AM INTERMEDIATE FREQUENCY VARIABLE GAIN AMPLIFIER CIRCUIT, VARIABLE GAIN AMPLIFIER CIRCUIT, AND SEMICONDUCTOR INTEGRATED CIRCUIT THEREOF

摘要

A variable gain amplifier circuit operable with a low power supply voltage and exhibiting less noise therein. A differential amplifier circuit is constituted by two MOS transistors, and a third MOS transistor is connected between the sources of those MOS transistors. A DC bias voltage is supplied to the gate of the third MOS transistor so as to render the third MOS transistor operative in a non-saturated region. When the output voltage of the AM variable gain amplifier circuit increases, a control voltage is applied so as to reduce the source-to-drain resistance of the third MOS transistor, thereby reducing the gain of the AM intermediate frequency variable gain amplifier circuit.