown) of a photo detector, e.g. an equivalent electrical model of a photodiode, for example, to a circuit comprising a resistor and/or capacitor.

(49) For completeness it is noted that the photo detector, e.g. photo diode 309, is connected to VDD for illustration purposes only, other connections, such as anode to ground, are also possible, depending on the nature of the photo diode.

(50) FIG. 4 exemplary shows a time series 400 of an exemplary input current signal 401, e.g. an exemplary photocurrent signal from a photo detector, e.g. a photo diode (not shown).

(51) This figure illustrates the transient evolution of a transmission signal for a given average optical light power level (i.e., for a given fiber length, temperature, process, etc). The time scales shown can, for example, be of the order of hundred of MHz or GHz.

(52) As shown, the exemplary input current signal 401, can vary between a maximum input current level 402 and a minimum input current level 404, and may have an average input current level denoted by the reference numeral 403.

(53) The difference between the maximum input current level 402 and the minimum input current level 404 can define the input voltage swing or variation of the input current.

(54) For example, in case of an analog transmission, between these two values 402, 404, the trans-mission signal can take any value, and the optical receiver (circuit) is in charge of interpreting it as the digital transmitted signal.

(55) The average current 403 exemplary represents the DC component of the received input signal. As this DC component is not necessarily needed to reconstruct the transmission signal in the receiver, it can be removed by, for example, a DC restoration component, such as for example the DC restoration component 308 of FIG. 3.

(56) Furthermore, the average input current 403 can provide a good estimation of the maximum input amplitude of the received photocurrent. In an optical transmission, the difference between I.sub.max and I.sub.min is given by the following relation:

(57) $I_{\mathrm{ma}x}-I_{min}=2.Math.\frac{\mathrm{ER}-1}{\mathrm{ER}+1}.Math.I_{\mathrm{avg}},$
where ER is known as the extinction ratio and can be defined for a given communication protocol.

(58) Therefore, for a given ER, the maximum input current swing can be calculated using the average current 403 and for adapting the equivalent transimpedance of the transimpedance amplifier to get a defined output voltage swing at the output of the transimpedance amplifier.

(59) It is important to remark that depending of the level of the received optical power, the average current variation can be up to three orders of magnitude or more.

(60) FIG. 5 exemplary shows a gain amplifier 500, for example, a gain amplifier of a first gain amplifier stage (not shown).

(61) In this example, the gain amplifier can comprise a cascoded transistor with a resistive load R.sub.load 501.

(62) In lieu of the resistive load R.sub.load, also a PMOS (p-channel metal-oxide-semiconductor) load may be used, when optimizing a desired balance between gain, input referred noise and corner variations.

(63) The cascoded transistor can be a cascoded NMOS (n-channel metal-oxide-semiconductor) field-effect transistor, which inter alia can improve the current noise characteristics of the transimpedance amplifier circuit (not shown), as the input referred noise of the transimpedance amplifier circuit can be inversely proportional to the equivalent transconductance of the input differential pair and the equivalent input capacitance.

(64) However, also other transistor types, such as PMOS (p-channel metal-oxide-semiconductor) field-effect transistor are possible.

(65) Furthermore, also other technologies, such as, for example, Bipolar (bipolar junction transistor technology), BiCMOS (combination of bipolar junction transistor technology and complementary metal-oxide-semiconductor technology), GaAs (Gallium Arsenide) based technology, etc. can be used in the implementation of the gain amplifier 500.

(66) A cascoded transistor can inter alia allow increasing the equivalent impedance of the input differential pair 504, 505 (of the transimpedance amplifier circuit) to obtain a higher gain.

(67) The gain and the output impedance of the gain amplifier can be scaled with the equivalent transimpedance of the transimpedance amplifier circuit (not shown).

(68) The bias current I.sub.bias, 503, of the input differential pair can inter alia be obtained from a transconductance control circuit (not shown) that keeps a constant transconductance along possible process/voltage/temperature (PVT) variations in the optical receiver circuit, thereby improving the stability control, linearity and noise performance of the optical receiver circuit under all conditions.

(69) In other words, bias current I.sub.bias, 503, can vary with the PVT variations to facilitate keeping the gain constant for all PVT variations, thereby inter alia facilitating the closed-loop response and keeping a similar performance in all corners.

(70) Furthermore, the gain amplifier 500 can comprise a common-mode control component 502 for controlling the reverse bias voltage of the input photo detector, i.e. the input photo diode. This can inter alia improve the control and stability of the output common mode voltage and can improve the performance of the possible following gain amplifier stage.

(71) While the above described architecture and topology can be implanted in the gain amplifier of a first gain amplifier stage to improve the input referred noise of the transimpedance amplifier circuit, the possible other subsequent stages can follow a similar architecture and topology.

(72) FIG. 6 exemplary shows an exemplary common-mode control circuitry component 600 that can be implemented in the gain amplifiers of a gain amplifier stage of a transimpedance amplifier circuit of an optical receiver circuit (not shown), i.e., for example, all gain amplifiers can have common-mode control circuitry component.

(73) A common-mode control circuitry component can serve to compensate the variations of the bias current I.sub.bias, without significantly influencing the amount of current going through the input differential pair and maintaining the transconductance properties and functionalities.

(74) In the present example, the common-mode control circuitry component 600 can sample the output node of the gain amplifier stage (not shown) by means of two large resistors 602, 603 in order to avoid modifying the output impedance of the gain amplifier stage (not shown).

(75) Said large resistors 602, 603 may, for example, have resistance values in the range of hundreds of kilo-ohms to few mega-ohms.

(76) Thereby the common-mode can be compared to a reference value 601, V.sub.CM, and the difference can be low-pass filtered, for example, by means of a 1 kHz transconductance-capacitance filter.

(77) The common-mode can then be adjusted by subtracting the corresponding current from the output nodes, for example, by means of a transistor, for example, an NMOS transistor.

(78) For completeness, it is noted that the common-mode control circuitry component topology is not limited to the proposed transconductance-capacitance scheme or MOS transistor, but that the common-mode control circuitry component could be implemented by other means performing the functionalities and steps described above.

(79) FIG. 7 exemplary shows possible steps of a control sequence 700 for controlling the equivalent transimpedance of the transimpedance amplifier circuit of an optical receiver circuit (not shown) in a stable manner.

(80) The exemplary sequence steps can, for example, be carried out by an automatic gain control component (not shown) and can include one, some or all of the following steps and in varying order of steps:

(81) Step 701: calculate an average input current from photo detector, e.g. photodiode

(82) Step 702: use the calculated input current to calculate the needed transimpedance

(83) Step 703: start reducing the equivalent transimpedance of the transimpedance amplifier circuit by start reducing the impedance of the programmable feedback resistor that is connected between the input and output of the transimpedance amplifier circuit, while keeping the feedback loop of the transimpedance amplifier circuit open

(84) Step 704: once the impedance of the programmable feedback resistor, that is connected between the input and output of the transimpedance amplifier circuit, is set to a given minimum value, reduce the resistive feedback of the gain amplifier stages, starting with reducing the resistive feedback of the at least one local programmable feedback resistor of the last gain amplifier stage.

(85) In parallel to step 703 and 704 the resistive feedback of the other local programmable feedback resistors of other gain amplifier stage can be reduced to reduce the gain of the other gain amplifier stages and to further control the stability of the transimpedance amplifier circuit of an optical receiver circuit.

(86) The described steps and sequences are exemplary only and other sequences comprising moving the gain of the different gain amplifier stages by means of programmable feedback resistors/shunt resistors are conceivable too, as well as other steps and means for reducing the impedance between the input and output of the transimpedance amplifier circuit are possible too.

(87) FIG. 8 exemplary shows an implementation of a programmable feedback resistor 800, e.g. of a local or global programmable feedback resistor or of a programmable feedback resistor connected between the input and output of the transimpedance amplifier circuit.

(88) For example, in an exemplary optical receiver circuit (not shown), some or each of the programmable feedback resistors can comprise a plurality of transistors 802, 803, 804, 805 connected in parallel and wherein the resistance of the programmable feedback resistors is controlled via the gate voltage(s) of their transistors 806, 807, 808, 809, and wherein some or all of the transistors 802, 803, 804, 805 of a programmable feedback resistor can have different characteristics, for example, can differ in scale or size, e.g. differ in their gate-width-to-gate-length ratio 810, 811, 812, e.g. in their gate-width-to-gate-length ratio from the first to the last transistor of the respective programmable feedback resistor.

(89) The possible different characteristics of said transistors can inter alia reduce linearity problems and improve the operation of the programmable feedback resistors at ohmic region for the full dynamic range.

(90) For example, MOS transistors, e.g. CMOS transistors, operate in ohmic region when V.sub.DS<V.sub.DS,Set, wherein V.sub.DS is the drain to source voltage and V.sub.DS,Set is the drain to source voltage when entering the saturation region and non-linear behavior occurs.

(91) transistors can, for example, be configured and designed for maximizing the V.sub.GE operating point to improve the linearity behavior for the whole range of equivalent impedances.

(92) Furthermore, a successively activation of the transistors 802, 803, 804, 805 from smaller gate-width-to-gate-length ratio to bigger gate-width-to-gate-length ratio is possible, as shown in exemplary sequence 801.

(93) Such exemplary successively activations of the transistors of the programmable feedback resistors of the optical receiver circuit can improve the linearity of equivalent impedance transitions, for example, the linearity of equivalent impedance transitions from high equivalent impedance values to low equivalent impedance values.

(94) FIG. 9 exemplary shows a possible linear decrease behavior 900 of the equivalent impedance 901 for a programmable feedback resistor (not shown) of an optical receiver circuit (not shown) when an exemplary gate-width to gate-length dependence 902 of the transistors (not shown) of a programmable feedback resistor is carried out.

(95) For example, an activation sequence analogous to the one described above, with an activation of the transistors from smaller gate-width-to-gate-length ratio to bigger gate-width-to-gate-length ratio, and wherein said transistors control the gate voltages 903 of the programmable feedback resistors (not shown).

(96) The design of the programmable feedback resistors described above and the sequential activation of the transistors operating in a sufficiently high and optimized V.sub.GS V.sub.tk regime can ensure a good linear behavior over the full dynamic range.

(97) For completeness, it is noted that the shown possible linear decrease behavior 900 can also be valid for the equivalent transimpedance behavior of the transimpedance amplifier circuit.

(98) FIG. 10a exemplary shows a possible DC restoration component architecture 1000, wherein the DC restoration component can comprise a low-pass filter 1001.

(99) The DC restoration component can remove the input DC current, i.e. the average current, I.sub.avg, of the current signal generated by the at least one photo detector, i.e. the photocurrent, for example, by means of a closed-loop control, which can involve a low-pass filtering of the output voltage of the transimpedance amplifier circuit (TIA, not shown) to calculate its DC component, V.sub.DC.

(100) The low-pass filter 1001 can be followed by a sequential voltage generator 1002 and a current source 1003 which generates the equivalent DC current to be subtracted from the transimpedance amplifier input and that can be built by means of a set of parallel transistor controlled by the sequential voltage control voltage V.sub.DC generated by the sequential voltage generator, which can be configured for a continuous control of the DC voltage.

(101) FIG. 10b exemplary shows the linear behaviour 1004 of the DC input current in dependence of the calculated DC voltage component V.sub.DC for a possible DC restoration component architecture, such as, for example, the DC restoration component architecture 1000.

(102) FIG. 10c exemplary shows the closed-loop control response behaviour 1006 of a possible transimpedance amplifier circuit architecture with a possible DC restoration component architecture together, such as, for example, the DC restoration component architecture 1000 and the transimpedance amplifier circuit architecture described above. The transimpedance amplifier circuit closed-loop response together with the DC restoration can form a band-pass filter which lower corner frequency or low frequency pole 1005, .sub.pl, can be given by .sub.pl=T.sub.z0.Math.g.sub.m,cs.Math.GBW.sub.LPF, wherein T.sub.z0 is the equivalent transimpedance, g.sub.m,cs is the equivalent transconductance of the current source, e.g. the set of parallel transistors of the current source component 1003 of FIG. 10a, and GBW.sub.LPF is the gain bandwidth product of the low-pass filter.

(103) Said lower corner frequency .sub.pl or low frequency pole 1005 can be kept constant for all the possible transimpedance range, thereby preventing inter alia a baseline wandering of the communication signal processed by the optical receiver circuit. Furthermore, the equivalent transconductance g.sub.m,cs can move together with the equivalent transimpedance T.sub.z0.

(104) The DC current generated by the DC restoration component, i.e. the DC current signal outputted by the DC restoration component, can, for example, be generated by means of an array of scaled transistors that can be activated sequentially as the DC voltage output increases as, for example, the ones shown in FIG. 10a.

(105) In addition the equivalent transconductance g.sub.m.cs of the current source can be reduced to reduce the current noise of the DC restoration injected at the input of the transimpedance amplifier. The use of the sequential voltage activation of the current source can help to minimize the current noise injection.

(106) The described architecture inter alia can allow keeping a constant T.sub.z0.Math.g.sub.m,cs product and at the same time reducing the amount of noise injected into the transimpedance amplifier circuit input, as the transistors used in this architecture can, for example, as described above, e.g. FIG. 8, be configured and designed for maximizing their gate source voltage V.sub.GS operating point.

(107) For example, the higher the gate source voltage V.sub.GS, the smaller the g.sub.m/I.sub.D, the so called inversion coefficient, resulting in less spectral noise density of the transistors and better overall performance of the optical receiver circuit.

(108) It is again noted for completeness that the transistors that can be used are not limited to a MOS (metal-oxide-semiconductor), e.g. CMOS (complementary metal-oxide-semiconductor), architecture based implementation.

(109) FIG. 11 exemplary shows a possible sequential voltage control component 1100 for continuous voltage control which, for example, can be used in DC restoration component (not shown) of an optical receiver circuit as described above.

(110) The exemplary possible sequential voltage control component 1100 can receive an input current reference I.sub.ref and a signal V.sub.dc coming from a/the low pass filter (not shown), which can be translated into a current I.sub.in proportional to the output DC voltage of the DC restoration component (not shown).

(111) Said translated input voltage-dependent current I.sub.in can then be copied by means of a current mirror 1104 along an array of scaled of copies of the reference current I.sub.ref.

(112) As shown a diode-connected transistor, for example, a diode-connected NMOS transistor, can convert exceeding current into voltage and thereby building the sequential bits, e.g. rising from a minimum to a maximum value sequentially, along the array.

(113) For example, in case the current input voltage V.sub.dc is small, all the output voltages 1105, 1106, 1107, are low.

(114) Thereby typical voltages can be in the range of 1 to 5V.

(115) Due to the different width-to gate length ratios of the MOS transistors, as shown in exemplary sequence 1101, as the voltage V.sub.dc grows, the different output voltages V.sub.DC <i>, 1105, 1106, 1107 (with i being a natural number greater 1), are sequentially activated.

(116) In addition a proper sizing, i.e. a proper dimensioning of the width-to-gate length ratios, can exactly control the activation sequence.

(117) FIG. 12 exemplary shows an automatic gain control component 1200. The automatic gain control carried out by automatic gain control component 1200 can be based on a/the DC current 1209 generated by the DC restoration component (not shown) and a scaled version of the main transimpedance amplifier circuit (TIA), called dummy transimpedance amplifier circuit (dummy TIA) 1208.

(118) In other words the exemplary automatic gain control component 1200 can, for example, comprise at least one dummy gain amplifier stage with a dummy gain amplifier 1205 and dummy programmable feedback resistors 1206, 1207, as well as an output buffer.

(119) In fact the dummy transimpedance amplifier circuit 1208 can have, for example, the same number of dummy gain amplifier stages, the same number of dummy gain amplifiers and the same number of dummy programmable feedback resistors as the main transimpedance amplifier circuit (not shown), but their characteristic values and properties can be scaled such as to have a lower power consumption but the same DC characteristics (e.g. same equivalent DC gain and same equivalent transimpedance) to facilitate correct calibration of the optical receiver circuit.

(120) Furthermore, the DC current, i.e. the average current, I.sub.avg, of the current signal generated by the at least one photo detector, i.e. the photocurrent, can be proportional to the amplitude of the AC (alternating current) component of the input signal.

(121) The reference signal 1201 can, for example a reference voltage, represent a/the maximum output voltage amplitude allowed at the main transimpedance amplifier circuit output (not shown).

(122) The possible block following the dummy transimpedance amplifier circuit 1208 can have a high gain, e.g. of up to 60 dB or more, and can generate an output voltage proportional to the difference between the dummy transimpedance amplifier circuit 1208 and the reference voltage 1201.

(123) Said block may further comprise a sequential voltage generator 1203 and that can be similar to the possible sequential voltage generator of the DC restoration component.

(124) Said sequential voltage generator 1203 can create sequentially voltage control signals 1202 (R.sub.ctrl<1:N>) that can drive the programmable feedback resistors of both the dummy transimpedance amplifier circuit 1208 and the main transimpedance amplifier circuit.

(125) FIG. 13 exemplary shows an optical communication system 1300 comprising an optical transmitter 1301, an optical fiber link 1302, e.g. a plastic fiber, and an optical receiver 1303.

(126) In this exemplary optical communication system 1300 a light emitting device, e.g. light emitting diode (LED) 1306 driven by an LED driving circuit 1305, of the optical transmitter, outputs an optical signal that is fed into an optical fiber link 1302, e.g. a plastic fiber, which guides the optical signal to a light receiving device, the optical receiver 1303, where the light is for example received by a photo diode 1307. The light received by the photo diode 1307 generates a photocurrent that is converted, for example, by a trans-impedance amplifier circuit (TIA) 1308 according to and consistent with the exemplary architecture(s) described above, into an electrical voltage output signal 1309.

(127) Furthermore, in this exemplary optical communication system 1300 the optical receiver 1303 comprises an optical receiver circuit 1304 according to and consistent with the exemplary architecture(s) of an optical receiver circuit described above.

(128) For completeness it is noted that the exemplary architecture(s) of an optical receiver circuit described above is/are not limited to a MOS (metal-oxide-semiconductor) architecture based implementation. The design of the architecture of an optical receiver circuit exemplary described above is also compatible with any other technology, such as, for example, Bipolar (bipolar junction transistor technology), BiCMOS (combination of bipolar junction transistor technology and complementary metal-oxide-semiconductor technology), GaAs (Gallium Arsenide) based technology, etc.

(129) Followed by 13 sheets comprising 13 figures.

(130) The reference numerals identify the following components:

(131) 100 Exemplary schematic architecture of parts of an optical receiver circuit

(132) 101 Exemplary transimpedance amplifier circuit

(133) 102 Exemplary input/input signal, e.g. input current signal, of transimpedance amplifier circuit

(134) 103 Exemplary output/output signal, e.g. output voltage signal, of transimpedance amplifier circuit

(135) 104 Exemplary photo detector, e.g. a photo diode

(136) 105 Exemplary local/global programmable feedback resistor

(137) 106 Exemplary local programmable feedback resistor

(138) 107 Exemplary local programmable feedback resistor

(139) 108 Exemplary local programmable feedback resistor

(140) 109 Exemplary local/global programmable feedback resistor

(141) 110 Exemplary local programmable feedback resistor

(142) 111 Exemplary local programmable feedback resistor

(143) 112 Exemplary local programmable feedback resistor

(144) 113 Exemplary programmable feedback resistor for controlling the equivalent transimpedance of the transimpedance amplifier circuit

(145) 114 Exemplary programmable feedback resistor for controlling the equivalent transimpedance of the transimpedance amplifier circuit

(146) 115 Exemplary programmable feedback resistor for controlling the equivalent transimpedance of the transimpedance amplifier circuit

(147) 116 Exemplary programmable feedback resistor for controlling the equivalent transimpedance of the transimpedance amplifier circuit

(148) 117 Exemplary programmable feedback resistor for controlling the equivalent transimpedance of the transimpedance amplifier circuit

(149) 118 Exemplary programmable feedback resistor for controlling the equivalent transimpedance of the transimpedance amplifier circuit

(150) 119 Exemplary gain amplifier

(151) 120 Exemplary gain amplifier

(152) 121 Exemplary gain amplifier

(153) 122 Exemplary gain amplifier

(154) 123 Exemplary (first) gain amplifier stage

(155) 124 Exemplary gain amplifier stage

(156) 125 Exemplary gain amplifier stage

(157) 126 Exemplary (last) gain amplifier stage

(158) 200 Exemplary optical receiver circuit

(159) 201 Exemplary transimpedance amplifier circuit

(160) 202 Exemplary input/input signal, e.g. input current signal, of transimpedance amplifier circuit

(161) 203 Exemplary output/output signal, e.g. output voltage signal, of transimpedance amplifier circuit

(162) 204 Exemplary automatic gain component

(163) 205 Exemplary DC restoration component

(164) 206 Exemplary dark (dummy) photo detector, dark (dummy) photo diode, or equivalent electrical model of a photo detector/photo diode

(165) 207 Exemplary photo detector, e.g. a photo diode

(166) 208 Exemplary possible low-pass filter

(167) 209 Exemplary (first) gain amplifier stage

(168) 210 Exemplary gain amplifier stage

(169) 211 Exemplary (last) gain amplifier stage

(170) 212 Exemplary (first) gain amplifier

(171) 213 Exemplary gain amplifier

(172) 214 Exemplary (last) gain amplifier

(173) 215 Exemplary local/global programmable feedback resistor

(174) 216 Exemplary local/global programmable feedback resistor

(175) 217 Exemplary local programmable feedback resistor

(176) 218 Exemplary local programmable feedback resistor

(177) 219 Exemplary local programmable feedback resistor

(178) 220 Exemplary local programmable feedback resistor

(179) 221 Exemplary fixed (global) resistor between input and output of transimpedance amplifier circuit

(180) 222 Exemplary unity gain amplifier, exemplary buffer

(181) 223 Exemplary unity gain amplifier, exemplary buffer

(182) 224 Exemplary shunt programmable resistor

(183) 225 Exemplary fixed resistor between input and output of transimpedance amplifier circuit

(184) 226 Exemplary (global) programmable feedback resistor for controlling the equivalent transimpedance of the transimpedance amplifier circuit

(185) 227 Exemplary (global) programmable feedback resistor for controlling the equivalent transimpedance of the transimpedance amplifier circuit

(186) 300 Exemplary optical receiver circuit

(187) 301 Exemplary input/input signal, e.g. input current signal, of transimpedance amplifier circuit

(188) 302 Exemplary output/output signal, e.g. output voltage signal, of transimpedance amplifier circuit

(189) 303 Exemplary transimpedance amplifier circuit

(190) 304 Exemplary gain amplifier

(191) 305 Exemplary (local/global) programmable feedback resistor

(192) 306 Exemplary (local/global) programmable feedback resistor

(193) 307 Exemplary automatic gain component

(194) 308 Exemplary DC restoration component

(195) 309 Exemplary photo detector, e.g. a photo diode

(196) 310 Exemplary dark (dummy) photo detector, dark (dummy) photo diode, or equivalent electrical model of a photo detector/photo diode

(197) 311 Exemplary output buffer

(198) 400 Exemplary time series of an exemplary input current signal

(199) 401 Exemplary input current signal, e.g. photocurrent signal

(200) 402 Exemplary maximum input current level, exemplary maximum input current signal

(201) 403 Exemplary average input current level, exemplary average input current signal

(202) 404 Exemplary minimum input current level, exemplary minimum input current signal

(203) 500 Exemplary gain amplifier, e.g. exemplary gain amplifier of a first gain amplifier stage

(204) 501 Exemplary resistive load, for example resistive load of a cascoded transistor

(205) 502 Exemplary common-mode control component

(206) 503 Exemplary bias current

(207) 504 Exemplary part of differential input pair

(208) 505 Exemplary part of differential input pair

(209) 600 Exemplary common-mode control circuitry component

(210) 601 Exemplary reference value

(211) 602 Exemplary (first) large resistor, exemplary resistance to calculate the output common-mode

(212) 603 Exemplary (second) large resistor, exemplary resistance to calculate the output common-mode

(213) 700 Exemplary control sequence

(214) 701 Exemplary control sequence step

(215) 702 Exemplary control sequence step

(216) 703 Exemplary control sequence step

(217) 704 Exemplary control sequence step

(218) 800 Exemplary implementation of a programmable feedback resistor

(219) 801 Exemplary sizing of gate-width-to-gate-length ratios

(220) 802 Exemplary transistor

(221) 803 Exemplary transistor

(222) 804 Exemplary transistor

(223) 805 Exemplary transistor

(224) 806 Exemplary gate control voltage of a programmable resistance

(225) 807 Exemplary gate control voltage of a programmable resistance

(226) 808 Exemplary gate control voltage of a programmable resistance

(227) 809 Exemplary gate control voltage of a programmable resistance

(228) 810 Exemplary gate-width-to-gate-length ratio

(229) 811 Exemplary gate-width-to-gate-length ratio

(230) 812 Exemplary gate-width-to-gate-length ratio

(231) 900 Exemplary behavior of linear decrease of equivalent impedance

(232) 901 Exemplary equivalent impedance of programmable feedback resistor

(233) 902 Exemplary transistor dimensions dependence of the programmable resistor(s)

(234) 903 Exemplary gate control voltages of the programmable feedback resistor(s)

(235) 1000 Exemplary DC restoration component architecture

(236) 1001 Exemplary low-pass filter

(237) 1002 Exemplary sequential voltage generator

(238) 1003 Exemplary sequential current source component, for example, set of parallel transistors

(239) 1004 Exemplary behaviour of DC input current

(240) 1005 Exemplary low frequency pole

(241) 1006 Exemplary closed-loop control response behaviour

(242) 1100 Exemplary sequential voltage control component

(243) 1101 Exemplary dependence of gate-width-to-gate-length ratios

(244) 1102 Exemplary input current reference, I.sub.ref

(245) 1103 Exemplary signal from low-pass filter

(246) 1104 Exemplary current mirror

(247) 1105 Exemplary output voltage

(248) 1106 Exemplary output voltage

(249) 1107 Exemplary output voltage

(250) 1200 Exemplary automatic gain control component

(251) 1201 Exemplary reference signal, e.g. representing maximum output voltage amplitude allowed at the main transimpedance amplifier circuit output

(252) 1202 Exemplary voltage control signals

(253) 1203 Exemplary sequential voltage generator

(254) 1204 Exemplary output buffer

(255) 1205 Exemplary dummy gain amplifier

(256) 1206 Exemplary dummy programmable feedback resistor

(257) 1207 Exemplary dummy programmable feedback resistor

(258) 1208 Exemplary dummy transimpedance amplifier circuit

(259) 1209 Exemplary DC (dummy) current signal generated by DC restoration component

(260) 1300 Exemplary optical communication system

(261) 1301 Exemplary optical transmitter

(262) 1302 Exemplary optical fiber link

(263) 1303 Exemplary optical receiver

(264) 1304 Exemplary optical receiver circuit

(265) 1305 Exemplary LED driver circuit

(266) 1306 Exemplary light emitting device, e.g. light emitting diode (LED)

(267) 1307 Exemplary photo detected, e.g. photo diode

(268) 1308 Exemplary transimpedance amplifier circuit

(269) 1309 Exemplary output signal, e.g. voltage output