Patent GB-970867-A (GB)
Title: Improvement relating to circuit arrangements employing semiconductor four-zone diodes
Inventor: DELLER WILLIAM BERNARD
IPC: H03K17/64
Filed: 1961-04-20
Abstract: 970,867. Transistor gating circuits; automatic exchange systems. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. April 4, 1962 [April 20, 1961], No. 14327/61. Headings H3T and H4K. In a semi-conductor switching system comprising four-layer diode or triode switches (e.g. as cross-point switches in a co-ordinate switch ing arrangement) and in which a certain minimum holding current flow is necessary to maintain a four-layer device conductive, there is provided in series in each transmission path controlled by a four-layer device a rectifier diode poled so as to conduct holding current for the device and associated therewith two biasing- current paths each comprising the emittercollector path of a respective transistor and being respectively connected to opposite sides of said rectifier diode, the arrangement being such that the transistor in the path connected to the diode on the side nearer the corresponding four-layer device is at all times conductive so that said path at all times supplies an adequate holding current to the said four-layer device while the transistor in the other path may be rendered selectively conductive and non-conductive, in the former case supplying current to render the rectifier diode conductive and so complete the transmission path controlled by the four-layer device and in the latter case supplying no current so that said diode becomes non-conductive and the transmission path is interrupted without, however, interrupting the supply of holding current to the four-layer device. Thus in a telephone-exchange system holding current can be supplied to selected switching devices for coupling, as appropriate, voice-frequency supervisory signals within the exchange while the rectifier diodes may be maintained non-conductive to prevent the supervisory signals from going beyond the points at which they are intended to be received. In the arrangement illustrated, connection between terminal circuits such as LC1 and LC<SP>1</SP>n in respective switching arrays S, S<SP>1</SP> may be effected over links such as L1 and Lm by rendering selected appropriate ones of four-layer semi-conductor diodes such as D, D<SP>1</SP> conductive by the application of suitable voltages between appropriate ones of the terminals x1-xm and y1-yn in array S and between corresponding terminals in array S<SP>1</SP>. In order to supply holding current for the four-layer diodes biasing circuits BC (of which one is shown in detail) are provided. During the setting up of a call at array S the switch A is closed. Earth potential is applied to the base of transistor VT2 which consequently conducts and supplies holding current for a corresponding conductive four-layer diode. The base of transistor VT3 however is held at a positive potential by means of emitter-follower transistor VT1 energized from potential divider R4, R5. The base of transistor VT3 is hence positive with respect to its emitter electrode and the transistor VT3 is cut off. Diode Rf1 is consequently also cut-off and voice-frequency supervisory currents, which may be injected into the array S at terminal t are unable to pass via link Lm to array S<SP>1</SP>. On the completion of the setting-up procedure, switch A is opened whereupon the base of transistor VT2 assumes a potential only slightly positive to that at the base of transistor VT3 due to conduction in diode Rf3 which is now forward biased. Both transistors conduct, the emitter of VT2 being slightly positive to that of VT3, thus cutting diode Rf2 off. Both transistors now supply holding current and diode Rf1 is conductive so placing a selected one of terminal circuits LC1, LCn in communication with link Lm and hence with a corresponding terminal circuit in array S<SP>1</SP> assuming that switch A<SP>1</SP> is likewise opened. The circuit shown prevents accidental switching off of a four-layer diode by large amplitude signals, such as transients appearing on the link Lm and acting in opposition to the holding current since such large amplitude signals will cause switching-off of diode Rf1, transistor VT2 continuing to supply the necessary holding current. In order to prevent such accidental switching off by signals produced at the terminal circuits, these circuits are provided with limiters such as catcher diodes or double-ended Zener diodes in parallel therewith, Fig. 3 (not shown). Fig. 2 (not shown) depicts an alternative form of array S comprising four-layer triodes, i.e. semi-conductor controlled rectifiers, in place of the fourlayer diodes. Specification 970,868 is referred to.