Patent GB-1510387-A (GB)
Title: Electronic switches
IPC: H03K17/74
Filed: 1975-12-12
Abstract: 1510387 Semi-conductor switching circuit and current sources WESTERN ELECTRIC CO Inc 12 Dec 1975 [27 Dec 1974] 51108/75 Heading H3T In a bridge of semi-conductor devices, such as diode connected transistors, switching the passage of a signal between opposite nodes 2, 3 of the bridge 1 by means of a bias current between the other pair of nodes 26, 27, e.g. for time division switching, distortion is minimized by applying bias current symmetrically via matched sources, turned on by a common current signal, to respective nodes 26, 27 as in the block diagram (Fig. 1, not shown). Reference current source 7 connected to the + 9 v supply and a regulated + 7 v supply allows a fixed current to flow through transistor 35, set by the drop across resistor 39 balanced against the 7 v supply minus the base emitter drops of transistors 34, 35, 36; a multiple-e.g. twice, the fixed current passes through transistor 37, depending on the ratio of resistor 38 to resistor 41, the emitter areas of transistors 37 and 35 being proportional to equalize their base-emitter drops in the "current mirror" circuit. The current through transistor 37 is passed to the symmetrical turn on/off unit 6 unless any one of the multiple emitters of diode-connected TTL gate 8 is grounded, shunting the current to ground and causing the bridge 1 to switch the signal OFF. If all emitters of gate 8 are at about + 5 v, the bridge 1 is ON, allowing signals to pass from nodes 2 to 3, the current in lead 42 passing to ground via transistors 58 and 59, the two baseemitter drops and the p.d. across resistor 62 being matched in a further "current-mirror" by those in transistor 56, 57 and resistor 61 when passing a multiple, e.g. four times, the current in lead 42. The common turn-on current through resistor 61 flows from the +9 v supply, through transistor 80 of matched current source 4, through unit 6 and through transistor 95 in the complementary matched current source 5 to the -9 v supply; the base-emitter drop in transistor 80 and the p.d. across resistor 89 in unit 4 and similar drops in unit 5 are matched in further "current-mirrors" against the drops in larger transistors 81 and 96 and smaller resistors 92, 107 to fix the current through nodes 26, 27 of the bridge as a multiple, e.g. six times, the current through resistor 61 of unit 6. When unit 6 is turned off, transistors 80 and 81 in unit 4 and 95 and 96 in unit 5 block, presenting a high impedance to the bridge which no longer conducts signal through the four pairs of diode connected complementary transistors 18-25. The bridge when turned off is reverse biased to about half the breakdown level of the bridge devices, giving maximum permissible signal amplitude without breakthrough, by current from unit 12 via resistor 129 to the diode connected transistors 158a, 158b, 158c, of which 158a and 158c break down, via resistor 152 to unit 13. When the current sources 4, 5 are cut off, units 12, 13 associated with reference voltages from units 16, 17 immediately conduct to discharge charge stored in the bridge junctions in order to reduce delay in the switch-off of the bridge. Saturation of transistors 138, 148 in units 12, 13 would prevent switch off of these units when units 4, 5 turn on, but is prevented by units 14, 15 biased by the drops across resistors 111, 119.