Abstract: A transconductance cell is disclosed that includes a first transistor and a second transistor. A regulator regulates an average voltage in response to a reference voltage, where the average voltage corresponds to the average of a voltage at a bias terminal of the first transistor and a voltage at a bias terminal of the second transistor. A mixer including the transconductance cell is also disclosed. In the mixer, the transconductance cell receives a differential input voltage and produces a differential output current. The mixer also includes one or more switches that multiply the differential output current with an oscillator signal. A method is disclosed that includes measuring an average voltage of a voltage at a bias terminal of a first transistor and a voltage at a bias terminal of a second transistor and regulating the average voltage responsive to the measured average voltage and a reference voltage.

Abstract: A method and system to compensate for DC and low frequency current produced by a photodiode that is illuminated with an optical data stream is described. An optical data stream ideally produces no current from a photodiode when the bit is a 0 and produces a current proportional to the optical power when the bit is a 1. Thus, the current produced from the photodiode consists of a DC component, which is typically half the current of a 1 bit (if there is an equal number of 1s and 0s in the data), and a high frequency component that carries the data. The DC component can interfere with the signal path's ability to process the information carrying component of the photodiode current, by causing a fixed offset to propagate and be amplified through it. This offset distorts the voltage signal at the output of the signal path, and must therefore be cancelled early in the path; usually in the first transimpedance stage or just after it.