OPTICALLY COUPLED APPARATUS

摘要

1522903 Transistor amplifiers WESTERN ELECTRIC CO Inc 4 Feb 1976 [10 Feb 1975] 04475/76 Heading H3T Two optically coupled circuits, e.g. for a bilateral communication system, each comprises an amplifier, e.g. 62, having a light-emissive device 78 coupled to its output, and light detectors 64, 80 disposed to receive said light, one 80 being coupled in an output circuit and the other 64 providing negative feedback to the amplifier 62, whereby non-linearity between an input signal to the amplifier and the corresponding signal from the output circuit is reduced, the apparatus further including an input/output terminal 56 coupled to an input of amplifier 62 via summing impedance R50, the output 66 of the other circuit being coupled to an input of 62 via a cancelling impedance Z2', for cancelling signals from 66 at amplifier 62. The output amplifiers are 84, 68, the input amplifier corresponding to 62 for the opposite direction being 92. An input signal from a first telephone set (not illustrated) connected at T, R is fed via input terminal 56 to the output of 68 and to an input 60 of 62, in parallel, the impedance Z11 being a terminating impedance for the input line. The output terminal 66 of amplifier 68 is a virtual earth, and hence the input signal cannot reach the other input 70 of amplifier 62. After passing through the optical coupling 78, 80 and amplifier 84 the signal emerges at a second telephone set connected to Tl, Rl. An impedance Z3' (corresponding to Z21) acts as a hybrid cancellation impedance to ensure that the signal components reaching the differential inputs of amplifier 92 are equal, so that no reflected signal is produced. Networks Zol', Zo2' are gain control feedback impedances switchable by S1, S2 respectively to provide alternative values of gain in amplifiers 68, 84. R64, C16 and R65, C17 serve for determining the gain and shaping the frequency response of amplifiers 62, 92. Capacitors C28 and C29 compensate for the stray capacitances CS1, CS2 in parallel with the optical couplings, so as to prevent common mode signals from propagating through the system. Resistors R53, R79 in Zol', Zo2' respectively may be varied in value to compensate for mispositioning of the light sources with respect to the light detectors. The light sources may be gallium phosphide LED's and the detectors silicon diodes operating in the reverse biased mode. The system may be used as an optical transmission system by optical fibre couplings. The input and power circuitry 57 comprises a polarity guard (102), Fig. 5 (not shown), consisting of a diode bridge, and a high impedance constant current sink which provides the supply voltages � Vx and Vx ref. A Zener diode (Dzl) protects two transistors (Q3, Q4) against excessive supply voltage. The input and power circuitry 59 comprises a chopper (S3) driven by an oscillator to produce A.C. from a D.C. supply, Fig. 6 (not shown), to provide the supply voltages �Vy and Vy ref, the circuit including three thermistors (Th1-3).