Patent GB-1027080-A (GB)
Title: Improvements in or relating to signal amplifiers
IPC: H03G3/12
Filed: 1964-06-18
Abstract: 1,027,080. Transistor amplifying circuits. TEKTRONIX Inc. June 18, 1964 [June 18, 1963], No. 25306/64. Heading H3T. A wide-band amplifier which is more particularly intended as a horizontal sweep amplifier for cathode-ray oscilloscopes comprises a current amplifier stage feeding an operational amplifier via a low-impedance transmission line. The sweep voltage is applied to the base of an emitter-follower 10, the bias of which is determined by Zener diode 14. Transistor 10 feeds the base of one transistor 18 of an emittercoupled pair 18, 34 the second transistor 28 of which receives on its base a reference voltage from potentiometer 30 via a second emitterfollower 28 and coupling diode 29. The transistors 18, 34 are provided with D.C. negative feedback by means of resistors 26, 28, coupling respective collectors and bases; the common emitter circuit comprises a # network of resistors 22, 24, 40, 42, 44 in which resistor 44 is adjustable to set the gain and resistor 42 comprises a potentiometer for adjusting the balance. The transistors 18, 34 feed push-pull sweep voltages to operational output amplifiers via low-impedance transmission lines 54, 60 respectively. Each output amplifier comprises an emitter follower 64, 82 respectively driving a common emitter output stage 68, 84 respectively overall negative feedback being provided by resistors 74, 86 and ganged gain-setting adjustable resistors 76, 88 respectively. The frequency response is modified by top-peaking inductor 72 in the collector circuit of transistor 68 and may be adjusted by means of variable feedback capacitors 80, 90. Saturation of this transistor is prevented by a voltage limiting diode 46 connected between the collector of transistor 18 and earth and coupling diode 46 in the series path which transmits only negative signals. In addition, means are provided to prevent transistor 84 being driven to cut-off, comprising a PNP control transistor 94 acting as a common base amplifier with its base connected to a source of constant positive potential formed by Zener diode 96. When the collector of transistor 84 becomes more positive than the emitter of transistor 94, a positive voltage is applied to the latter via diode 106, whereby its collector is driven positive so that an amplified positive pulse is applied via Zener diode 102 and a diode 104 to the base of transistor 82 such as to oppose further increase in the positive potential at the collector of transistor 84. In addition, a clamping diode 108 connected between Zener diode 96 and the collector of transistor 68 prevents the latter being driven to an excessively positive potential. In a modification (Fig. 2, not shown), the transistors 18, 34 are of the PNP type and each feeds its corresponding transmission line via an NPN transistor connected as an emitter follower, having its base directly connected to the collector of the preceding PNP transistor and its collector connected to the emitter of that transistor via an oscillation suppressing resistor. Capacitors are connected between emitter and collector of each of the transistors 18<SP>1</SP>, 34<SP>1</SP> and assist in preventing oscillation. In this embodiment the gain-setting variable resistor 44 is omitted, gain being varied by switching alternative resistors in place of resistors 24<SP>1</SP>; also overload disconnect diodes are included in the connections to the bases of transistors 18<SP>1</SP>, 34<SP>1</SP>.