SEMICONDUCTOR INTEGRATED CIRCUIT WITH BUILT-IN VARIABLE GAIN AMPLIFIER

PROBLEM TO BE SOLVED: To provide a new variable gain amplifier or a variable gain amplifier with a frequency-dependent small gain in a wide band. SOLUTION: The variable gain amplifier includes a bias circuit (BC) 1, a matching circuit (MC) 2, a variable gain resistor feedback amplifier (FA) 3, and an output follower (EA) 4. Resistance values of a load resistor Rc and a feedback resistor Rf are changed in cooperation. To achieve a high gain in a low noise amplifier, when a load resistor Rc is of high resistance, a feedback resistor Rf is also of high resistance. A feedback time constant of a closed loop of a resistor negative feedback amplifier 3, τfb (cl) 2π×RfCbe/(1+gmRc) becomes nearly constant, and the amplifier includes a frequency-dependent small gain in a wide band. To achieve a low gain in a low noise amplifier, when a load resistor Rc is of low resistance, a feedback resistor Rf is also of low resistance. By a feedback resistor Rf of low resistance, the amount of negative feedback is increased, and a gain becomes low. A load resistor Rc is also of low resistance and a feedback time constant τfb (cl) becomes nearly constant, and a gain does not decrease furthermore in a high frequency area. COPYRIGHT: (C)2009,JPO&INPIT