Here's an FM transmitter that I found on the web:

FM transmitter

Here's a regenerative receiver that I found:


The capacitor connected to the base of the transistor in each circuit is different. Why isn't the capacitor for the FM transmitter also at 10uF or why isn't the capacitor for the receiver also 1nF?

I'm trying to understand why such values are chosen for grounding capacitors.


In both cases a capacitor is needed to ground the Base at RF frequencies. However in the transmitter the Base is also the input for FM modulation, so the capacitor must be small enough to not attenuate the audio signal.

In the receiver the Base is fed with a DC voltage from the regen pot, so the capacitor can be larger. 10uF might be a bit too large for high frequencies though, as the capacitor's parasitic inductance will start to resonate with its capacitance and make it less effective. At 15MHz it's probably OK, but at 100MHZ a smaller capacitor might work better.

So one answer is:- because they work on different frequencies.

  • The Tx has a 4k7 input impedance for the mic which uses 1nF as a LPF to reject RF and lowers the emitter output impedance of Zc by hFE for RF to improve gain, sufficient to make it oscillate.
  • The Rx uses Q1 as a regenerative Colpitts oscillator where the DC bias current is control by a 10K Pot and a 10uF C1 as a LPF for DC only.
  • The breakpoint frequencies are different for LPF and HPF which require different values for C to be computed.

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