GB1244548A - Improvements in or relating to microphone amplifiers - Google Patents

Abstract

1,244,548. Substation circuits; transistor amplifiers. SIEMENS A.G. 16 Dec., 1969 [17 Dec., 1968 (2)], No. 61159/69. Headings H3T and H4K. Amplifier arrangement for the microphone in a telephone subset comprises two amplifier circuits arranged in opposed parallel relation and supplied over the line so that whichever polarity of supply potential is applied a respective amplifier is cut off while the other amplifier is made operational. As described, Fig. 1 signal output is taken, and power supplied, over lines A1 and A2, the two amplifiers, comprising transistor T1 to T4 and T5 to T8, are identical, and the input signal from microphone W is supplied in parallel to the two first stages. The non-operational amplifier is biased off by the polarity supplied to the emitter circuits of the transistors and the connection from the microphone to the A.C. negative line is via one or the other of diodes D1 or D2 rendered conductive by the appropriate supply polarity. In a modification, Fig. 2 (not shown) the microphone is connected across the two input circuits connected in series with arrangements to render the input circuit of the inoperative amplifier low impedance according to the supply polarity. An alternative arrangement Fig. 3, uses similar parallel opposed amplifier output stages, T21 to T26, with parallel opposed differential amplifier input stages T17 to T20 rendered alternatively operative by the diodes D5 to D8. It is suggested that by using a symmetrical F.E.T. circuit one of the input stages could be eliminated. The arrangement is suitable for production as an integrated monolithic semi-conductor circuit.