OPTICAL RECEIVER WITH A CASCODE FRONT END

Abstract

An optical receiver (1) comprises a differential TIA (4) linked with a photodiode (2, 3) providing a current sense signal (I.sub.sig_tia). The receiver is configured to provide to the TIA a sense signal as a sense TIA input (I.sub.sig_tia) and a second input (I.sub.dark_tia) which is a proportion of the maximum sense signal. The proportion input is half of said maximum sense signal. The inputs to the TIA are via cascode circuits (5, 6), thereby providing the advantages of a low input impedance for large area photodiodes at the TIA input, while creating a fully differential signal at the output, and the reduction of TIA bandwidth in burst mode applications, which filters out high frequency noise.

Claims

1. An optical receiver comprising: a differential TIA linked with a photodiode providing a current sense signal, a first cascode circuit configured to provide to the TIA a sense signal as a sense TIA input, and a circuit configured to provide to the TIA a proportion of a maximum sense signal as a proportion TIA input, a replica circuit, wherein the first cascode circuit is configured to provide to the replica circuit a copy of the sense signal, based on replicating the TIA load, and said replica circuit is configured to provide a signal from which said proportion TIA input is derived, and a peak detector to peak detect said signal from the replica circuit, and the peak detector provides directly or indirectly said proportion TIA input.

2. The optical receiver as claimed in claim 1, wherein the replica circuit comprises a replica amplifier, a dummy transimpedance load, and a transconductance block.

3. The optical receiver as claimed in claim 1, wherein the peak detector comprises a transconductance block.

4. The optical receiver as claimed in claim 1, wherein the differential TIA gain is regulated with an automatic gain control AGC loop.

5. The optical receiver as claimed in claim 1, wherein the cascode circuit is a regulated gate cascode RGC circuit.

6. The optical receiver as claimed in claim 1, wherein the cascode circuit is biased with a bias current.

7. The optical receiver as claimed in claim 1, wherein the peak detector is configured to generate a received signal strength indicator.

8. The optical receiver as claimed in claim 1, wherein the receiver comprises a transconductance block driven by the peak detector output, and a received signal strength indicator is an output of the transconductance block.

9. The optical receiver as claimed in claim 1, wherein the photodiode is a monolithic integrated photodiode.

10. The optical receiver as claimed in claim 1, wherein the peak detector provides the proportion TIA input directly into an AGC within the TIA.

11. The optical receiver as claimed in claim 1, wherein the peak detector comprises a transconductance component to provide a current sink signal for said second cascode circuit, configured to provide the proportion TIA input.

12. The optical receiver as claimed in claim 1, wherein the peak detector comprises a transconductance component to provide a current sink signal for said second cascode circuit, configured to provide the proportion TIA input; and wherein the cascode circuits are configured to provide the proportion TIA input as half of said maximum sense signal.

13. The optical receiver as claimed in claim 1, wherein the peak detector comprises a transconductance component to provide a current sink signal for said second cascode circuit, configured to provide the proportion TIA input; and wherein the peak detector is configured to use a replicated current signal to generate half the maximum received current, and the second cascode circuit is arranged to produce a fully differential output voltage for incoming received light.

14. The optical receiver as claimed in claim 1, wherein the peak detector comprises a transconductance component to provide a current sink signal for said second cascode circuit, configured to provide the proportion TIA input; and wherein the first cascode circuit is arranged to provide a generated half the maximum received current signal as a current source to the positive input to the differential TIA or as a current sink to the negative input to the differential TIA, to produce a fully differential output voltage for incoming received light.

15. The optical receiver as claimed in claim 1, wherein the peak detector comprises a transconductance component to provide a current sink signal for said second cascode circuit, configured to provide the proportion TIA input; and wherein the optical receiver further comprises a dark photodiode or an equivalent element, and said second cascode circuit is connected to receive a signal from said dark photodiode to provide the proportion TIA input.

16. The optical receiver as claimed in claim 1, wherein the peak detector comprises a transconductance component to provide a current sink signal for said second cascode circuit, configured to provide the proportion TIA input; and wherein said AGC is configured to use positive and negative TIA outputs as feedback signals to dynamically modify its gain.

17. The optical receiver as claimed in claim 1, wherein the receiver further comprises a pseudo differential to differential amplifier connected at its input to an output of the TIA, said amplifier being configured to produce a fully differential output voltage for incoming received light.

18. The optical receiver as claimed in claim 1, wherein the receiver further comprises a pseudo differential to differential amplifier connected at its input to an output of the TIA, said amplifier being configured to produce a fully differential output voltage for incoming received light; and wherein the TIA is configured to use said pseudo differential to differential amplifier output signals to dynamically adjust the AGC gain using feedback control.

19. An electronic device comprising a processing circuit and an optical receiver as claimed in claim 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

Brief Description of the Drawings

[0054] The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only with reference to the accompanying drawings in which:

[0055] FIG. 1 is a set of plots to illustrate background to the invention, as set out above;

[0056] FIG. 2 is a circuit diagram of an optical receiver of the invention;

[0057] FIG. 3 is a set of plots for operation of the optical receiver;

[0058] FIG. 4 is a diagram showing the cascode arrangement in more detail;

[0059] FIG. 5 is a diagram of a transconductance circuit used in the receiver in some embodiments;

[0060] FIGS. 6 and 7 are circuit diagrams of alternative optical receivers of the invention; and

[0061] FIG. 8 is a series of plots for these receivers.

DESCRIPTION OF THE EMBODIMENTS

[0062] Referring to FIG. 2 an optical receiver 1 comprises a signal photodiode (PD) 2, a dark PD 3, and a differential TIA 4. There are cascode circuits 5 and 6, a replica circuit 10, and a peak detector circuit 11. The output from the series combination of replica circuit 10, and a peak detector circuit 11 is provided to the TIA 4 as I.sub.peakDet, which creates a reference when added to I.sub.dark to produce I.sub.dark_tia which creates a fully differential TIA output when compared to the data signal I.sub.sig_tia.

[0063] The signal input to the TIA 4 is via the cascode device 5 which: [0064] receives a V.sub.cas_sig_gate control signal, [0065] receives the PD 2 signal, I.sub.sig, [0066] receives a copied current I.sub.copy, from the replica circuit 10, [0067] delivers a signal I.sub.sig_tia tia to the TIA 4, [0068] delivers a signal I.sub.copy_tia to a replica load in the TIA 4.

[0069] The reference input to the TIA 4 is via the cascode device 6 which: [0070] receives a control input V.sub.cas_dark_gate, [0071] receives the PD 3 reference signal I.sub.dark, [0072] receives a reference signal I.sub.peakDet from the peak detector 11 and delivers a signal I.sub.dark_tia to the TIA 4, which is a sum of I.sub.dark and I.sub.peakDet.

[0073] The peak detector 11 receives an input from the replica circuit 10, and provides a feed-forward signal to the transconductance circuit (tc) tc2. This feed forward signal causes the circuit tc2 to produce an output I.sub.peakDet, which is an input signal to the TIA 4, via the cascode device 6. This feedforward current creates a fully differential TIA output in combination with other inputs. Feed forward control is beneficial because of its speed in taking predefined action depending on the strength of the sense signal, which is particularly relevant in a burst mode receiver application.

[0074] In more detail, the signal receiving photodiode 2 and the dark photodiode 3 are both connected to the cascode circuits 5 and 6 that feed into the differential TIA 4 whose gain can be regulated with an AGC control loop. A copy of the signal current is generated by the replica circuitry 10, and is used by the peak detector circuit 11 to output half the maximum input signal current. The peak detector circuit 11 output is connected to the dark (or minus) (in an alternative embodiment can be connected to the active side) side of the differential TIA 4. As illustrated in FIG. 3 supplying a differential TIA with a signal and dark current in this manner produces a fully differential output signal.

[0075] Advantageously, the TIA 4 receives a proportion TIA input which is half of the maximum signal, rather than for example being zero. This achieves a fully differential output.

[0076] The differential TIA 4 input signal current (I.sub.sig_tia) is regenerated by the cascode circuit 5, where the gate source voltage is copied in the replica circuit 10 with a replica amplifier (Amp) and transconductance block tc1 produces a replica of the signal current (I.sub.copy) sinked from a replica load in the TIA 4 by I.sub.copy_tia.

[0077] The cascode devices 5(a) and 5(b) are used to produce a copied current I.sub.copy. If the gate, source and drain voltages of the cascode device 5(a) are equal to corresponding voltages of the cascode device 5(b) then the current will be the same through both devices. The gate voltage V.sub.cas_sig_gate is common to both devices. The Amp and transconductance block tc1 of the replica circuit 10 ensure that the source voltages V.sub.SA and V.sub.SB are the same, and the replica load input to the TIA 4 (which is the same as the plus input) ensures that the drain voltage V.sub.DA and V.sub.DB will be the same if I.sub.copy is the same as I.sub.sig.

[0078] The output voltage of the Amp in the replica circuit 10 is peak detected by the peak detector circuit 11, to output half the maximum received current (I.sub.peakDet) by using a transconductance block tc2 which is half the size of the transconductance block tc1. This current I.sub.peakDet equals half of maximum I.sub.sig_tia tia and is connected to the minus (or dark) side of the differential TIA via a second cascode device 6, as shown in FIG. 2. The I.sub.dark_tia current is equal to the sum of the PD 3 reference I.sub.dark and the peak detector current I.sub.peakDet.

[0079] The benefit of providing to the TIA 4 a replica load input is that it helps to provide a stable loop for the replica circuitry 10 to reproduce the sense signal. The benefit of providing to the TIA a sense signal and a signal which is a fixed proportion of a maximum sense signal, is that this architecture produces a fully differential output TIA signal which is generated with minimum delay. It is because of this reason that this architecture is suitable for DC to multiple megabits per second (Mbps) applications. It has the advantage of processing the signal at the front end so that the bandwidth of the differential TIA 4 can be reduced for a low power receiver, and if an AGC was implemented with the TIA it would have limited implications on this architecture.

[0080] An additional aspect is that the peak detector output voltage can drive another (or multiple) transconductance block(s) tc3 to produce a Received Signal Strength Indication (RSSI) current I.sub.RSSI_OUT. If the transconductance block tc3 equals tc1, then this current is an accurate copy of the received photodiode current.

[0081] FIG. 4 shows how a static bias current can be used in a cascode circuit to enhance the cascode circuit 5 to improve speed, where I.sub.sig_tia=I.sub.sig+I.sub.bias. Likewise, this bias current would be symmetrically used in the dark cascode to enhance the circuit 6, and as a consequence its content must be removed from the peak detector current I.sub.peakDet, in order for the receiver to output an accurate differential signal.

[0082] Referring to FIG. 5 a transconductance circuit is illustrated for one or more of the components tc1, tc2, or tc3. This transconductance circuit comprises a source follower 102, a current sink 103 for the source follower, a low g.sub.m current sink 104, and a high g.sub.m current sink 105. The actual g.sub.m are not important, the important point being that the sink 105 has a higher g.sub.m that the sink 104.

[0083] This transconductance circuit improves the dynamic range of a single current sink, to improve the accuracy at low output currents and ensure it has the dynamic range to output higher currents if required.

[0084] The manner in which the two current sinks 104 and 105 are connected to the inputs by the components 102 and 103 is advantageous because it combines the advantages of both a low g.sub.m current sink 104, and a high g.sub.m current sink 105 in one circuit. At low V.sub.IN only the low g.sub.m device 4 is on, as the gate source voltage drop across the source follower (102) ensures that the high g.sub.m device (105) is off. As the V.sub.IN increases the high g.sub.m device 105 is switched on. The sizing of the low g.sub.m and high g.sub.m devices 4 and 5 dictate the accuracy at lower voltage and range at the high voltage, and helps to linearize the output current over the V.sub.IN range.

[0085] The components 102 and 103 provide a voltage drop for the bias of the current sinks 104 and 105, ensuring that the sink 105 turns on later. Such a voltage drop may be provided by another means such as a combination of a resistor with a current source/sink, with the use of an amplifier to buffer the input from any current dissipation.

[0086] In various embodiments, the transconductance circuit which is used may have at least two current sinks. One sink is a single current sink which is sized to have low transconductance and to have good accuracy for low output currents, but would have a poor dynamic range. The other is a single current sink sized to have high g.sub.m and a wide dynamic range but poor accuracy for low output current. The transconductance circuit 1 combines both of these current sinks into one circuit to achieve high accuracy for low output current, a wide dynamic range for high current, and improved the linearity across the range of output currents.

[0087] It will be appreciated that the transconductance circuit improves the dynamic range (due to the high g.sub.m current device), accuracy and tolerance to mismatch of the output current versus a single transconductance current sink (due to the low g.sub.m current device).

[0088] In the transconductance circuit the NMOS devices may be replaced with PMOS devices, so the output transconductance current sinks are now transconductance current sources. Also, the NMOS or PMOS MOSFET devices may be replaced with NPN or PNP bipolar transistors creating bipolar based output transconductance current sources or current sinks.

[0089] Also, the optical receiver 1 achieves low EMI because of the differential architecture of the TIA and subsequent stages. Another reason is that the signal processing is implemented at the front end, which allows the reduction of the TIA bandwidth in burst mode applications to filter out high frequency noise.

[0090] An optical receiver 100 of another embodiment is shown in FIG. 6. Like parts are given the same reference numerals.

[0091] In the circuit 100 there is an additional proportion TIA input to the differential TIA 4, a signal I.sub.peakDet_2, from the transconductance circuit 12. This current is a proportion of the maximum sense signal. This signal provides a feed forward current to the automatic gain control (AGC) block 13. In this case a second or more inputs, which are a proportion of the maximum sense signal are provided to the TIA.

[0092] In more detail, the output of the transconductance circuit 12 is used as an input to an AGC block 13 which is incorporated in the differential TIA. Known TIAs include such AGC blocks, the difference here being the connection to it from the transconductance circuit 12. This signal, a third input current (I.sub.peakDet_2), is a proportion of the maximum sense signal. This can be used to speed up the AGC using feed forward control to adjust the approximate optimal gain of the differential TIA in a pre-defined manner The AGC 13 may use the TIA outputs TIA_plus and TIA_minus signals to dynamically adjust the AGC gain using feedback control, to attain the required output amplitude.

[0093] The main advantage of this embodiment is that for high received power the AGC 13 can quickly achieve the required differential TIA gain via feedforward control as to improve pulse width distortion (PWD) of the received data. This embodiment can be used to reduce the variation of the settling time of the AGC over process, temperature and voltage.

[0094] An alternative optical receiver 200, is shown in FIG. 7. Again, parts similar to those of the other embodiments are given the same reference numerals.

[0095] Again, there is a proportion TIA input to the differential TIA where it's current is a proportion of the maximum sense signal.

[0096] In the FIG. 2 embodiment the proportion TIA input (I.sub.dark_tia) is used to produce a fully differential output voltage for incoming received light as the output of the peak detector 11 was fed into the second cascode device 6.

[0097] This is not the case in the optical receiver 200 (FIG. 7) embodiment as a pseudo differential to differential amplifier 14 is now used to produce a fully differential output voltage for incoming received light. This proportion TIA input (I.sub.peakDet_2) to the differential TIA is used as an input to the automatic gain control (AGC) block 13 which is incorporated in the differential TIA. This input current (I.sub.peakDet_2) is a proportion of the maximum sense signal. This can be used to speed up the AGC using feed forward control to adjust the approximate optimal gain of the differential TIA in a pre-defined manner.

[0098] The AGC may use the pseudo differential to differential amplifier 14 outputs Diff_plus and Diff_minus signals to dynamically adjust the AGC gain using feedback control.

[0099] FIG. 8 is a plot of the signals in the circuit 200, where the TIA output is a pseudo differential signal, and the differential signal is outputted from the pseudo differential to differential amplifier 14. Again similar to the embodiment of FIG. 6; the same advantages apply here.

[0100] An extra advantage of this embodiment is that the total accuracy of the front end maximum current sense circuitry may be relaxed depending on the accuracy needed for the feed-forward control of the AGC. The mismatch of devices and variation over temperature no longer have a large impact on PWD of the receiver output.

[0101] The invention is not limited to the embodiments described but may be varied in construction and detail. For example, the current signal may not represent a light signal, but represents another measurable signal from a transducer. The photodiode may be discrete or a monolithic integrated photodiode. The receiver may operate with transducers other than photodiodes