GB1493591A - Microphone arrangements for use in telephone apparatus - Google Patents

Abstract

1493591 Microphone arrangements GENERAL ELECTRIC CO Ltd 8 Jan 1975 [15 Jan 1974] 01846/74 Heading H4J A microphone arrangement for telephone apparatus comprises a microphone having a resonance peak 16 (Fig. 2) in its frequency response in the middle or lower portion of the telephone voice frequency range. The output signal from the microphone is amplified by an amplifier having an input impedance which combines with the electrical impedance of the microphone to provide an amplifier gain which rises with frequency substantially over the whole of the frequency range (curve 17). This frequency response combines with the microphone resonance peak to provide an overall frequency response which rises with frequency over the lower portion of the voice frequency range and is substantially constant over the upper portion of said range. The microphone may use a multimorph piezoelectric ceramic element or an electret comprising a permanently charged film of P.T.F.E. Microphone capsule 1 (Fig. 1) has a plastics body 2 and an aluminium cover 3. The edge of cover 3 is turned over to retain a metal foil diaphragm 4. This has a conical centre portion whose apex bears against piezoelectric element 5 which rests on pillars 6. Electrical connections 7 from element 5 lead through two stakes 8. The base of body 2 carries an integrated circuit amplifier (Fig. 3) associated with a printed circuit board (not shown). Capsule 1 and the printed circuit are enclosed in an outer cover of which a gasket 9 forms part. Cover 3 has a single aperture 10 closed by a disc 11 of cloth or porous metal forming an acoustic resistance. Gasket 9 has a single aperture 12 which communicates with a limited number of apertures 13 (e.g. one) in the mouthpiece 14. This construction provides the low resonance peak 16 (Fig. 2). Element 5 has a source capacitance of 500 picofarads and when coupled to an amplifier having an input impedance of 100 kilohms has a change in characteristic at about 3200 Hertz. This frequency response 17, together with the response 16 provides an overall response 19. The amplifier (Fig. 3) comprises two output transistors 21, 22 to both of which amplified signals from microphone 1 are applied from a preamplifier stage 23. One of the transistors 21, 22 controls the current flow between output terminals 24, 35, depending on the polarity of the energizing supply to terminals 24, 25. The other transistor is shunted by a respective diode 26 or 27. Diodes 26, 27 may be Zener diodes, protecting against voltage surges in the supply to terminals 24, 25.