GB1004531A - Push-pull amplifier circuit - Google Patents

Abstract

1,004,531. Transistor amplifying circuits. RADIO CORPORATION OF AMERICA. Dec. 7, 1962 [Jan. 2, 1962], No. 46350/62. Heading H3T. A push-pull amplifier comprises a pair of transistors, a signal source having an impedance which is low compared with the base emitter impedance of the transistors, and a semiconductor diode thermally coupled to the transistors and being poled so that the voltage across it provides forward bias for the baseemitter junction of the transistor, the arrangement being such that the diode and source provide substantially all the impedance connected across the input to the transistors. As described, Fig. 1, a gramophone amplifier for a crystal pick-up 5 has a tone control comprising capacitors 8, 10 and resistor 9 and a volume control 28 feeding a field effect transistor preamplifier stage 12, the input impedance of which is high. The output from this stage is fed to driver amplifier 39 in the output circuit of which is a transformer 73 giving an impedance step-down in the order of 50: 1, and having a push-pull output winding. In order to maintain a low source impedance for the push-pull output stage, the transistors 70, 71 are biased by a pair of parallel connected diodes 91 the impedance of which is low, and negative feed-back is provided over a first path comprising resistors 53, 54, 56 and capacitor 55, a second path comprising resistor 61 and capacitor 60 and a third path comprising resistor 66 and capacitor 65. The diodes 91 are mounted in thermal contact with the transistors 70, 71 to provide temperature compensation. In a modification, Fig. 2, the diodes 91 are connected to earth via separate variable resistors 110, 111 so that the biases on the transistors 70, 71 may be individually adjusted. Specification 1,004,532 is referred to.