GB856105A - Improvements in or relating to transistor oscillator circuits - Google Patents

Abstract

856,105. Transistor oscillators. GENERAL ELECTRIC CO. Ltd. Nov. 21, 1958 [Dec. 12, 1957], No. 38666/57. Class 40(6). [Also in Group XXXV] A transistor oscillator comprises a plurality of similar transistors having their emittercollector paths connected in series in the same sense. There being a capacitor connected between the emitter and collector of each transistor and all the capacitors being of substantially the same value, the transistors being connected in series with a resonant circuit between the terminals of a power supply and a regenerative feedback signal derived from the resonant circuit being applied between the emitter and base of each transistor. The circuit illustrated serves as a power supply for a low-pressure mercuryvapour lamp 14 and is supplied from a 28-volt source 6; this source may, however, rise to 60 volts for short periods whereby damage would be done to the circuit if a single transistor alone were used. The transistors 1, 2 are arranged in series with the source 6 and the primary winding 3 of a transformer having a tuned secondary 11, further secondary windings 19, 20 supplying feedback voltages to the transistors 1, 2. Since the transistors 1, 2 are either both conducting or both non-conducting the voltage between the emitter of transistor 1 and the collector of transistor 2 should be equally divided between the two transistors; equal-value capacitors 7, 9 are provided to ensure that this is so despite differences between the characteristics of the transistors. Resistors 8, 10 may assist the potential-divider action of the capacitors 7, 9. When the circuit is first switched on and before the lamp conducts, the feedback to the emitterbase circuits of the transistors is of sufficient magnitude to drive the transistors well into saturation and a non-sinusoidal output voltage is developed. Such a voltage, together with the overshoot which tends to occur in each cycle when the transistors commence conducting, assists in starting the discharge in the lamp 14. Thereafter the load provided by the lamp 14 reduces the amount of feedback with the result that each transistor is sent just into saturation once per cycle and a sinusoidal output voltage is developed across the secondary winding 11. The emitter-base circuits of the transistors are biased during operation by the potentials developed across capacitors 22, 24 as a result of the rectifying action of the emitter-base diodes; as a result the transistors normally operate under slightly class-C conditions. It is stated that the circuit may be modified to comprise two series groups of transistors operating in push-pull. Specification 854,206 is referred to.