TRANS-IMPEDANCE AMPLIFIER WITH FAST OVERDRIVE RECOVERY

Abstract

A high-speed low-noise trans-impedance amplifier (TIA) with fast overdrive recovery is suitable for use in light detection and ranging (LIDAR) receivers.

Claims

1. A light detecting and ranging (LIDAR) system comprising a plurality of detection channels, each detection channel having an avalanche photo-diode (APD) for sensing light to provide an input signal, a trans-impedance amplifier (TIA) for amplifying the input signal and a timing discriminator for detecting an event in the amplified input signal, the TIA comprising: an input stage including a common-emitter configured transistor having a collector terminal and a base terminal, the base terminal of the common-configured transistor being coupled to receive the input signal from the APD; an output stage including a common-base configured transistor having an emitter terminal, a collector terminal and a base terminal, the collector terminal of the common-base configured transistor being coupled to provide the amplified input signal and the base terminal of the common-base configured transistor receiving a first bias voltage; a resistor coupled between the input and output stages of the TIA; and a current limiting circuit, the current limiting circuit comprising a programmable current source coupled to the emitter terminal of the common-base configured transistor and a diode matrix coupled between the emitter terminal of the common-base configured transistor and the collector terminal of the common-emitter configured transistor.

2. The LIDAR system of claim 1, wherein the diode matrix limits the emitter current of the common-base configured transistor such that the common-base configured transistor operates in a linear region.

3. The LIDAR system of claim 1, wherein the input stage is part of a differential input stage.

4. The LIDAR system of claim 3, wherein the differential input stage comprises a current source that limits the emitter current in the common-emitter configured transistor.

5. The LIDAR system of claim 3, wherein the differential input stage further comprises a second common-emitter configured transistor having a base terminal that receives a second bias voltage.

6. The LIDAR system of claim 1, further comprises a voltage clamp circuit that maintains the input signal at no less than a first predetermined voltage or no greater than a second predetermined voltage.

7. The LIDAR system of claim 6, wherein the TIA is coupled between first and second supply voltages and wherein the first and second predetermined voltages may each be one of the supply voltages or a reference voltage.

8. The LIDAR system of claim 6, wherein the voltage clamp circuit comprises a diode.

9. The LIDAR system of claim 6, wherein the voltage clamp circuit comprises a bipolar transistor.

10. The LIDAR system of claim 1, wherein the common-base configured transistor comprises a SiGe PNP transistor.

11. A trans-impedance amplifier (TIA) having an input terminal and an output terminal, comprising: an input stage including a common-emitter configured transistor having a collector terminal and a base terminal, the base terminal of the common-configured transistor being coupled to the input terminal of the TIA; an output stage including a common-base configured transistor having an emitter terminal, a collector terminal and a base terminal, the collector terminal of the common-base configured transistor being coupled to the output terminal of the TIA and the base terminal of the common-base configured transistor receiving a first bias voltage; a resistor coupled between the input and output terminals of the TIA; and a current limiting circuit, the current limiting circuit comprising a programmable current source coupled to the emitter terminal of the common-base configured transistor and a diode matrix coupled between the emitter terminal of the common-base configured transistor and the collector terminal of the common-emitter configured transistor.

12. The TIA of claim 11, wherein the diode matrix limits the emitter current of the common-base configured transistor such that the common-base configured transistor operates in a linear region.

13. The TIA of claim 11, wherein the input stage is part of a differential input stage.

14. The TIA of claim 13, wherein the differential input stage comprises a current source that limits the emitter current in the common-emitter configured transistor.

15. The TIA of claim 13, wherein the differential input stage further comprises a second common-emitter configured transistor having a base terminal that receives a second bias voltage.

16. The TIA of claim 11, further comprises a voltage clamp circuit that maintains the input terminal of the TIA at no less than a first predetermined voltage or no greater than a second predetermined voltage.

17. The TIA of claim 16, wherein the TIA is coupled between first and second supply voltages and wherein the first and second predetermined voltages may each be one of the supply voltages or a reference voltage.

18. The TIA of claim 16, wherein the voltage clamp circuit comprises a diode.

19. The TIA of claim 16, wherein the voltage clamp circuit comprises a bipolar transistor.

20. The TIA of claim 11, wherein the common-base configured transistor comprises a SiGe PNP transistor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1(a) is a block diagram illustrating the principles of operation in a conventional LIDAR system 10.

[0013] FIG. 1(b) shows a TIA output signal in a LIDAR detection channel when a single laser pulse is emitted towards a highly reflective target.

[0014] FIG. 1(c) shows desirable detections of additional reflections of the same emitted laser pulse in a TIA with a sufficiently fast overdrive recovery.

[0015] FIG. 2(a) is a block diagram of conventional trans-impedance amplifier 100, whose trans-impedance gain is determined by the resistance RF of resistor 101.

[0016] FIG. 2(b) shows conventional TIA 200, which includes a common-emitter input stage and a common-collector output stage.

[0017] FIG. 3 shows TIA circuit 300 that includes diode 301 that clamps the voltage across feedback resistor 302.

[0018] FIG. 4 shows TIA circuit 400 which includes MOSFET 401 across feedback resistor 402.

[0019] FIG. 5 shows such a low-noise folded cascode TIA 500 with a SiGe PNP transistor 501.

[0020] FIG. 6 illustrates the operation of NPN common-base stage 600, which includes NPN transistor 601 that provides an output current which is linear within a limited specific range.

[0021] FIG. 7 illustrates the operation of PNP common-base stage 700, which includes PNP transistor Q0 with an output current that is linear within a limited specific range, similar to that illustrated for NPN common-base stage 600 of FIG. 6.

[0022] FIG. 8 shows folded cascode amplifier 800, which incorporates PNP common-base stage 801 with programmable current limiter implemented by current source 805 and diode matrix 804, in accordance with one embodiment of the present invention.

[0023] FIG. 9 shows folded cascode TIA 900 with a differential input stage that includes common-emitter transistor Q1, programmable current source 902 and reference biased common-emitter transistor Q3, in accordance with one embodiment of the present invention.

[0024] FIG. 10 shows TIA 1000 in which PNP transistor Q4 is provided as a voltage clamp at input terminal 803, in accordance with one embodiment of the present invention.

[0025] FIG. 11 shows TIA 1100 in which diode-connected PNP transistor Q4 is provided as a voltage clamp at input terminal 803, in accordance with one embodiment of the present invention.

[0026] To facilitate cross-referencing among the figures, like elements are provided like reference numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] The present invention provides a low-noise high-speed TIA with a current signal limiter to facilitate fast overdrive recovery.

[0028] The common-collector output stage of conventional TIA 200 of FIG. 2(b) recovers slowly from an overdrive condition when a negative input current is present that is of a sufficiently large amplitude. However, a modem complementary SiGe process may provide fast PNP transistors that allows a low-noise folded cascode TIA to achieve similar bandwidths as conventional TIA 200. FIG. 5 shows low-noise folded cascode TIA 500 with a SiGe PNP transistor 501 biased by a fixed reference voltage V.sub.REFP. According to one embodiment of the present invention, the collector current in common-base PNP transistor 501 of TIA 500 can be conditioned using a programmable linear signal limiter, as discussed below. Such a TIAwhere the common-base transistor can be kept out of the saturation region during an overdrive conditionrecovers quickly.

[0029] U.S. Pat. No. 4,808,858 (Stoops), entitled Dual Limit Programmable Linear Signal Limiter, to J. F. Stoops, issued on Feb. 28 1989, discloses an NPN common-base stage which provides an output current that is linear within a limited specific range, but can absorb a considerably larger input current. Based on the teachings in Stoops, FIG. 6 illustrates the operation of an example NPN common-base stage 600, which includes NPN transistor 601 that provides an output current that is linear within a limited specific range. In FIG. 6, programmable current sources 601 and 602 and diode matrix 603 (formed by diodes D1, D2, and D3) together provide a current limiting mechanism. In FIG. 6, NPN transistor 601 is biased by a fixed biased voltage V.sub.REFN. As shown in FIG. 6, programmable current sources 601 and 602 are set to provide currents X+Y and 2Y respectively, for suitable X and Y values. Output current I.sub.outthe common-base collector currentranges from 0 to 2Y.

[0030] At minimum output current, i.e., I.sub.out=0, the corresponding input current I.sub.in is given by (XYdI), for a base current dI. In this regime, diode D3 is not conducting. As input current I.sub.in increases from (XYdI) to (XY+dI), output current I.sub.out increases linearly from 0 to dI, with the current in diode D1 increases from (2YdI) to (2YdI), at which point diode D2 ceases to conduct and diode D3 is turned on. As input current I.sub.in increases from (XY+dI) to (X+Y+dI), output current I.sub.out increases linearly from dI to 2Y, with the current in diode D1 decreases from (2YdI) to 0, while the current in diode D3 increases from 0 to dI. Table I below summarizes the common-base collector current I.sub.out and the currents in diodes D1, D2 and D3 of the diode matrix, versus input current I.sub.in:

TABLE-US-00001 Input Output Current in Current in Current in Region Current Current D1 D2 D3 Minimum X Y dI 0 2Y dI dI 0 current limit Linear range X Y + dI dI 2Y dI 0 0 Maximum X + Y + dI 2Y 0 0 dI current limit

[0031] The method illustrated by the NPN common-base stage of FIG. 6 may be adapted for a PNP common-base stage, such as that shown in FIG. 7. FIG. 7 illustrates the operation of PNP common-base stage 700, which includes PNP transistor Q0 with an output current that is linear within a limited specific range. In FIG. 7, PNP transistor Q0 is biased by a fixed bias voltage V.sub.REFP and provided a diode matrix including diodes D1, D2 and D3 for the current limiting operation. FIG. 7 is a configuration that is suitable for use in LIDAR receiver applications, in which it is common to couple an APD to the input terminal of TIA 700 to provide a unipolar negative current output pulse. The current-limiting operation of PNP common-base stage 700 of FIG. 7 is substantially the same as that illustrated for NPN common-base stage 600 by FIG. 6, described above, and summarized in Table I above.

[0032] FIG. 8 shows folded cascode trans-impedance amplifier 800, which incorporates PNP common-base stage 801 with a programmable current limiter implemented by current source 805 (with current I2) and diode matrix 804, in accordance with one embodiment of the present invention. As shown in FIG. 8, the maximum collector current in transistor Q0 can be limited by programmable current source 805 and diode matrix 804 (implemented by diodes D1, D2 and D3) to maintain the maximum output voltage across resistor 802 to below the reference bias voltage V.sub.REFP at the base terminal of transistor Q0, when an input current pulse with a sufficiently large amplitude is applied to input terminal 803. With transistors Q0 and Q1 kept out of their respective saturation regions under overdrive conditions, TIA 800 recovers very quickly immediately following removal of the large input current pulse from input terminal 803. As diode matrix 804 does not degrade the speed or the noise performance of low-noise, high-speed folded cascode TIA 800, the resulting fast overdrive recovery makes TIA 800 suitable for LIDAR receiver applications.

[0033] In addition to keeping the common-base transistor Q0 of the folded cascode TIA out of the saturation region when a negative pulse is applied, it is also desirable to keep transistor Q1 at the common-emitter input stage of the TIA out of saturation as well, when a unipolar positive current input pulse is present. One approach is to make transistor Q1 part of a differential pair. FIG. 9 shows folded cascode TIA 900 with a differential input stage that includes common-emitter transistor Q1, programmable current source 902 and reference biased transistor Q3, in accordance with one embodiment of the present invention. Transistor Q3 is biased by reference voltage V.sub.REFN. In this configuration, current source 902 (with current I3) sets the maximum emitter current in transistors Q1 and Q3. With suitable selections in folded cascode TIA 900 of bias voltages V.sub.REFP and V.sub.REFN, bias currents I2 and I3 and resistor values R0, RF and R1, both transistors Q0 and Q1 can be kept out of the saturation, so that fast overload recovery under both positive and negative input currents may be achieved.

[0034] A voltage clamp circuit may be provided to folded cascode TIA 900 to prevent the voltage at input terminal 803 from going below the negative supply V.sub.EE. FIG. 10 shows folded cascade TIA 1000 in which PNP transistor Q4 is provided as a voltage clamp circuit at input terminal 803, in accordance with one embodiment of the present invention. Alternatively, the voltage clamp circuit may be provided by a diode-connected NPN transistor, such as illustrated in folded cascade TIA 1100 of FIG. 11 by diode-connected NPN transistor Q4.

[0035] The above detailed description is provided to illustrate specific embodiments of the present invention and is not intended to be limiting. Numerous variations and modifications within the scope of the present invention are possible. The present invention is set forth in the accompanying claims.