Patent ID: 7398070

Claim:
A variable gain amplifying circuit comprising: a semiconductor amplifying stage using a semiconductor amplifying device having a control electrode, a first main electrode, and a second main electrode having a low output impedance characteristic, wherein, in the semiconductor amplifying device, an RF signal is input to the control electrode and a bias circuit for generating a bias voltage is connected to the control electrode, the first main electrode is alternately grounded, and an amplified RF signal is output from the second main electrode, and wherein the bias circuit adjusts the bias voltage of the control electrode corresponding to a supplied AGC voltage, and as the amplitude of the input RF signal becomes large, the bias voltage of the control electrode decreases and a main current flowing through the first main electrode and the second main electrode decreases to thereby decrease a signal gain of the semiconductor amplifying device, but as the amplitude of the input RF signal becomes small, the bias voltage of the control electrode increases and the main current increases to thereby increase the signal gain of the semiconductor amplifying device.