Automatic gain control loop
In conventional optical receivers the dynamic range is obtained by using variable gain amplifiers (VGA) with a fixed trans-impedance amplifier (TIA) gain. To overcome the SNR problems inherent in conventional receivers an improved optical receiver comprises an automatic gain control loop for generating at least one gain control signal for controlling gain of both the VGA and the TIA. Ideally, both the resistance and the gain of the TIA are controlled by a gain control signal.
1. An electrical circuit comprising:
a transimpedance amplifier (TIA) comprising:
a variable gain feed-forward amplifier configured to generate a feed forward gain, and
a feedback resistor; and
a gain control loop operatively coupled to the variable gain feed-forward amplifier and configured to vary the feed forward gain thereof.
2. The electrical circuit according to claim 1 comprising a gain amplifier electrically coupled to the TIA and configured to amplify an output signal thereof.
3. The electrical circuit according to claim 1 wherein the gain control loop is further configured to vary the feedback resistor.
4. The electrical circuit according to claim 3 , wherein the gain control loop is configured to simultaneously vary the feedback resistor and the feed forward gain so as to keep a ratio of values thereof generally constant.
5. The electrical circuit according to claim 3 , wherein the feedback resistor comprises a control transistor and a fixed feedback resistor connected in parallel, and wherein the gain control loop is coupled to a gate of the control transistor.
6. The electrical circuit according to claim 2 , wherein the gain control loop is configured to generate a first gain control signal in dependence on a first reference voltage for controlling the feed forward gain of the variable gain feed-forward amplifier.
7. The electrical circuit according to claim 6 , wherein the gain control loop includes:
a peak detector capable of measuring an amplitude of an output signal of the gain amplifier; and
a device capable of comparing the amplitude of the output signal with the first reference voltage.
8. The electrical circuit according to claim 6 , wherein the gain control loop is configured to vary the feedback resistor and the feed forward gain so that a gain of the TIA varies linearly with the first gain control signal.
9. The electrical circuit according to claim 8 , wherein the gain control loop is configured to vary the feedback resistor to have a larger value for a comparatively small input electrical signal to the TIA and a smaller value for a comparatively large input electrical signal to the TIA.
10. The electrical circuit according to claim 6 , wherein the gain amplifier comprises a variable gain amplifier (VGA), and wherein the gain control loop is configured to control a gain of the VGA.
11. The electrical circuit according to claim 2 , wherein the gain control loop comprises a signal conditioning circuit.
12. The electrical circuit according to claim 10 , wherein the gain control loop is configured to vary the gain of the VGA based on a second reference voltage.
13. The electrical circuit according to claim 12 , wherein the gain control loop comprises: a peak detector capable of measuring an amplitude of an output signal of the VGA, and an error amplifier configured to compare the amplitude to the second reference voltage; and wherein the gain control loop is configured to generate a second gain control signal based at least in part on an output signal of the error amplifier for controlling at least one of the feedback resistor of the TIA and the gain of the VGA.
14. The electrical circuit according to claim 2 , further comprising an analog to digital converter (ADC) for converting an output signal of the gain amplifier, or a signal obtained therefrom, into a digital signal.
15. The electrical circuit according to claim 1 , further comprising a photodetector configured to generate an input electrical signal of the TIA in dependence on input light.