Patent GB-1130360-A (GB)
Title: Communication circuits
Inventor: GAUNT WILMER BARSEAU
IPC: H04M3/56
Filed: 1965-12-23
Abstract: 1,130,360. Transistor amplifying circuits; telephone exchanges. WESTERN ELECTRIC CO. Inc. 23 Dec., 1965 [28 Dec., 1964], No. 54528/65. Headings H3T and H4K. Lines are connected in conference by connecting them to active elements, such as transistors 10, 11, 12, which are connected as a loop of amplifiers with a gain of approximately unity round the loop. By coupling the lines to collector and emitter circuits three transistors can couple six lines or a fewer number if the spare places are equivalently terminated. Impedances Z in the collector circuits (such impedances being at their simplest a capacitor or a resistor and capacitor in series) are used to ensure stability in face of the nature of the lines particularly outside the audio-frequency range. Zener diodes 14, 15, 16, having a 12-volt breakdown provide a quiescent base potential of 12 volts when the supply terminal 30 is 24 volts. The gain of each stage approaches unity and the loop provides an insertion loss close to zero and a high return loss. It is demonstrated that, despite variations of gain from stage to stage or variations in line impedance, the resulting insertion and return losses do not give rise to instability. The lines may be coupled with capacitors, rather than with transformers as shown, and allows the circuit to be miniaturized. Biasing current may be supplied in the collector and emitter paths of each transistor, such as 10, 11, 12, by the inclusion of additional transistors in series with each path (Fig. 8, not shown). The coupling between each stage may also include the baseemitter path of a further transistor so that while transmitting the full signal voltage each collector of the transistors, such as 10, 11, 12, sees a high impedance (Fig. 8, not shown). Where the switching network connecting the lines introduces series resistance, compensation is provided by a gain greater than unity introduced into each stage by reciprocal connections of base and collector with an additional transistor of opposite type in each stage (Fig. 10, not shown). This arrangement must comprise an odd-number of stages to be stable and can terminate lines in the emitter circuits only. Where, however, the switching network introduces shunt resistance as well as series resistance the circuit can be employed to terminate lines on both collector and emitter circuits. In such a circuit arrangement the output from each stage can be taken from the emitter circuit of the additional, reciprocally coupled, transistor of opposite type (Fig. 16, not shown). If the emitter circuits in question have a potential divider, only a fraction of the voltage need be passed to the base of the next stage and choice of this fraction provides a further control over insertion and return losses. Compensation for the impedance introduced by the switching network may be effected by the insertion of negative impedance pads between the lines and their coupling to the circuit. The base bias for the amplifiers in the loop may be provided by potential dividers rather than by the Zener diodes and a slight amount of positive feedback may be introduced into each stage to obtain a gain nearer to unity.