Electret Microphone Circuit
The ‘
FM Remote Control Receiver’ (available on this website in Infra-red circuits section) has a connector where an analogue output is made available.
To make a simple intercom or P.A. system the associated
transmitter needs a microphone pre-amplifier that outputs a signal at the correct level. And that is exactly the function of this circuit. Actually, this design is adapted from a circuit published last year (‘AM Modulator for Intercom’). A few things have been changed so that it can work with the 5 V supply from the transmitter module. The OTA (IC1) used here is the single version (CA3080), which has slightly different characteristics from the dual CA3280.
Electret Microphone Curves
The quad opamp is the same rail-to-rail TS924IN, made by ST. The turnover frequency of the filter (3rd order 1 dB Chebyshev) has been increased slightly to improve the intelligibility of speech and is now about 5.5 kHz. The filter now amplifies the signal by a factor of 10. In practice it is possible that due to various tolerances and the fact that the opamp is not perfect, the filter characteristic shows some deviation from that required. In our prototype it was necessary to change R15 into 2k7 to straighten the response curve. The DC current variation at the output of the OTA and the resulting offset variation at the output of current/voltage converter IC2d is such that the gain of IC2d has to be substantially smaller than in the ‘old’ design.
Otherwise the output could easily rise to the
supply voltage at low signal levels. The value of R6 has therefore been made smaller by a factor of 10. This has reduced the gain of the circuit by 20 dB, which is compensated for in the filter. The amplitude of the signal from IC2d is fed back as a control current to the OTA by peak rectifier D1/C3 and inverting amplifier IC2b. R7 limits the loading on IC2d. P1 can be used to adjust the amplifier between a fixed gain and maximum compression. Figure A shows clearly what effect the circuit has. 0 dBr corresponds to 100 mV. The maximum gain, with P1 set to maximum compression, is about 48 dB (250 Ω) for small signals.