Microphone AGC amplifier

A homebrew 2M transceiver I wanted to use it mobile so I wouldn`t have been able to use a fist mike. I had the bright idea of using one of those cheap personal hands free kits you can buy for mobile phones, however once installed reality struck home - hardly any audio! Like the microphone amplifier of many Ex PMR radios and lowcost hand helds, the circuit
Microphone AGC amplifier - schematic

I used had fixed amplification. Gain could be increased but it would then lead to the problem of distortion if the microphone position changed or I shouted to get over vehicle noise. The simple circuit described here gives additional gain so increasing the microphone sensitivity, enabling low cost mobile phone accessories to be used and also adds AGC (automatic gain control) action. Whilst developing the circuit it was obvious that the same design would make a good desk microphone, the prototype allowed me to obtain good quality audio from anywhere in the workshop. A few years back one of my transceiver designs incorporated the Plessey VOGAD IC type SL6270 to good effect, unfortunately they now seem to be deleted from distributor`s lists. This design is not totally original with elements previously being used elsewhere1, but it does provide a cost-effective alternative to the Plessey solution. Figure 1 shows the circuit diagram, IC1a amplifies the incoming microphone signal with the gain set by R8 and the combination of R9 and the drain / source resistance of Q1. Minimum negative feedback hence maximum gain occurs when Q1 is at minimum resistance. The output from IC1a at pin 1 is applied to the voltage doubler circuit of C2, D1, D2 and smoothing capacitor C3. This feedback voltage is applied to the gate of Q1. Minimum source to drain resistance occurs when the gate is at (or below) the source potential, as the op amp AC...

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