GB1287127A - Improvements in or relating to power amplifiers - Google Patents

Abstract

1287127 Transistor amplifying circuits MARCONI CO Ltd 14 Oct 1969 [23 Jan 1969] 50342/69 Heading H3T A power amplifier comprising at least one class A transistor stage incorporates an additional transistor which has its collector circuit connected to the base of the amplifier transistor and which has a negative feedback circuit which includes a resistor which is common to the emitter circuit of the additional transistor and the collector circuit of the amplifier transistor, the additional transistor being arranged to act as a current generator. The arrangement is described as applied to a transmitting amplifier comprising an initial stage Tr 5 coupled by transformer T to a first class A stage Tr7 which is followed by a further class A stage Tr9 and two class C stages Tr11, Tr12 respectively, coupling being by means of low-pass filters pb1-pb3. The emitters of transistors Tr7, Tr9 are earthed, and they obtain base biases via auxiliary transistors Tr6, Tr8 respectively, which are provided with negative feedback by means of resistors R1, R2 respectively. The collector currents of transistors Tr7, Tr9 pass through resistors R1, R2 respectively, so completing D.C. negative feedback loops which render the collector currents of these transistors substantially constant. The biases for the bases of transistors Tr6, Tr8 are derived from a network including the collector current path of a further transistor Tr13 to which a bias can be applied to render transistors Tr6, Tr8 and hence transistors Tr7, Tr9 non- conductive, cutting off the output of the amplifier. This may be utilized to provide overload protection, or if the amplifier is utilized in a radio relay station, transistor Tr13 can be arranged to be controlled by known automatic insertion circuitry for bringing the transmitter of the station into operation when the receiver thereof picks up a signal.