GB1261070A - Improvements in or relating to amplifier circuits - Google Patents

Abstract

1,261,070. F.E.T. amplifiers; microphone circuits. AKUSTISCHE U KINO-GERATE G.m.b.H. 2 July, 1969 [5 July, 1968], No. 33422/69. Headings H3T and H4J. An amplifier circuit for a sound transducer M comprises a field, effect transistor having controlled electrodes 1, 2 serving as the source or drain each connected to a load resistor R4, R5 of the same value, the output being taken from across the controlled electrodes and means are provided for applying to the gate electrode 3 a bias voltage which depends on the polarity of'the supply voltage UB and which serves to keep the voltage difference between the gate and the controlled electrode which serves as the source the same for either polarity of the supply voltage so that the operation of the circuit is independant of the polarity of the supply voltage. In the amplifier for a capacitor microphone M shown when UB is positive the diode D conducts to lower the gate potential 3 whereas when UB is negative thediode D is non- conducting so as to raise the gate potential 3 and make the potential between 3 and 2 the same as that between 3 and 1 when UB is positive. In a modified gate bias arrangement (Fig. 2, not shown), the diode D is replaced by an auxiliary voltage source such as a battery (Uh). The FET may be of barrier layer or insulated gate type.