Transimpedance amplifier with selective DC compensation

• US 20030218508A1
• Filed: 04/02/2003
• Published: 11/27/2003
• Est. Priority Date: 04/09/2002
• Status: Active Grant

• Current Assignee: Ip Gem Group, LLC
• Original Assignee: Microsemi Corporation (Microchip Technology Incorporated)
• Inventors: Chiou, Chii-Fa, Yoshida, Yuji, Isobe, Yuji
• Granted Patent: US 6,784,750 B2
• Time in Patent Office: Days
• US Class Current: 330/308
• CPC Class Codes: H03F 3/087 with IC amplifier blocks H03F3/085 takes precedence
• CPC Class Codes: H03F 3/211 using a combination of several amplifiers H03F3/60 takes precedence
• CPC Class Codes: H03F 3/45475 using IC blocks as the active amplifying circuit
• CPC Class Codes: H03F 3/45973 by using a feedback circuit

Abstract

A transimpedance amplifier selectively activates DC compensation to optimize a signal-to-noise ratio for an optical receiver. The optical receiver includes a photodiode that converts a light signal to an electrical current signal, and the transimpedance amplifier converts the electrical current signal to a pair of differential voltage signals for further processing. The electrical current signal is provided to the transimpedance amplifier by connecting a cathode of the photodiode to a first input amplifier via a DC blocking capacitor and by directly connecting an anode of the photodiode to a second input amplifier. The transimpedance amplifier includes a DC correction circuit that generates a correction current when an output of the first input amplifier exceeds a predefined threshold. The correction current is added to an input of the second input amplifier to adjust a DC offset at an output of the second input amplifier.

Claims

1. A pseudo-differential transimpedance amplifier with selective DC compensation, the pseudo-differential transimpedance amplifier comprising:

a first input amplifier configured to convert a first polarity of an input current to a first voltage;

a second input amplifier configured to convert a second polarity of the input current to a second voltage; and

a DC compensation circuit configured to adjust a DC level of the second voltage when the amplitude of the input current exceeds a predetermined threshold.

2. The pseudo-differential transimpedance amplifier of claim 1, wherein the first input amplifier is AC-coupled to an optical detector, and the second input amplifier is DC-coupled to the optical detector.

3. The pseudo-differential transimpedance amplifier of claim 2, wherein the first input amplifier is coupled via a capacitor to a cathode of a photodiode in the optical detector, and the second input amplifier is coupled to an anode of the photodiode.

4. The pseudo-differential transimpedance amplifier of claim 1, wherein the amplitude of the input current is detected indirectly by monitoring the first voltage, and wherein the DC level of the second voltage is adjusted by adding a compensation current to an input of the second input amplifier.

5. The pseudo-differential transimpedance amplifier of claim 4, wherein the DC compensation circuit comprises:

an amplitude detection circuit coupled to an output of the first input amplifier and configured to output a control voltage indicating when the first voltage exceeds a predefined threshold voltage; and

a current generator coupled to an output of the amplitude detection circuit and configured to generate the compensation current in response to the control voltage.

6. The pseudo-differential transimpedance amplifier of claim 5, wherein the amplitude detection circuit is a comparator with an internal peak-hold circuit.

7. The pseudo-differential transimpedance amplifier of claim 5, wherein the amplitude detection circuit is a comparator coupled to a logic-level-hold circuit.

8. The pseudo-differential transimpedance amplifier of claim 5, wherein the current generator is a voltage-controlled current source.

9. The pseudo-differential transimpedance amplifier of claim 5, wherein the current generator is a voltage-controlled resistor.

10. The pseudo-differential transimpedance amplifier of claim 5, wherein the current generator is a current mirror circuit.

11. An optical receiver comprising:

a photo detector that generates an input current in response to a light signal;

a first amplifier that converts a first polarity of the input current to a first voltage;

a second amplifier that converts a second polarity of the input current to a second voltage; and

a compensation circuit that generates a correction current when the first voltage is above a predetermined threshold voltage, wherein the correction current is added to an input of the second amplifier to adjust a DC level of the second voltage.

12. The optical receiver of claim 11, wherein the first amplifier is AC-coupled to the photo detector and the second amplifier is DC-coupled to the photo detector.

13. The optical receiver of claim 11, wherein the compensation circuit further comprises:

an amplitude detection circuit coupled to an output of the first amplifier and configured to provide a control voltage when the first voltage is above the predetermined threshold voltage; and

a current generator coupled to an output of the amplitude detection circuit and configured to provide the correction current to the input of the second amplifier in response to the control voltage.

14. A pseudo-differential transimpedance amplifier comprising:

a first input amplifier configured to receive an AC-coupled input current and to output a first voltage;

a second input amplifier configured to receive a DC-coupled input current and to output a second voltage; and

a DC compensation circuit coupled between an output of the first input amplifier and an input of the second input amplifier to selectively adjust a DC offset in the second voltage.

15. The pseudo-differential transimpedance amplifier of claim 14, wherein the DC compensation circuit generates a correction current for the input of the second input amplifier when the first voltage exceeds a predefined voltage threshold.

16. The pseudo-differential transimpedance amplifier of claim 15, wherein the correction current increases in discrete steps as the first voltage exceeds different voltage thresholds.

17. A method for selectively adjusting a DC offset in a pair of differential signals, the method comprising the acts of:

providing a first input signal of a first polarity to a first amplifier to generate a first output voltage;

providing a second input signal of a second polarity to a second amplifier to generate a second output voltage;

detecting the amplitude of the first output voltage;

generating a correction current if the amplitude of the first output voltage exceeds a predetermined threshold; and

combining the correction current with the second input signal at an input of the second amplifier to adjust the DC offset of the second output voltage.

18. The method of claim 17, wherein the first input signal and the second input signal are generated by a photodiode in response to a light signal.

19. A transimpedance amplifier comprising:

means for converting an AC-coupled current signal to a first voltage;

means for converting a DC-coupled current signal to a second voltage;

means for generating a compensation current when the first voltage exceeds a predefined threshold; and

means for adding the compensation current to the DC-coupled current signal to adjust a DC offset in the second voltage.

20. The transimpedance amplifier of claim 19, wherein the AC-coupled current signal and the DC-coupled current signal are of opposite polarities.