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 optical receiver comprising: a photodetector for converting an input optical signal into an input electrical current signal; a transimpedance amplifier (TIA) for converting the input electrical current signal into an input voltage signal, the TIA including a feedback resistor R F and a variable gain feed-forward amplifier for generating a feed forward gain A o ; a gain amplifier for amplifying the input voltage signal to a desired voltage level; and an automatic gain control loop for generating at least one gain control signal capable of varying the value of the feedback resistor R F and the feed forward gain A o of the variable gain feed-forward amplifier.
2. The optical receiver according to claim 1 , wherein the at least one gain control signal comprises a first gain control signal for varying a value of the feedback resistor R F and the feed forward gain A o simultaneously to control a gain of the TIA.
3. The optical receiver according to claim 2 , wherein the first gain control signal is capable of varying the value of the feedback resistor R F and the feed forward gain A o so as to keep a ratio R F /A o generally constant.
4. An optical receiver comprising: a photodetector for converting an input optical signal into an input electrical current signal; a transimpedance amplifier (TIA) for converting the input electrical current signal into an input voltage signal, the TIA including a feedback resistor; a gain amplifier for amplifying the input voltage signal to a desired voltage level; and an automatic gain control loop for generating a first gain control signal capable of varying a value of the feedback resistor and feed forward gain of the variable gain feed-forward amplifier; wherein the feedback resistor Rr comprises a control transistor connected in parallel with a fixed feedback resistor; wherein the automatic gain control loop is capable of applying the first gain control signal to a gate of the control transistor to control the fixed feedback resistor.
5. The optical receiver according to claim 2 , wherein the first gain control signal is based on a first reference voltage, which is based on the desired voltage level.
6. An optical receiver comprising: a photodetector for converting an input optical signal into an input electrical current signal; a transimpedance amplifier (TIA) for converting the input electrical current signal into an input voltage signal, the TIA including a feedback resistor and a variable gain feed-forward amplifier; a gain amplifier for amplifying the input voltage signal to a desired voltage level; and an automatic gain control loop for generating a first gain control signal for varying at least one of a resistance value of the feedback resistor and feed forward gain of the variable gain feed-forward amplifier; wherein the automatic gain control loop includes: a peak detector capable of measuring an amplitude of an output voltage signal from the gain amplifier; and an amplifier capable of comparing the amplitude of the output voltage signal with a reference voltage, wherein the first gain control signal is based at least in part on the comparison between the amplitude of the output voltage signal and the reference voltage.
7. The optical receiver according to claim 2 , wherein the first gain control signal is capable of adjusting a value of the variable feedback resistor R F , whereby the TIA gain varies linearly with a level of the first gain control signal.
8. The optical receiver according to claim 7 , wherein the TIA gain is consistent with the input PD current, whereby the automatic gain control loop is capable of generating the first gain control signal whereby the variable feedback resistor R F has a larger value for small input PD current and a smaller value for large input PD current.
9. The optical receiver according to claim 2 , wherein the gain amplifier comprises a variable gain amplifier (VGA), and wherein the at least one gain control signal also comprises a second gain control signal for controlling the gain of the VGA.
10. The optical receiver according to claim 9 , wherein the automatic gain control loop comprises a signal conditioning circuit for generating the first gain control signal based at least in part on the second gain control signal.
11. The optical receiver according to claim 9 , wherein the automatic gain control loop is capable of generating the second gain control signal, whereby the VGA gain varies linearly with a level of the second gain control signal.
12. The optical receiver according to claim 9 , wherein the second gain control signal is based at least in part on a reference voltage, which is based on the desired voltage level.
13. The optical receiver according to claim 12 wherein the automatic gain control loop includes: a peak detector capable of measuring an amplitude of an output voltage signal of the VGA; and an error amplifier capable of comparing the amplitude of the output voltage signal with a reference voltage; wherein the second gain control signal is based at least in part on a comparison between the amplitude of the output voltage signal and the reference voltage.
14. The optical receiver according to claim 1 , further comprising an analog to digital converter (ADC) for converting the amplified input voltage signal into a digital signal.