GB817530A - Improvements in or relating to automatic gain control arrangements for electric amplifiers - Google Patents

Abstract

817,530. Automatic voltage control systems. STANDARD TELEPHONES & CABLES Ltd. Feb. 24, 1958, No. 5965/58. Class 38 (4). [Also in Group XL (c)] The output level of a valve oscillator is stabilized by an automatic gain control circuit in which a transistor D.C. amplifier is used. The invention is described as applied to the frequency-changer in transmission measuring apparatus, the oscillator of which comprises a pair of valves coupled by tuned circuit 5 and back-coupled via a frequency-determining crystal 9 interconnecting the cathodes. Frequency changing is performed by a voltage doubling circuit comprising rectifiers 14, 15, the signal being applied at terminal 20 and I.F. taken out across resistor 17 in series with rectifier 15. The negative D.C. voltage across resistor 16 in series with rectifier 14 is applied to the emitter of a transistor D.C. amplifier 22 in common base connection. The collector is connected to a negative source 28 via resistor 27, the base being also connected to the source via voltage divider 25, 29. The amplified negative control voltage at the collector is used to control the gain of the valve 1 being applied to the grid via smoothing network 30, 31. The standing current through the frequency-changing rectifiers 14, 15 is monitored by a meter 18 which, with resistor 16, is decoupled by condenser 19. As shown, the transistor 22 is of P-N-P type; the N-P-N type may, however, be used, in which case the polarity of the source 28 is reversed and resistor 10 chosen so that the standing cathode potential of valve 1 is slightly greater than that of the source 28. Specification 816,902, [Group XL (c)], is referred to.