by Frank v Hughes

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 range could be increased by adding a PA stage as shown on the FM bug V5 (SM0VPO web pages).

If you have any questions related to this project then please refer them to the author Frank V. Hughes.

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