Patent ID: 6894564

Claim:
A variable-gain amplifier (VGA) comprising: four first matched bipolar transistors arranged as an input pair and an output pair, each transistor connected at its base to the base of a corresponding transistor in the other pair; an input-current source connected to the emitters of the input pair; an output-current source connected to the emitters of the output pair; a loop amplifier coupled between the collectors of the input pair and the connected bases of the input and output pairs, the loop amplifier comprising a gain transistor pair, each transistor in the gain pair having a base coupled to a corresponding input pair transistor collector; a controllable gain-current source connected to the emitters of the gain pair; and a gain-control circuit coupled to the gain-current source, the gain-control circuit adjusting the gain-current-source current such that the product of the input pair transconductance and the gain pair transconductance remains approximately constant.