Patent GB-1041748-A (GB)
Title: Improvements in or relating to transistor switching circuits
Inventor: DAVIS JOHN CHRISTOPHER HAMMOND
IPC: H03K3/281
Filed: 1962-06-15
Abstract: 1,041,748. Semi-conductor switching; bi-stable circuits. BRITISH TELECOMMUNICATIONS RESEARCH Ltd. June 6, 1963 [June 15, 1962], No. 23178/62. Heading H3T. [Also in Division G4] In a multi-stable state transistor circuit having a plurality of similar stages and each stage including a pair of transistors of opposite conductivity type reciprocally coupled, the transistors are switched from one state to another by means of one or more further transistors each having its emitter-collector path in parallel with that of one of the pair of transistors. In the bi-stable circuit of Fig. 1 the circuit is in a first state when transistors VT13, VT14 conduct and transistors VT2, VT7 are cut off and a second state when VT2, VT7 conduct and VT13, VT14 are cut off. If the circuit is in its second state and transistor VT16 is rendered conducting by a signal applied to its base the current through resistor R8 tends to increase resulting in the emitter of transistor VT2 becoming slightly more negative. The current through it decreases and the base of transistor VT7 becomes more negative and its collector current is reduced and the base of transistor VT2 is driven more positive. Transistors VT2, VT7 are now cut off and the current through resistor R8 now flows through transistor VT16 and resistors R17, R18 the voltage drop across the latter causing transistor VT13 to be switched on so that when VT16 is switched off transistors VT14, VT13 conduct and the circuit is in its first state. In the circuit of Fig. 2 (not shown) transistor VT2 is not connected to transistor VT7 in a regenerative manner. The circuit of Fig. 4 shows the application of the toggle of Fig. 2 to a feedback encoder for a pulse code modulation system. During the time Tx a pulse is applied to base of transistor VT3 to cause this transistor to switch on and transistors VT6, VT9 to cut off. During the period Tx + 1 a negative pulse is applied to transistor VT10 which conducts if the base of transistor VT12 is at - 0À6 v. but not if it is at - 1À8 v. dependent on the output from a comparator M. If transistor VT10 conducts then transistors VT9, VT6 do so as well at the end of the period Tx + 1. When all the stages have been dealt with a reset pulse is applied to transistor VT1 to switch it off if it is not already off and transistors VT6, VT9 are switched on. Transistor VT1 may be replaced by a diode and the output from the switch is taken from terminal A. Fig. 5 shows a number of switches which may form a ring counter if the last stage is connected to the first stage. If transistors VT4r and VT1Or in the r<SP>th</SP> stage are initially conducting capacitor C6r is charged. A negative driving pulse to transistor VT2 results in the emitters of the PNP transistors going negative and these transistors being cut off, and diode D4 being forward biased so that the NPN transistors are also cut off. Capacitor C6r now discharges through resistors R7r, R1s, R11r, R9r so that when the driving pulse ceases the base of transistor VT4S has the most negative potential on its base causing stage S to switch on. Capacitor C6S is now charged through resistor R7s, diode D3t, and resistor R2t and is primed to switch on stage t when the next driving pulse arrives. In order to reset the counter to a particular state a PNP transistor may be connected with its emitter-collector path in parallel with the PNP transistor of the particular state.