Radio Transmitter

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Friday, March 27, 2009

FM Transmitter with 4 Transistors

This circuit is an FM transmitter use four transistors. It provides an FM modulated signal with an output power of around 500mW. The input Microphone preamp is built around a couple of 2N3904 transistors, audio gain limited by the 5k preset.

The oscillator of this fm transmitter is a colpitts stage, frequency of oscillation governed by the tank circuit made from two 5pF capacitors and the inductor. Frequency is around 100Mhz with values shown.

Audio modulation is fed into the tank circuit via the 5pf capacitor, the 10k resistor and 1N4002 controlling the amount of modulation. The oscillator output is fed into the 3.9 uH inductor which will have a high impedance at RF frequencies.

The output stage operates as a class D amplifier , no direct bias is applied but the RF signal developed across the 3.9 uH inductor is sufficient to drive this stage. The emitter resistor and 1k base resistor prevent instability and thermal runaway in this stage.

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