GB813944A - Improvements in or relating to transistor amplifiers - Google Patents

Abstract

813,944. Transistor amplifier and oscillator circuits. TELEFONAKTIEBOLAGET L. M. ERICSSON.: Jan. 2, 1957 [Jan. 2, 1956], No. 245/57. Class 40 (6). An amplifier, suitable for use in an oscillator and having a high input impedance comprises a first transistor arranged in an emitter-follower circuit with its output fed to the base of a further transistor having its emitter connected to the collector of the input transistor, the output of the amplifier being derived from the emitter circuit of the second transistor. In Fig. 5 the input emitter-follower transistor 1 feeds a further transistor 2 also arranged as an emitter follower and coupled to the collector of the first transistor. The collector voltage of the input transistor therefore follows the input signal and current flow between the base and collector of the input transistor is reduced thereby increasing the input impedance of the circuit. The input impedance may be further increased by connecting a capacitor between the emitter of the second transistor and a tap on the resistor Zel (Fig. 6, not shown). A yet further increase in input impedance may be obtained as shown in Fig. 7 by connecting the lower end of the emitter resistor Ze<SP>1</SP> to the emitter of the second transistor and by providing a further transistor 3 arranged as an emitter follower with its input fed from the output of the second transistor and its output coupled to the output terminal and to the collector of the second transistor. Similar circuits in which the transistors are connected in series across the D.C. supply are illustrated in Figs. 1, 2 and 3 (not shown) and in Fig. 4 In the latter figure additional feedback to the base of the second and third transistors is provided by capacitors Ce21, Ce31 and Ce32. The second transistors in Figs. 5, 1 and 2 may be replaced by transistors of the opposite conductivity type by small changes in the circuits, Figs. 8, 9 and 13 (not shown). Oscillator circuits may be formed by connecting appropriate network between output and input terminals. One suitable network (Fig. 10) comprises a circuit consisting of L1 and C1 (which may be replaced by a crystal) and two capacitors C2, C3, rectifiers L1, and L2 being provided to limit the output amplitude. In Fig. 11 the network comprises a crystal, a thermistor and a transformer while in Fig. 12 the network comprises a crystal in series with a trimming capacitor connected across two chokes, amplitude control being effected by a lamp La and a rectifier circuit L3, C5, R6. The crystal Kl can either work inductively or capacitively depending upon the setting of capacitor Ct. The lamp or a thermistor in the circuit can be fed with D.C. proportional to the oscillatory amplitude, for example by connecting it to the emitter of one of the transistors.