GB905598A - Transistor amplifier of high or very high input impedance - Google Patents

Abstract

905,598. Transistor amplifying circuits. MURRAY, J. S. Feb. 17, 1959 [Feb. 21, 1958], No. 5724/58. Class 40 (6). An amplifier of high or negative input impedance comprises a first transistor stage 12 of emitter-follower type which is also provided with a collector load 13 to which is coupled, via potentiometer 19, 20 the base of an auxiliary transistor 21. This is in common-emitter connection, the emitter being grounded via adjustable resistor 23 and the collector connected to the negative line via resistor 22 and also directly to the base of transistor 12. A further transistor 15 operating as an emitter-follower is coupled to the transistor 12 and its emitter load 16, 17 is so chosen that the common point is at the same D.C. potential as the common point of resistors 19, 20 to which it is connected by way of variable resistor 24. Resistor 23 is used to set up the correct D.C. working conditions for the amplifier and resistor 24 is then used to control, by negative feed-back, from transistor 15, the positive feed-back introduced into the circuit input by transistor 21. The relative amount of positive feed-back at low frequencies may be reduced by connection of a high-pass filter in the base circuit of transistor 21, e.g. a condenser may be connected across resistor 19. The circuit may be simplified by omission of transistor 15 and resistors 16, 17, 24, the degree of positive feed-back being adjusted by making resistor 13 variable. The Specification also describes a modification in which the transistors 15, 21 are of opposite conductivity type to transistor 12. A pair of amplifiers may be operated in push-pull.