Improvements in push-pull amplifiers


1,070,703. Transistor amplifying circuits. FERGUSON RADIO CORPORATION. Nov. 3, 1965, No. 46622/65. Heading H3T. The invention relates to an amplifier of the type comprising a pair of transistors VT3, VT4 with their emitter-collector paths connected in series and the common output load 15 connected between their common point and the junction of two capacitors 17, 18 which are connected in series across the power supply. According to the invention, the base of an input stage transistor VT1 is connected to the junction of two resistors 12, 13 connected across the power supply: these resistors form with capacitors 17, 18 a balanced bridge network having the power supply across one diagonal and the amplifier across the other, so that fluctuations in power supply are not communicated to the amplifier. Overall D.C. negative feedback is obtained by means of lead 19 interconnecting the load 15, which must conduct D.C., and the emitter of transistor VT1. The signal is fed in from source 20 via capacitor 21; in a modification (Fig. 2, not shown) the base of the input transistor (VT1) is earthed via a capacitor and the signal source is conductive to D.C. and is connected in the emitter lead. In this embodiment, a resistor (21) is connected between the emitters of the output transistors (VT3, VT4) and the emitter of the input transistor (VT1) to provide A.C. negative feedback; a resistance-capacitance network (22-26) is also provided to convert the configuration of the output transistors (VT3, VT4) to common emitter.




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