FET AMPLIFIER

PURPOSE:To realize a small-sized and light weight FET amplifier by varying a drain bias voltage and a drain bias current simultaneously so as to apply temperature compensation of the gain of a FET. CONSTITUTION:A voltage from a constant voltage source Vb22 is divided by a thermister 23 and a resistor 24 and the divided voltage is applied to a buffer amplifier 25. An output of the buffer amplifier 25 is divided by resistors 27, 28 and the base of a transistor(TR) 29 is connected to the dividing point. The drain bias voltage VD of a FET 19 depends on the TR 29. When the ambient temperature rises, an output voltage V1 of the buffer amplifier 25 is increased and a drain bias voltage VD and a drain bias current I1 of the FET 19 are also increased. As a result, the gain decrease of the FET 19 due to the increased ambient temperature is compensated by the gain increase according to the change in the bias condition.