GB1028591A - Improvements in or relating to capacitor microphone amplifiers - Google Patents

Abstract

1,028,591. Transistor amplifying circuits; microphone circuits; frequency modulation. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. June 5, 1964 [June 7, 1963], No. 23411/64. Headings H3R, H3T and H4J. [Also in Division H2] In a head amplifier for a capacitor microphone of the type in which oscillations from a high-frequency generator are frequency-modulated from the microphone, the modulation is not effected directly but through the intermediary of a voltage variable capacitor. Transistor 1 is connected as a Clapp oscillator, the emitter being connected to the join of capacitors 2, 3 which are connected in series and tune inductor 4 which is between base and ground. Chokes 5, 6 allow the passage of D.C. to respectively the emitter and a pair of voltage-variable capacitor diodes 8, 9 oppositely connected in series with inductor 4. The capacity of diodes 8, 9 is varied by the audio signal from microphone 10, which is fed with D.C. over a highvalue resistor 10 and coupled to the join of diodes 8, 9 via D.C. blocking capacitor 11 and H.F. choke 12: the diodes are provided with a standing bias from source 14, via resistor 15. The source 14 may comprise a decoupled resistor in the emitter circuit of transistor 1. The choke 5 may comprise the primary of a step-up transformer, the secondary of which feeds a rectifier 21 supplying the D.C. to microphone 10. The frequency-modulated output from the oscillator is applied to a discriminator 16; negative feedback may be obtained by using the output as source 14. Choke 12 may also be replaced by a resistor: in addition, terminal a may be connected to earth or a small negative potential and a high negative potential applied to the lower electrode of the microphone, in which case source 14 and resistor 15 may be omitted.