Patent GB-970868-A (GB)
Title: Improvements relating to feeding transmission bridge circuits for telephone systems and the like
Inventor: DELLER WILLIAM BERNARD
IPC: H03K17/66
Filed: 1961-06-07
Abstract: 970,868. Transistor gating circuits; automatic exchange systems. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. June 5, 1962 [June 7, 1961], No. 20571/61. Addition to 970,867. Headings H3T and H4K. In a balanced-line transmission system incorporating a semi-conductor switching arrangement as described in the parent Specification for selectively completing a transmission path interconnecting respective terminal circuits and comprising inserted in one line a rectifier diode poled so as to conduct holding current for the respective four-layer semiconductor diode or triode switch together with two holding-current paths each including the emitter-collector paths of respective transistors of one conductivity type and respectively connected to opposite sides of the rectifier diode so that holding current is supplied to the respective four-layer device whether or not the rectifier diode is conductive, the return line includes a further current path comprising the emittercollector path of a further transistor of the other conductivity type and serving as the return path for the holding current supplied by said holding- current paths, there being further provided a feed-back connection from a potential divider connected between said lines and serving to counteract any tendency for the current flowing in said further path to differ from the sum of the currents supplied by the holding-current paths. As described in the parent Specification closure of switch A during setting up a call causes transistor VT5 and diode D3 to become non- conductive so that the whole of the holding current for the four-layer semi-conductor switching device coupling the terminal circuit to the line L1 is supplied by transistor VT6, the terminal circuits and four-layer devices not being shown, while opening switch A permits both transistors VT5, VT6 and diode D3 to become conductive. In the balanced line system shown, the holding current I supplied by transistor VT5, VT6 is returned to the supply source B via a transistor VT2 of opposite conductivity type. In order to prevent saturation from occurring in one or other of the transistors VT2, VT5 and VT6 due to a lack of balance between the components a stabilizing feed-back connection is provided. The feed-back is taken from the junction point of resistors R7, R8 connected as a potential divider between lines L1, L2. In the circuit shown it is connected via a potentialdropping Zener diode Zd and emitter follower transistor VT4 to the base of transistor VT2 so that when, for example, conduction in transistor VT2 tends to increase, the potential at its base becomes more negative, thus counteracting the tendency. The feed-back may alternatively be taken to the bases of transistors VT5 or VT6. During setting up of a call with switch A closed voice-frequency supervisory signals may be introduced via transformer T. When the call is set up and switch A is opened to complete the transmission path, the junction point of resistors R7, R8 is de-coupled to earth as regards alternating current by means of capacitor C5.