Patent GB-1048015-A (GB)
Title: Transistor push-pull amplifier
IPC: H03F1/34
Filed: 1963-08-14
Abstract: 1,048,015. Transistor amplifying circuits. TELEFUNKEN PATENTVERWEKTUNGS G.m.b.H. Aug. 14, 1963 [Aug. 22, 1962], No. 32111/63. Heading H3T. In a transistor amplifier comprising a pushpull output stage with combined current and voltage feedback therefrom to an earlier stage of the amplifier through a variable attenuator whereby the amount of feedback may be varied but with the feedback so provided that the output impedance of the amplifier does not alter as the amount of feedback is varied, the output stage comprises two transistors T 1 , T 2 , Fig. 2, connected in series as regards direct current, fed on their input sides by respective secondary windings 2, 3 of a push-pull input transformer and coupled on their output sides to a common load by respective primary windings 4<SP>1</SP>, 4<SP>11</SP> of an output transformer, the primary windings 4<SP>1</SP>, 4<SP>11</SP> being connected together and to earth for alternating currents by the series combination of a capacitor C 3  and resistor R 3  and a potential divider R 1 , R 2  being connected across one of the windings 4<SP>11</SP>, the combined feedback being taken from between the junction point of resistors R 1  and R 2 , and the earth point connected to the junction between the other of the windings 4<SP>1</SP> and capacitor C 3 . In this circuit the current feedback is provided by resistor R 3  through which flows the sum of the output currents of the transistors and the voltage feedback is provided by the voltage developed across resistor R 2 . The capacitors in the circuit shown present negligible reactance to alternating current; hence it may be shown that a bridge circuit can be drawn having as two adjacent arms resistors R 1 , R 2 , and as the other two arms the parallel combination of resistor R 3  and biasing resistors R 7 , R 8  and the parallel combination of the output impedances of the transistors, the amplifier output being taken from across one diagonal and the feedback from across the other. Hence when the bridge is balanced variations of impedance in the feedback circuit do not influence the impedance presented across the output terminals. Fig. 4 shows an alternative circuit in which the source of supply is provided with a centre tap so that each transistor may be separately supplied. Biasing resistors R 10 , R 11 , R<SP>1</SP> 10 , R<SP>1</SP> 11  are provided for class A working; for class B and class C operation resistors R 10 , R<SP>1</SP> 10  are omitted and resistors R 11 , R<SP>1</SP> 11  are replaced by short circuits. For class A working decoupling capacitors may be connected across resistors R 11 , R<SP>1</SP> 11 . Resistors R E1 , R E2  may be similarly decoupled so rendering them effective to provide D.C. stabilizing feedback only. It is stated that transistor common-emitter and common-base amplifiers may be used in place of the commonemitter circuits shown. Prior-art circuit, Fig. 1.-In this circuit current feedback is likewise provided by resistor R 3  and voltage feedback by resistor R 2 . However, in this circuit the emitter circuit of transistor T 2  is connected to a point of high alternating-current potential and consequently stray capacitance from the circuit can reduce the high-frequency response of the circuit. In the circuits of Figs. 2 and 4, on the other hand, the emitter circuit of T 2  is at a comparatively low alternating-current potential, represented by that developed across resistor R 3 . Induction L prevents loading of the output circuit by bias resistors R 4 , R 5 , R 6 .