GB945192A - Improvements in and relating to electronic switching circuits - Google Patents

Abstract

945,192. Transistor amplifying; switching circuits; two-way transmission systems. TELEPHONE RENTALS Ltd. May 3, 1962 [May 19, 1961], No. 18307/61. Headings H3R and H4T. A transistor switching arrangement comprises two cross-coupled transistors 7, 8 including a further transistor 6 in series with the path of one of the couplings and arranged to be switched from a conducting state to a non-conducting state in dependence upon whether a signal level applied to transistor 6 is above or below a given threshold. Initially in the absence of a signal at terminal 3, transistor 8 is switched on and transistors 6, 7 switched off. When an input signal is applied to terminal 3 the circuit remains in the initial state until the -level of the input signal is such as to switch transistor 6 on and so cause transistor 7 to conduct and transistor 8 to be cut-off. The output potentials at terminals 4, 5 are reversed until the input signal falls below the level to maintain transistor 6 conducting, when the circuit reverts to its initial state with transistor 8 on and transistor 7 off, the potential at terminal 4 then being substantially at earth potential and that at terminal 5 at potential of supply line 1. In the circuit of Fig. 7 transistor 80 including a relay 83 is switched on and off under the control of pulses applied to its base from terminal 4 of Fig. 1. The relay contacts 86 of relay 83 may be included in a further relay circuit, Fig. 8, in which circuit with contacts 86 open capacitor 91 is discharged and transistor 92 switched off. When relay 83 operates under the control of an input signal on terminal 4 contacts 86 are actuated and capacitor 91 charges over rectifier 95 until the base of transistor 92 is sufficiently negative for this transistor to conduct and relay 93 to operate. When relay 83 releases capacitor 91 discharges through transistor 92 and variable resistor 97 until base-emitter current ceases and relay 93 is released. The switch of Fig. 1 may also be used as the switching element in an R.C. time delay circuit, Fig. 9 (not shown). The switching circuit of Fig. 1 may be used in association with a two-way telephone system comprising a TRANSMIT circuit, Fig. 2, and a RECEIVE circuit, Fig. 3. The TRANSMIT circuit comprises A.F. amplifiers 29, 30 and control transistor 32, the D.C. output from the emitter of this latter transistor being fed as a control signal to terminal 3 of the switching circuit of Fig. 1. The base of transistor 25 is connected to terminal 4 of Fig. 1 which is normally at earth potential in the absence of a signal at terminal 3 and hence no signals are passed from the TRANSMIT circuit to the line 23, 24. The RECEIVE circuit comprising amplifier 40 and push-pull amplifier 41 is connected to the TRANSMIT circuit by terminals 20, 21 and thus to the line at 23, 24. Due to the negative bias on the base of transistor 40 from the normally negative potential terminal 5 of Fig. 1 the RECEIVE circuit is normally open. When signals are picked up over microphone 28 a signal is fed from transistor 29 through control transistor 30 to the base-emitter circuit of transistor 32 causing the latter transistor to conduct and a D.C. control signal to be applied to terminal 3, which signal causes the circuit of Fig. 1 to switch over and terminal 4 to be at a negative potential and terminal 5 at earth potential. The TRANSMIT circuit is thus switched on and the RECEIVE circuit switched off, until the control signal at the terminal 3 falls below a predetermined level and the circuits revert to their initial conditions. In order to prevent the circuit of Fig. 1 switching due to background noise part of the signal from the RECEIVE circuit is passed to an isolating stage 44 rectified and fed from terminal 22 to the input circuit of discriminator transistor 32 so that this transistor is shunted by rectifier 35 and hence backbiased. Thus transistor 32 only becomes forwardly biased when there is no signal in the RECEIVE circuit and a signal from the microphone 28. To provide for break-in a further transistor stage is inserted between terminals 20, 21 and transistor 40, Fig. 4 (not shown), which stage provides a D.C. voltage at terminal 122 regardless of whether the RECEIVE circuit is on or off, which voltage shunts the transistor 32 in a pause in the conversation or at the end of a word, causing the circuit of Fig. 1 to change states and open the RECEIVE circuit and shut the TRANSMIT circuit. The power for the circuits of Figs. 1-4 may be derived from the loop current in the telephone switchboard line circuit, Fig. 5 (not shown). The circuit of Fig. 6 (not shown), describes how the circuit of Fig. 1 may be connected with a sound or vibration amplitude detector.