GB1184968A - High Efficiency RF Power Amplification - Google Patents

Abstract

1,184,968. Transistor amplifying circuits; amplitude modulation; pulse modulation. COLLINS RADIO CO. 28 June, 1968, No. 31021/68. Headings H3R, H3T and H4L. In a high-efficiency RF power amplifier and modulator, " on-off " switching means are provided in the D.C. power supply to control the power supply to an RF amplifier and are driven in accordance with signals from a modulating signal source: a low-pass filter is connected between the switching means and RF amplifier. Audio signals from source 15, Fig. 4, are fed over level-controlling potentiometer 30 to an amplifying transistor 33, the collector of which is coupled to the base of transistor 42 of a pair of transistors 42, 46 which are cross-coupled to form a multivibrator. Pulses which are of constant amplitude but width modulated in accordance with the audio signals appear across resistor 48 and are applied to an emitterfollower output amplifier stage comprising transistors 54, 55. The output from emitter-follower 54, 55 is applied over transformer 59 to the base of transistor 16<SP>1</SP> which forms a switch, its collector-emitter path being connected in the path from D.C. power supply 17 to the collector of RF power amplifying transistor 68. D.C. restoration is applied to the output from transistor 16<SP>1</SP> by means of diode 62 and the D.C. pulses are smoothed to produce an amplitudemodulated collector potential by means of lowpass filter 64, 65, 66. The arrangement is used to economize D.C. power: the audio signal may be derived from an envelope and syllabic filter 24, Fig. 2, coupled to a low power amplifying stage 21 in a radio transmitter, the input to which comprises modulated RF signal and may be of single-side band type. The output from envelope detector 24 is combined with a feedback signal, derived from the output of lowpass filter 18 via feedback regulator 25, in mixer 26, before application to the powersupply switching means 27. In another embodiment, transistors 83, 86, Fig. 6 are crosscoupled to form a free-running multivibrator; the coupling capacitors 88, 89 are given widely different values so that the mark-space ratio of its output is small. The collector-emitter path of a further transistor 76 forms the base circuit resistor of transistor 86: its impedance is varied in accordance with the audio signal from source 15<SP>1</SP> so variably decreasing the space period while maintaining the widths of the pulses constant. Audio signal is applied to the emitter of transistor 76 via Zener diodes 77, 78, switch 79 providing for shorting diode 77 so as to change the basic D.C. modulation level. The output from transistor 86 is applied over amplifier 91 to the base of D.C. switching transistor 16<SP>11</SP> and audio signal is returned by a negative feedback path comprising resistors 74, 75 to the base of transistor 76.