US Patent 7639085 Automatic gain control circuit

A first MOS transistor (M1) and a second MOS transistor (M2) constitute a cascode amplifier. The second MOS transistor (M2) is in a differential connection with a gain control MOS transistor (M4), which has its gate supplied with an AGC control voltage (VAGC), and it is arranged that the device area ratio of the second MOS transistor (M2) to the gain control MOS transistor (M4) is one to N (where N≧1). In this way, even in a region where the AGC control voltage (VAGC) is small, abrupt variations of the gain can be suppressed, while the drain current of the first MOS transistor (M1) can be kept constant independently of the gain control.