Patent ID: 7400197

Claim:
A gain control circuit that reduces a change in gain due to a variation in ambient temperature, comprising: a variable resistor element that is connected to a power supply terminal, the variable resistor element including a first P-channel field effect transistor and a second P-channel field effect transistor; a control unit that controls the resistance value of the first and second variable resistor elements on the basis of a gain control voltage, the control unit including a first operational amplifier; an amplifying transistor that is supplied with a power supply voltage through the variable resistor element; a current detecting resistor that is interposed between the first P-channel field effect transistor of the variable resistor element and a collector of the amplifying transistor, the collector of the amplifying transistor being connected to the second P-channel field effect transistor; and a voltage detecting unit that detects a voltage dropped by the current detecting resistor, the voltage detecting unit including a second operational amplifier, wherein the dropped voltage detected by the voltage detecting unit is fed back to the control unit to control the resistance value of the variable resistor element such that a current flowing through the current detecting resistor does not vary, wherein the control voltage and an output voltage proportional to the dropped voltage detected by the second operational amplifier are input to the first operational amplifier, and an output voltage of the first operational amplifier is input to a gate of the first P-channel field effect transistor and to a gate of the second P-channel field effect transistor.