Wide dynamic range auto-AGC transimpedance amplifier

Abstract

An automatic gain control (AGC) transimpedance amplifier (TIA) uses a differential structure with feedback PIN diodes to adjust the loop gain of the amplifier automatically to maintain stability over a wide dynamic range when converting optical power using a photodiode to an electrical signal. A stable DC current derived from the photodiode current sets the voltage gain of the amplifier. The use of ultra-linear long carrier lifetime PIN diodes assures the transimpedance feedback resistance is linear. The AGC function adjusts the gain of the TIA to provide a linear stable differential transresistance controlled by the photodiode current; a linear stable AGC function using current supplied by the photodiode; an improvement of about 10 db of the transresistance dynamic range; and reduces the need for internal and external circuitry needed to provide the same function. The TIA is applicable to CATV optical systems which have very strict linearity requirements.

Claims

1. A circuit comprising: a photodiode for generating an optical signal from a light source; a transimpedance amplifier (TIA) having a pair of input terminals and a pair of output terminals for converting the optical signal received at the input terminals to an electrical signal output by the pair of output terminals, the TIA including: a gain stage having the pair of input terminals for receiving and amplifying the optical signal to generate a first signal; an amplifier stage having: the output terminals for amplifying the first signal to generate a second signal at the output terminals; and a transistor for receiving the first signal at an input of the transistor; and an output differential buffer stage for buffering the second signal to generate a buffered output signal; and a pair of feedback resistors, with each feedback resistor connected between a respective output terminal and a respective input terminal of the TIA.

2. The circuit of claim 1, wherein the feedback resistors adjust a loop gain of the TIA automatically.

3. The circuit of claim 1, wherein the feedback resistors include diodes.

4. The circuit of claim 3, wherein the diodes are PIN diodes.

5. The circuit of claim 1, wherein the output differential buffer stage performs impedance matching with an output load.

6. The circuit of claim 1, wherein the gain stage includes a pseudomorphic high-electron-mobility transistor (pHEMT).

7. The circuit of claim 6, wherein the pHEMT is composed of GaAs.

8. The circuit of claim 1, further comprising: a capacitor connected between an output of the transistor and one of the pair of input terminals.

9. The circuit of claim 1, wherein the gain stage receives the optical signal from the photodiode; and wherein the output differential buffer stage generates the buffered output signal proportional to the optical signal.

10. A method comprising: receiving light from a light source at a photodiode; generating an optical signal from the light using the photodiode; receiving the optical signal at a pair of input terminals of a transimpedance amplifier (TIA), wherein the TIA includes: a gain stage having the pair of input terminals for receiving and amplifying the optical signal to generate a first signal; an amplifier stage having: the output terminals for amplifying the first signal to generate a second signal at the output terminals; and a transistor for receiving the first signal at an input of the transistor; and an output differential buffer stage for buffering the second signal to generate a buffered output signal; providing a pair of feedback resistors, with each feedback resistor connected between a respective output terminal and a respective input terminal of the TIA; converting the optical signal received at the input terminals to an electrical signal; and outputting the electrical signal by the pair of output terminals of the TIA.

11. The method of claim 10, further comprising: providing a pair of feedback resistors, with each feedback resistor connected between a respective output terminal and a respective input terminal of the TIA; and adjusting a loop gain of the TIA automatically using the feedback resistors.

12. A device comprising: a photodiode for generating an optical signal from light, wherein the optical signal has a direct current (DC) component; a first resistor having a resistance and connected to the photodiode; an amplifier having an open loop gain and connected to the first resistor, wherein the amplifier includes: a gain stage having the pair of input terminals for receiving and amplifying the optical signal to generate a first signal; an amplifier stage having: the output terminals for amplifying the first signal to generate a second signal at the output terminals; and a transistor for receiving the first signal at an input of the transistor; and an output differential buffer stage for buffering the second signal to generate a buffered output signal; and a pair of feedback resistors, with each feedback resistor connected between a respective output terminal and a respective input terminal of the amplifier; wherein the DC component changes the resistance to adjust the open loop gain, thereby maintaining stability of operation of the device.

13. The device of claim 12, wherein the resistor is a diode.

14. The device of claim 13, wherein the diode is selected from a PIN diode, a Schottky diode, and a PN-junction diode.

15. The device of claim 13, wherein the diode includes a three-terminal transistor connected to operate as a two-terminal diode.

16. The device of claim 15, wherein the three-terminal transistor is selected from a field-effect transistor (FET), a bipolar junction transistor (BJT), and a heterojunction bipolar transistor (HBT).

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The foregoing summary, as well as the following detailed description of presently preferred embodiments of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.

(2) In the drawings:

(3) FIG. 1 illustrates a TIA circuit with a PIN diode in the prior art;

(4) FIG. 2 illustrates a Maxim 3854 fixed feedback resistor differential TIA in the prior art;

(5) FIG. 3 illustrates a differential TIA with feedback PIN diodes of the present invention;

(6) FIG. 4 illustrates a differential TIA of the present invention with PIN feedback diodes and PIN diodes sharing photocurrent for AGC stability control;

(7) FIG. 5 illustrates a single-ended equivalent circuit used to describe stability in the TIA of the present invention;

(8) FIG. 6 illustrates Bode plots used to describe stability in the TIA of the present invention;

(9) FIGS. 7A-7B illustrate voltage gains of the present invention; and

(10) FIG. 8 illustrates the performance of a TIA circuit 30 with and without PIN diode AGC resistors.

(11) To facilitate an understanding of the invention, identical reference numerals have been used, when appropriate, to designate the same or similar elements that are common to the figures. Further, unless stated otherwise, the features shown in the figures are not drawn to scale, but are shown for illustrative purposes only.

DETAILED DESCRIPTION

(12) Certain terminology is used in the following description for convenience only and is not limiting. The article a is intended to include one or more items, and where only one item is intended the term one or similar language is used. Additionally, to assist in the description of the present invention, words such as top, bottom, side, upper, lower, front, rear, inner, outer, right and left may be used to describe the accompanying figures. The terminology includes the words above specifically mentioned, derivatives thereof, and words of similar import.

(13) As shown in FIG. 3, an example circuit 10 has a differential TIA 12 of the present invention with feedback PIN diodes 14, 16 for converting an optical signal received by a photodiode 18 into an analog signal on electrical lines 20, such as an analog voltage across the lines 20. The lines 20 may be connected to an inductor 22 which may be a component of a transformer 24 for augmenting the analog signal. In an example embodiment, the TIA 12 may use a GaAs high electron mobility field-effect transistor (HFET), commercially available from GLOBAL COMMUNICATION SEMICONDUCTORS LLC (GCS), and the PIN diodes 14, 16 may be silicon PIN diodes.

(14) This circuit 10 can provide about +3 dbm to about 12 dbm of optical dynamic range. Because the circuit 10 is differential, it can provide good second order distortion performance. Any low optical power performance is limited by stable gain and bandwidth considerations.

(15) The transimpedance amplifier (TIA) of the present invention is shown in greater detail in FIG. 4, with PIN feedback diodes as well as PIN diodes sharing photocurrent for AGC stability control. The circuit 30 includes the differential TIA 12 having output lines 20, and internally includes an amplifier 32 connected to transistors 34, 36, which in turn are connected to a buffer 38. The transistors 34, 36 may optionally be connected to blocking capacitors 40, 42, respectively. Variable resistors 44-50 are connected in series to the photodiode 18, and have resistances RLA, RFBA, RFBB, and RLB, respectively. One or more of the variable resistors 44-50 may be or may include diodes.

(16) The circuit 30 of FIG. 4 offers an inventive solution for optical-to-electrical operations with wide-bandwidth, low-noise, high stability, and a wide dynamic range. The circuit 30 has three gain stages: an output differential buffer stage using the buffer 38; an input amplifier stage using the transistors 34, 36; and a gain stage using the amplifier 32. The output differential buffer stage is used to provide a good output impedance match to an output load, such as the inductor 22 or the transformer 24 in FIG. 3, which is virtually independent of the value of the feedback resistance R.sub.f. The amplifier 32 of the gain stage includes a GaAs pHEMT transistor which is low noise and high input impedance. The voltage gain of the amplifier 32 in this stage is denoted by the label A1, and has the value A.sub.1. In this embodiment the feedback elements formed by the resistors 46, 48, and having resistance values RFBA and RFBB, respectively, are connected to the output drains of the transistors 34, 36, having transconductances labeled gm.sub.A and gm.sub.B, respectively. Capacitors 40, 42 are included at the output drains of the transistors 34, 36, respectively, and are meant to be large and essentially an AC short over the frequencies of use.

(17) As shown in FIG. 4, the circuit 30 of the present invention has PIN diodes as linear variable resistors 44-50. A DC bias voltage at pin 52, labeled P1, is provided to the PIN photodiode 18, labeled X3. A major aspect of the circuit 30 of the present invention is that the same photodiode current flows through all of the devices 18 and 44-50, and not to the input of the first stage 32 due to its high input impedance. DC blocking capacitors 40, 42 could be added at the drains of the transistors 34, 36, respectively, which may be BJT input transistors. The current sources 54, 56, labeled IB1, IB2, respectively, provide the bias currents for the transistors 34, 36, respectively.

(18) Referring to FIG. 4, the operation of the circuit 30 is as follows: an optical power signal is applied to photodiode 18. The current produced by the photodiode 18 will be I.sub.X3=P.sub.optR where P.sub.opt is the optical power, and R is the responsivity of the photodiode 18. For 1310 nm and 1550 nm photodiodes, R is close to unity. The DC value of I.sub.X3 will flow through all four PIN diodes 44-50, causing a value of equivalent feedback resistance R.sub.f proportional to the current for PIN diodes 44-50. The output differential voltage V.sub.out, assuming A.sub.ol is high and RFBA=RFBB, will be:
V.sub.out=2(I.sub.X3RFA)(BUFFER)(1)
where BUFFER is a factor associated with the buffer 38. For gm.sub.A=gm.sub.B,
A.sub.ol=2A.sub.1gm.sub.ARLA(2)
where A.sub.ol is the open loop gain, and A.sub.1 is the gain of the amplifier 32.

(19) Because the capacitors 40, 42 are an AC short, the resistor 44 appears to be directly as the load resistor element for the second gain stage of the TIA 12. At high optical powers, all of the PIN diode resistors 44-50 have resistances of small values and offer high bandwidth and low distortion since V.sub.out will follow the above equations.

(20) In fact, since the feedback resistance R.sub.f for the PIN diodes 44-50 changes one-to-one (1:1) with DC current, V.sub.out will remain constant, hence removing the need for an external AGC loop in the circuit 30. As the P.sub.opt is reduced, all of the resistances of the PIN diode resistors 44-50 will increase in value, offering a high A.sub.ol and low noise.

(21) Referring to FIGS. 3-4, the devices and circuits shown therein use a DC component of the optical signal from the photodiode 18 changes the resistance to adjust the open loop gain of the amplifiers 12, 32, thereby maintaining stability of operation of the device or circuit. The photodiode 18 generates an optical signal from light from a light source, and the optical signal has a direct current (DC) component. At least one resistor or other component has a resistance and is connected to the photodiode 18. Each amplifier 12, 32 in FIGS. 3-4, respectively, has an open loop gain and is connected to the resistor, so that the DC component changes the resistance to adjust the open loop gain, thereby maintaining stability of operation of the device or circuit. The resistor may be a diode such as one or more of the diodes 14, 16 in FIG. 3, and the diodes 44-50 in FIG. 4. Each diode may be a PIN diode, a Schottky diode, and a PN-junction diode. Alternatively, the diode may be a three-terminal transistor connected to operate as a two-terminal diode in a manner known in the art. The three-terminal transistor may be a field-effect transistor (FET), a bipolar junction transistor (BJT), and a heterojunction bipolar transistor (HBT).

(22) FIG. 5 illustrates a single-ended equivalent circuit 60 used to describe stability in the TIA 12 of the present invention. In FIG. 5, the amplifier 62 is connected in parallel with the elements R.sub.f and C.sub.f, with R.sub.f being the feedback resistance, and C.sub.f being the feedback capacitance, while the input capacitance is C.sub.i which is parallel to a current source I.sub.p representing the input optical power signal. Referring to the simplified single-ended TIA circuit 60 of FIG. 5, one can show the trade-offs between transimpedance bandwidth and noise:
Z.sub.in=R.sub.f/((1+sC.sub.fR.sub.f)(1+A.sub.ol)(3)
C.sub.i=C.sub.photodiode+C.sub.amplifier(4)
f.sub.3 db=1/(2Z.sub.inC.sub.i)(5)
then a bigger open loop gain A.sub.ol gives a higher f.sub.3 db, and then a bigger R.sub.f which gives a lower f.sub.3 db. In addition,
.sup.2=4KTf/R.sub.f(6)
then a bigger R.sub.f results in lower noise.

(23) From the above Equations (3)-(6), one can see the tradeoff between noise and bandwidth. In order to achieve the best bandwidth and the best circuit performance with the lowest noise as possible, circuits require a TIA with a very large feedback resistance R.sub.f and a very large amplifier open loop gain A.sub.ol.

(24) Another tradeoff which arises with wide dynamic systems is stability at high optical powers. With an optical power as high as +3 dbm the typical PIN diode would offer an R.sub.f of about 100 ohms. This would cause a lot of feedback from the input to the output. If there is an input frequency f.sub.i in the amplifier 62 in FIG. 5, where the total phase shift is 360 before A.sub.ol reaches unity, the circuit 60 will become unstable and oscillate. The equations for beta, the feedback factor, and f.sub.zf are given below:

(25) $\begin{array}{cc}=\frac{1+R_f{C}_{f}s}{1+R_f(C_i+C_{f)}s}\mathrm{and}& \left(5\right)\\ f_{\mathrm{zf}}=\frac{1}{2R_f(C_i+C_{f)}}& \left(6\right)\end{array}$

(26) FIG. 6 illustrates Bode plots used to describe stability in the TIA of the present invention. As shown in FIG. 6, the current-to-voltage gain of the TIA, measured as a function of R.sub.fI.sub.p, is very linear over a wide range of frequencies, until around the input frequency f.sub.i at which the operation of the TIA would become unstable.

(27) FIGS. 7A-7B illustrate voltage gains of the present invention. FIG. 7A measures voltage gains for the second stage in FIG. 4 using photocurrents I.sub.X3 of 2 mA, 500 A, 100 A and 10 A, in which the voltage gain is measured by the product A.sub.1gm.sub.ARLA. For the photocurrents I.sub.X3 of 2 mA, 500 A, 100 A and 10 A, the gain gm.sub.ARLA is 0.94, 2.9, 7 and 10 in the mid-band response, respectively. FIG. 7B measures the total voltage gain using photocurrents I.sub.X3 of 2 mA, 500 A, 100 A and 10 A. For the measure of stability in the transresistance response, a peaking of about 3 db is the highest allowable in a circuit.

(28) FIG. 8 illustrates the performance of the TIA circuit 30 with and without PIN diode AGC resistors 44, 50 in FIG. 4. At a photocurrent I.sub.X3 of 250 A, labeled I.sub.ph in FIG. 8, the circuit 30 without the resistors 44, 50 shows 7 db of peaking while the circuit 30 with the resistors 44, 50 is at 2.9 db. At higher I.sub.X3 of 2.5 mA, labeled I.sub.ph in FIG. 8, the circuit 30 without the resistors 44, 50 shows less peaking of about 2 db, while the other circuit with the resistors 44, 50 shows 16 db of peaking. Such results represent a 10 times or 20 db transresistance improvement in performance.

(29) The circuit of FIG. 6 can be modified using two extra PIN diode resistors connected to the first gain stage via DC block capacitors to further improve performance. In addition, a number, N, of gain stages using this technique may be used, with N being greater than one.

(30) The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention, therefore, will be indicated by claims rather than by the foregoing description. All changes, which come within the meaning and range of equivalency of the claims, are to be embraced within their scope.