DC and offset cancellation for fully differential optical receiver
A method and apparatus that cancels or reduces DC offset in a fully-differential optical receiver. The method includes receiving differential optical signals, converting, with photodetectors, the differential optical signals to differential current signals representative of the differential optical signals, converting, using a transimpedance amplifier, the differential current signals to differential intermediate voltage signals, amplifying, using a main amplifier, the differential intermediate voltage signals to generate differential output voltage signals, and cancelling a DC component of the differential output voltage signals using a fully differential DC cancellation circuit. Output offset may also be cancelled or reduced using digital control.
1. A method comprising:
receiving differential optical signals;
converting, with photodetectors, the differential optical signals to differential current signals representative of the differential optical signals;
converting, using a transimpedance amplifier, the differential current signals to differential intermediate voltage signals;
amplifying, using a main amplifier, the differential intermediate voltage signals to generate differential output voltage signals;
cancelling a DC component of the differential output voltage signals using a fully differential DC cancellation circuit; and
regulating, fully differentially, common-mode voltage of the differential intermediate voltage signals by detecting common mode voltage at both an input and an output of the transimpedance amplifier using, at least, a voltage divider spanning inputs to the transimpedance amplifier.
2. The method of claim 1 , further comprising detecting the common mode voltage at the output of the transimpedance amplifier using a voltage divider spanning outputs of the transimpedance amplifier.
3. The method of claim 1 , further comprising feeding back the differential output voltage signals to the fully differential DC cancellation circuit via low pass filters.
4. The method of claim 1 , further comprising averaging, using capacitors, the DC component of the differential output voltage signals.
5. The method of claim 1 , further comprising controlling transistors, which shunt photocurrent from the differential current signals, based on the DC component of the differential output voltage signals.
6. The method of claim 1 , further comprising adjusting a residual-voltage offset between the differential output voltage signals by applying a selectable voltage to a first transistor of a differential pair of transistors while a second transistor of the differential pair of transistors is biased at a predetermined set voltage.
7. A method comprising:
converting, using a transimpedance amplifier, differential current signals to differential intermediate voltage signals;
amplifying, using a main amplifier, the differential intermediate voltage signals to generate differential output voltage signals;
cancelling a DC component of the differential output voltage signals using a fully-differential DC cancellation circuit;
regulating, fully differentially, common-mode voltage between an input of the transimpedance amplifier and an output of the transimpedance amplifier; and
detecting common mode voltage at both an input and an output of the transimpedance amplifier using a voltage divider spanning outputs of the transimpedance amplifier.
8. The method of claim 7 , further comprising detecting the common mode voltage at the input of the transimpedance amplifier using a voltage divider spanning inputs to the transimpedance amplifier.
9. The method of claim 7 further comprising feeding back the differential output voltage signals to the fully-differential DC cancellation circuit via low pass filters.
10. The method of claim 7 , further comprising controlling transistors, which shunt photocurrent from the differential current signals, based on the DC component of the differential output voltage signals.
11. An apparatus comprising:
a pair of photodetectors;
a transimpedance amplifier in communication with the photodetectors;
a main amplifier in communication with the transimpedance amplifier;
a fully-differential DC cancellation circuit in communication with an output of the main amplifier, the fully-differential DC cancellation circuit configured to cancel a DC component of the output of the main amplifier by processing signals from an input and an output of the transimpedance amplifier and from an output of the main amplifier; and
a residual offset-voltage adjustment circuit comprising a pair of differential transistors that controls a voltage offset of the output of the main amplifier.
12. The apparatus of claim 11 , further comprising low pass filters disposed between the output of the main amplifier and the fully-differential DC cancellation circuit.
13. The apparatus of claim 11 , wherein a first transistor of a differential pair of transistors is selectably controllable and a second transistor of the differential pair of transistors is biased at a predetermined set voltage.
14. The apparatus of claim 11 , wherein the fully-differential DC cancellation circuit comprises a pair of transistors that, responsive to an output of the main amplifier, shunts photocurrent generated by the photodetectors away from an input to the transimpedance amplifier.