POWER AMPLIFIER

PURPOSE:To keep the junction temperature of a FET to a permitted range by providing a means detecting the case temperature, a means comparing the case temperature with the reference temperature and a pulse generating means. CONSTITUTION:The junction temperature of a FET 34, that is, the case temperature rises, the resistance of a thermister 42 is decreased and an inverting input voltage of a comparator 40 is equal to a noninverting input voltage, the output of the comparator 40 is inverted to 'H'. Since a pulse generating means 45 generates an 'H' pulse at this leading edge, a transistor(TR) 46 is turned off and a gate bias voltage reaches -Va. In setting the voltage -Va to a pinch-off voltage of the FET 34 or below, since the FET 34 is turned off, the power consumed by the FET 34 and the drain bias power supply is almost less. Thus, the junction temperature of the FET 34 is kept within a permitted range even under any environmental condition.