VARIABLE GAIN CIRCUIT

摘要

PROBLEM TO BE SOLVED: To provide a variable gain circuit whose gain characteristic with respect to an AGC voltage has linearity over a wide range of the AGC voltages. SOLUTION: A 2nd amplifier 14 is a variable gain amplifier of a variable drive current and has a variable gain characteristic where linearity is higher at a high gain in comparison with that at a low gain. The 2nd amplifier 14 receives an IF transmission signal from a 1st amplifier of a constant drive current type with a variable gain characteristic having high linearity at a low gain in comparison with that at a high gain in the gain characteristic with respect to the AGC voltage through a coupling capacitor 13 and amplifies the IF transmission signal. The gain characteristic of the 2nd amplifier 14 is made constant at a low gain side with a gain clamp circuit consisting of TRs T33-T36 and resistors R35-R38.