A 50 dB variable gain amplifier using parasitic bipolar transistors in CMOS

Amplifiers with electrically controllable gain are essential parts of AGC loops in front-end analog circuits for telecommunications applications. They are typically implemented using binary weighted arrays of capacitors or resistors that are digitally selected as feedback elements around op amps. This approach can consume substantial chip area when a large range of variable gain is sought In contrast, amplifiers whose gain is determined by bias currents, like the Gilbert gain cell, can be compact because they take advantage of the linear dependence of the transconductance of bipolar transistors on emitter current