Solid state polarity change-over for telephones - uses transistor bridge coupled to complementary transistors to cause reversal of DC output

摘要

The processing of telephone call signals requires that a dc voltage applied to a telephone receiver has the polarity changed at a rate of 25 Hz. The 25 Hz signal provides sampling of the 36kHz carrier voltage. The polarity change-over of the voltage applied to output terminals (A, B) is provided by a bride circuit between de supply terminals (+, -/. One arm of the bridge has collector coupled transistors (T1, T2), with the collector connects to two resistors (R3, R4) across the bases of a similar bridge arm (T3, T4). The bases of the first arm are coupled over resistors (R1, R2) to a pair of complimentary transistors (T6, T7). In put (e) is connected to a gate transistor (T5) coupled to the bridge. Between the on period and off period of the 25 Hz sampling signal the polarity of the supply to the output (A, B) is inverted.