Transimpedance amplifier with automatic current control

Patent metadata

Field	Value
Publication number	US-11060906-B1
Application number	US-201715844305-A
Country	US
Kind code	B1
Filing date	Dec 15, 2017
Priority date	Dec 28, 2016
Publication date	Jul 13, 2021
Grant date	Jul 13, 2021

Abstract

In one embodiment, a method includes receiving, at an input of a transimpedance amplifier, an input electrical-current signal. The electrical-current signal includes a photodetector current and a DC cancellation current. The photodetector current corresponds to an input optical signal and includes an alternating-current (AC) portion and a direct-current (DC) portion. The method also includes performing, by the transimpedance amplifier, a transimpedance amplification of the input electrical-current signal to produce, at an output of the transimpedance amplifier, an output voltage signal corresponding to the input electrical-current signal. The method further includes providing, by a current-control circuit coupled to the input and the output of the transimpedance amplifier, the DC cancellation current to the input of the transimpedance amplifier, where the DC cancellation current is based on the output voltage signal.

First claim

What is claimed is:

1. A method comprising:
receiving, at an input of a transimpedance amplifier, an input electrical-current signal comprising:
a photodetector current corresponding to an input optical signal, wherein the photodetector current comprises an alternating-current (AC) portion and a direct-current (DC) portion; and
a DC cancellation current;

performing, by the transimpedance amplifier, a transimpedance amplification of the input electrical-current signal to produce, at an output of the transimpedance amplifier, an output voltage signal corresponding to the input electrical-current signal, wherein the output voltage signal produced at the output of the transimpedance amplifier is analyzed to wirelessly provide feedback information, via a free-space optical link, to an optical transmitter for one of signal acquisition or signal tracking, and wherein the feedback information that is wirelessly provided to the optical transmitter for signal tracking comprises information to adjust aiming of an optical beam for maintaining incidence of the optical beam on an optical receiver; and
providing, by a current-control circuit coupled to the input and the output of the transimpedance amplifier, the DC cancellation current to the input of the transimpedance amplifier, wherein the DC cancellation current is adjusted based on (1) the output voltage signal by the transimpedance amplifier and (2) whether the feedback information to the optical transmitter is to be used for one of signal acquisition or signal tracking, and wherein if the feedback information to the optical transmitter is to be used for signal tracking, then the DC cancellation current is increased to divert the DC portion of the photodetector current away from the input of the transimpedance amplifier.

2. The method of claim 1 , wherein:
if the DC portion of the photodetector current is greater than or equal to a threshold turn-on current value, then:
the DC cancellation current is greater than zero; and
the DC cancellation current is positively correlated with the DC portion of the photodetector current, wherein as the DC portion of the photodetector current increases above the threshold turn-on value, the DC cancellation current also increases;

else:
the DC cancellation current is approximately zero.

3. The method of claim 2 , wherein if the DC portion of the photodetector current is less than the threshold turn-on current value, then at least part of the current-control circuit is electrically isolated from the input of the transimpedance amplifier, wherein approximately zero electrical noise generated by the current-control circuit propagates to the input of the transimpedance amplifier.

4. The method of claim 2 , wherein the DC cancellation current is configured to divert at least part of the DC portion of the photodetector current away from the input of the transimpedance amplifier.

5. The method of claim 1 , wherein if the DC portion of the photodetector current is greater than or equal to a threshold offset current value, then:
a magnitude of the DC cancellation current is approximately equal to a magnitude of the DC portion of the photodetector current;
the DC cancellation current is configured to divert substantially all of the DC portion of the photodetector current away from the input of the transimpedance amplifier, wherein a sum of the DC cancellation current and the DC portion of the photodetector current is approximately zero; and
the input electrical-current signal received at the input of the transimpedance amplifier is approximately equal to the AC portion of the photodetector current.

6. The method of claim 1 , further comprising, by a photodetector coupled to the input of the transimpedance amplifier:
receiving the input optical signal, the input optical signal comprising a time-varying portion and a substantially constant portion; and
producing the photodetector current corresponding to the received input optical signal, wherein:
the time-varying portion of the received input optical signal corresponds to the AC portion of the photodetector current; and
the substantially constant portion of the received input optical signal corresponds to the DC portion of the photodetector current.

7. The method of claim 1 , wherein the output voltage signal comprises an AC-voltage portion and a DC-voltage portion, the AC-voltage portion corresponding to the AC portion of the photodetector current, and the DC-voltage portion corresponding to a sum of the DC portion of the photodetector current and the DC cancellation current, and further comprising:
producing, by the current-control circuit, the DC cancellation current, wherein the DC cancellation current is based on the DC-voltage portion of the output voltage signal.

8. The method of claim 1 , wherein the DC cancellation current has a direction, with respect to the transimpedance amplifier input, that is opposite a direction of the DC portion of the photodetector current.

9. The method of claim 1 , wherein the transimpedance amplifier has a transimpedance gain of approximately 10 5 volts per ampere to approximately 10 7 volts per ampere.

10. The method of claim 1 , wherein the current-control circuit comprises a filtering circuit and a current-source circuit, wherein:
the filtering circuit is configured to receive the output voltage signal from the output of the transimpedance amplifier and produce a control voltage based on the output voltage signal; and
the current-source circuit is configured to receive the control voltage from the filtering circuit and produce the DC cancellation current based on the received control voltage.

11. The method of claim 10 , wherein the filtering circuit comprises an operational amplifier configured to operate as an integrator.

12. The method of claim 10 , wherein the filtering circuit comprises an operational amplifier configured to operate as a low-pass filter.

13. The method of claim 10 , wherein the current-source circuit comprises a transistor, wherein:
a first terminal of the transistor is configured to receive the control voltage;
a second terminal of the transistor is coupled to a resistor; and
a third terminal of the transistor is coupled to the input of the transimpedance amplifier.

14. The method of claim 1 , wherein:
if the feedback information to the optical transmitter is to be used for signal acquisition, then the DC cancellation current is decreased to allow the DC portion of the photodetector current to the input of the transimpedance amplifier.

15. A system comprising:
a transimpedance amplifier configured to:
receive, at an input of the transimpedance amplifier, an input electrical-current signal comprising:
a photodetector current corresponding to an input optical signal, wherein the photodetector current comprises an alternating-current (AC) portion and a direct-current (DC) portion; and
a DC cancellation current; and

perform a transimpedance amplification of the input electrical-current signal to produce, at an output of the transimpedance amplifier, an output voltage signal corresponding to the input electrical-current signal, wherein the output voltage signal produced at the output of the transimpedance amplifier is analyzed to wirelessly provide feedback information, via a free-space optical link, to an optical transmitter for one of signal acquisition or signal tracking, and wherein the feedback information that is wirelessly provided to the optical transmitter for signal tracking comprises information to adjust aiming of an optical beam for maintaining incidence of the optical beam on an optical rec

Assignees

• Facebook Inc

Inventors

• Wang Shih-Cheng

Classifications

• H04B10/693

  Arrangements for optimizing the preamplifier in the receiver · CPC title

• H04B10/11

  Arrangements specific to free-space transmission, i.e. transmission through air or vacuum · CPC title

• H03F2203/45521

  the FBC comprising op amp stages, e.g. cascaded stages of the dif amp and being coupled between the LC and the IC · CPC title

• H03F3/087

  with IC amplifier blocks (H03F3/085 takes precedence) · CPC title

• H03F1/14

  by use of neutralising means · CPC title