GB1004621A - Improvements in transistor switching circuits, especially for use in modulators - Google Patents

Abstract

1,004,621. Transistor amplifying; switching circuits; amplitude modulation systems; pulse modulation circuits. HONEYWELL CONTROLS Ltd. Aug. 22, 1961 [Aug. 22, 1960], No. 29036/60. Headings H3R, H3T and H4L. A switching or modulator circuit comprises a transistor and a source of control voltage connected between the base electrode of the transistor and a junction between two resistances R1, R2 connected respectively to the emitter and collector electrodes. The resistances are adjusted in value so that at zero D.C. input applied across the emitter and collector the ratio of the forward diode resistance r 2 of the emitter-base diode to the resistance of the emitter-connected resistance R2 is equal to the ratio of the collector-base forward diode resistance r 1 and the resistance of the collector-connected resistance R1. The transistor is preferably a symmetrical junction one. A practical circuit is as shown in Fig. 4 wherein resistors R1, R2 are provided by two portions of potentiometer winding 26 and a diode network D1, D2, D3 is connected as shown to prevent positive half-cycles of the control waveform applied over transformer 16 from reaching the base of transistor 11. In order to overcome transient voltages induced into the secondary winding 19 the primary 20 is screened at 22 and in order to avoid the effect of the secondary to core capacity a further screen 23 is provided which is connected to the wiper 24 of potentiometer 25. To calibrate the circuit the input leads 12 are shorted and potentiometer 26 adjusted to give zero output, then with the input leads 12 open-circuited potentiometer 25 is adjusted to remove transient spikes from the output waveform. The control voltage may be sinusoidal. The modulator may be coupled to the following A.C. amplifier by a capacitor or transformer. Sine waves may be converted to pulses for driving the modulator using the circuit of Fig. 7 (not shown).