The transistor is self biased common emitter mode, but the output at the collector is fed back to the base via C2 and the bridge rectifier comprising D1 to D4 and the meter. This feedback reduces the overall gain of the circuit to slightly less than 1. R3 controls the amount of current through the meter and R3's value is determined by the maximum (wanted) full scale voltage reading divided by the meter full scale deflection current. So, to read 1V AC RMS with a 100uA meter R3 would be 1/0.1mA or 10k. In practise R3 would have to be reduced as the gain is less than one so you could use a 6.8k resistor and a 4.7k preset in series. With an input voltage of 1V RMS the preset would be adjusted for maximum reading. R3 also sets the input impedance for the circuit.
If you wanted the circuit to read 100mV then use an op-amp non-inverting or inverting amplifier with gain set to x10. As this is an AC voltmeter it does not matter if the first stage is inverting because the input is AC.
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