SINGLE FET VHF (88-108 MHz)
FM WIRELESS MIC

The circuit shown depicts a wide band FM wireless microphone.
Its power and range are similar to the commonly known circuits, which use bipolar transistors.
The simplicity of the design means it is certain to work first time.
One gate circuit of the MOSFET is a self excited RF oscillator, and the other gate circuit acts to bias the device and also as a microphone pre-amp and modulator.

Tuning can be effected by making Ct a trimmer/padder capacitor, or alternatively if this is a fixed value, then expanding compressing the coil should give some variation in frequency.
Almost any dual-gate N-Channel MOSFET should work;
suitable types are 40673, 40822, BF900, BF988.
A 9V battery is used as a power supply.



The picture shows a "working - model" constructed so as the mosfet is 'socketized' in an external socket.
This allows the circuit to be used as a test jig.
By plugging/unplugging a mosfet you can judge if it works by listening to the result on an FM radio.