GB1189010A - Improvements in Transistorised Power Amplifiers - Google Patents

Abstract

1,189,010. Transistor power amplifiers. COMPAGNIE FRANCAISE THOMSON-HOUSTON-HOTCHKISS BRANDT. 28 July, 1967 [29 July, 1966], No. 34923/67. Heading H3T. [Also in Division H1] In a power amplifier, comprising at least one power stage having a plurality of transistors 1-6 connected and fed in parallel and being simultaneously driven by an input signal applied to a common input terminal 7 connected to similar electrodes of the transistors, a transformer 18 is provided having a single secondary winding connectible to a load and a plurality of primary windings 21-26 corresponding in number to the transistors and each primary winding is connected to the output electrode of a different one of the transistors. The circuit shown is a power amplifier of a transmitter feeding an aerial 27 which is tuned by capacitor or inductor 20. Protection resistors 41-46 are provided in the collector circuit of each transistor to prevent thermal runaway and to limit their collector current as does resistor 29 which is switched out of circuit by switch 30 when the amplifier is receiving a signal which may be low, very high or ultra-high frequency. The parasitic admittance between the collector and base of the transistors may be neutralized by connecting an impedance 40 between the output and input of the amplifier and by arranging the transformer windings so that the input and output voltages at 7 and 19 are of the same phase but of opposite phase to that at the collectors. Negative temperature coefficient resistors or thermistors can be included in the collector circuits of the transistors to regulate the circuit against temperature changes. An inductor L in anti-oscillation elements 11-16 damps oscillations which may be produced by parasitic coupling in the circuit of a transistor or between different transistors. Several amplifiers of Fig. 1 may be connected in parallel (Fig. 2, not shown). Measurement and testing.-The current flowing through each collector circuit of the transistors may be measured in turn by switching a voltmeter calibrated in current across shunt resistors 51-56 in each collector circuit. Transformer construction and circuit mounting.-The transistors are mounted on a conductive earth plate (61, Figs. 3 and 4, not shown), around a toroidal transformer which has a coaxial output (57, 58) from a single turn secondary around which the primary windings 21-26 are wound.