GB895359A - Improvements relating to electrical control networks - Google Patents

Abstract

895,359. Transistor gating circuits; keying carrier frequencies. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. Feb. 10, 1961 [April 1, 1960], No. 11640/60. Classes 40(5) and 40(6). [Also in Group XL(b)] A circuit for keying voice frequency signals under the control of double current signals comprises a network which is such that between input and output terminals of this network there is a high impedance when the signals have one polarity, and a low impedance when the signals have the other polarity. The voice frequency generator S feeds a transformer TRA, the secondary of which is connected to a similar transformer TRB via the emitter/collector electrodes of a first pair of symmetrical transistors VT1, VT2. A second pair of transistors VT3, VT4 is connected in series with resistors R11, R12 and R13, R14 across the transformers. Double current signals are applied at terminal LT. When the signals have positive polarity transistors VT1, VT2 do not conduct but VT3, VT4 do so that the voice frequency signals do not pass to the transformer TRB and the output terminals. If the signals have negative polarity VT1, VT2 are conductive and VT3, VT4 are not, so that the voice frequency signals can pass to the output. The various resistors are so chosen that the impedance presented at the input and the output terminals is unchanged for the two conditions of the circuit. Should there be no signal present all the transistors are biased to their non- conductive condition by means of the 10 volt negative source indicated. If it is required that the voice-frequency signals be blocked when the signal has negative polarity and are passed when the polarity is positive, then links XA and YB must be replaced by links XB and YA.