There are two ways to connect an electret microphone to a circuit: source output and drain output. The source output is similar to the emitter output of a crystal transistor. Three lead wires are required. Drain electrode D is connected to the positive pole of the power supply. A resistor Rs is connected between source S and ground to provide source voltage, and the signal is output from the source through capacitor C. The braided wire is grounded to provide shielding. The output impedance of the source electrode is less than 2k, the circuit is relatively stable, and the dynamic range is large. But the output signal is smaller than the drain output. The drain output is similar to the common emitter placement of a crystal transistor. Only two outgoing lines are needed. The drain electrode D is indirectly connected to the positive pole of the power supply with a drain electrode resistance RD, and the signal is output by the drain electrode D through the capacitor C. The source pole S is grounded together with the braided wire. The drain output has a voltage gain, so the sensitivity of the microphone is higher than that of the source output, but the dynamic range of the circuit is slightly smaller.
The size of Rs and RD should be determined based on the power
supply voltage. It can generally be selected between 2.2-5.1k.
For example, when the power supply voltage is 6V, Rs is 4.7k and
RD is 2. 2k. In the output circuit, if the power supply is
grounded to the positive pole, only by swapping D and S, it can
still become a source and drain output. There are two different
connection methods for the source output and drain output of a
polarizable microphone in a preamplifier stage of a voice
controlled circuit. Finally, it should be noted that whether it
is the source output or the drain output, the polarizable
microphone must provide a DC voltage to operate because it is
equipped with a field-effect transistor.
