GB1265883A - - Google Patents

Abstract

1,265,883. Automatic gain control. PACIFIC PLANTRONICS Inc. 2 June, 1969, No. 27769/69. Heading H4R. A signal conditioner comprises a preamplifier 12 and a power amplifier 14 and connected between them is a signal attenuator 13, 18, 20 having a signal transmission characteristic which varies in response to an applied input signal. An amplifier detector 16 is connected to the preamplifier 12 for providing a control signal to the signal attenuator 13, 18, 20 to increase signal transmission therethrough when the signal at the output of the preamplifier 12 attains a selected value. Low level signals at the microphone 11 are amplified by preamplifier 12 and power amplifier 14. However, with low level signals, transistor 34 is conductive, but transistor 35 is non- conductive. Transistor 36 is conductive and high signal attenuation is provided by attenuator 13, 18, 20. As the signal level increases, transistor 34 becomes non-conductive and transistor 35 begins to conduct on half-cycles of alternating voice signals. A capacitor 75 then discharges and removes forward biasing at the base-emitter junction of transistor 36 after a few milliseconds. This renders the transistor 36 substantially non-conductive and reduces the attenuation provided by attenuator 13, 18, 20 after a brief delay. Thus, for input signals above a threshold, the apparatus provides substantially linear signal amplification at a higher gain level. The charging path for capacitor 75 is through a high resistor 73 and the charging time is sufficient to maintain transistor 36 non- conductive during syllable pauses. Operating bias voltages at terminals A, B, C for the active elements are derived from voltages appearing on a pair of conductors to which the output terminals 81, 83 are connected. Germanium diodes 37 to 40 ensure a unidirectional biasing signal on lines 82 and 84 irrespective of the polarity of the feed current. The output impedance of attenuator 13, 18, 20 is higher than the normal input impedance of amplifier 14 and as the operating bias voltage on lines 82, 84 approaches low values the two impedances closely match, thereby increasing transfer efficiency and compensating for reduction of amplification of amplifiers 12 and 14. Similarly, the value of resistor 70 is higher than the normal input impedance of transistor 34 so that with decreasing supply current the closer impedance match increases transfer efficiency, thereby compensating for reduction in amplification of amplifiers 12, 14. The overall transmission characteristics are thereby maintained substantially constant over a large range of supply current values.