| 摘要 |
PURPOSE:To drive a gain variable amplifier at a low voltage by adjusting a value of a constant current applied to a differential amplifier whose amplification gain is adjusted depending on the increase/decrease of an operating current in response to a control voltage. CONSTITUTION:An intermediate frequency signal from terminals 8, 20 is given to a base of a couple of transistors (TRs) 8, 10 of the differential amplifier 2. The operating current of the amplifier 2 is given by a TR28 of a constant current circuit 4, the amplification gain of the amplifier 2 is set, and an input signal is amplified based on the amplification gain and extracted from output terminals 22, 24. When the amplified output is detected and given to a control input terminal 54 as a gain control voltage Vagc, a current of a TR50 of a current control circuit 6 is absorbed into a TR42 by the current mirror effect. As a result, a synthesized current of the current I of a constant current source 32 and a current of the TR24 flows to a TR30 of the circuit 4 and the synthesized current adjusts the current of a TR28 by using the current mirror effect. Thus, a prescribed proportional relation is kept for the gain of the circuit 2 and the voltage Vagc and the AGC operation is performed by a low voltage. |
