2016-10-18 FM Transmitter Uses Power MOSFET

I watched this YouTube video where the author shows his FM transmitter using a big 60 volt power MOSFET. He also shows a very readable photo of the schematic. The video was made several years ago in 2012.

He is powering this transmitter by connecting it to eleven 9 volt batteries in series, totalling 99 volts. The power MOSFET he is using is rated for only 60 volts, which is not a good situation. For any LC oscillator, the power supply should never be more than half the maximum collector voltage rating of the transistor, in this case, 30 volts. The reason why this circuit has not failed is that the circuit has a 5k, 10 watt resistor in series with the negative supply. He says that it gets warm. If the 5k is connected directly across 99 volts, the maximum current would be 20 milliamps, so the current must be less than that. This circuit is wasting a large voltage (and therefore a lot of power) across the resistor. My guess is more than 50 volts, but he doesn’t say how much voltage is across the resistor. This is wasting more than half the battery power heating the resistor. The battery voltage should be much less, and this resistor should be much lower, only a few hundred ohms.

I have built quite a few of these 1 transistor FM transmitters, and I’m very familiar with the coils and capacitor values. I have a really handy tool called a FET dipmeter. It’s easy to find the frequency of resonance with this. I wound some wire around a AA battery just like he said and showed in the video. I connected a 30 pF capacitor across it and found the resonant frequency and it was between 45 and 50 MHz. I am reasonably certain that his transmitter is oscillating at about that frequency and what he is hearing on the FM radio is the second harmonic, between 90 and 100 MHz.

In order to get the fundamental frequency of his transmitter up to the FM band, the coil must be much smaller. The coil’s diameter should be smaller than a pencil, about 5 mm or 0.2 inches. And the number of turns should be reduced, 3 or 4 at most. The tuning capacitor he is using should have a maximum of 30 pF — he said that is the capacitor’s minimum. He could connect a 47 pF capacitor in series with the tuning capacitor to reduce the total capacitance.

I think it will be difficult for the MOSFET to get up to the frequency in the FM band. The typical power MOSFET has high internal capacitances, and these will change depending on the voltage. So the frequency will depend too much on the MOSFET, and as the battery voltage changes, so will the frequency.

I enjoyed watching this video, it is interesting that a power MOSFET can get up to fifty MHz. But I think it would be better to use a small transistor such as a 2N3904 for this project – it should be powerful enough to hear for a hundred feet (30 meters).

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