| 摘要 |
912,163. Transistor switching circuits; amplitude modulating and demodulating systems. SMITH & SONS (ENGLAND) Ltd., S. Sept. 22, 1961 [Sept. 30, 1960], No. 33743/60. Classes 40 (5) and 40 (6). [Also in Group XL (b)] A switching circuit includes a symmetrical junction transistor of the kind in which when the collector-base and emitter-base junctions are both forward biased the emitter-collector potential does not exceed 5 mv., and is arranged such that any base current flowing in the absence of a signal divides itself equally between the collector-emitter electrodes. In a first embodiment, Fig. 2, the circuit between a source 5, e.g. a thermocouple, and a meter 6 is completed by the emitter collector path of a symmetrical transistor 1. A switching signal is connected between the terminals 3, and the resistors 4 are adjusted so that the base current divides equally between the emitter and collector. In a modification. Fig. 3 (not shown) two such switching circuits are used to connect either of two thermocouples to a meter common to both circuits. In a second embodiment, Fig. 7, a signal source is connected to a load via a circuit including series transistor 41 and shunt transistor 42, the two transistors being of opposite conductivity types. In order to complete the circuit, the switching signal 47 is arranged to switch transistor 41 on and transistor 42 off. The two transistors may be made the same conductivity type, antiphase switching signals being then used, Fig. 8 (not shown). One or other of the two transistors may be omitted, Figs. 5, 6 (not shown). In the case of the shunt transistor being omitted, a third winding may be provided on one of the transformers which is short-circuited by a further transistor when the switching circuit is open, to give a low output impedance. A number of such circuits may be switched either to a common load, or to individual loads by a common switching circuit. Fig. 9 (not shown). In a balanced modulator or demodulator, Fig. 10, two symmetrical transistors 66, 67 are alternatively switched to their conducting state by signals applied between terminals 69, and 71. These switching signals may be derived from a square wave generator 72, Fig. 10a, feeding a transformer having two output windings 75, 76 from which the antiphase signals are derived. Two Zener diodes 77 may be connected back-toback across the transformer input as a limiter. |
