I made this circuit and it works properly but I need to make an FM receiver using the same idea of this circuit (common base).

Would you tell me what changes should I make so that the circuit is able to receive FM waves instead of transmitting it ?

I've tried to connect an antenna to the base of the transistor, and connect a 8-ohm speaker in parallel with the tank circuit but it doesn't work. Also, I tried to connect it in series it doesn't work as well ?!

I have one more question: When I receive radio wave from this circuit using a radio receiver device, It receives the waves on many frequencies such as 108 MHz and 95 MHz.

when I change the values of capacitor and coil of the tank circuit, the frequencies change but it still receives on many frequencies.

I connected a capacitor of 1000uF instead of C2 ( To be out of the range 87 - 108 MHz ) but the receiver still receives signals.

I connected a wire instead of the coil ( with C2 = 22 pF ) but it still also receives.

Is that normal ? or have I made something wrong ?

Thanks in advance,

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  • 1
    \$\begingroup\$ It is customary to ask ONE question per question. \$\endgroup\$ Mar 13, 2015 at 20:33
  • \$\begingroup\$ @WoutervanOoijen I thought that if I asked two questions about the same circuit, the second question will marked as a repeated or duplicated question :) \$\endgroup\$ Mar 13, 2015 at 21:59

1 Answer 1


It's a bit like asking how to build an internal combustion engine from a gearbox. It's just easier to get a receiver circuit and build it without reference to a transmitter circuit. AM or FM Receivers are usually more complex than an AM or FM transmitter. My advice is get a circuit by googling and disregard any commonalities with the transmitter you have built.

Why is it capable of being received on a whole load of frequencies even when you radically change tank capacitance and inductance?

Answer: because it's a badly designed transmitter with lots of harmonics being generated. You said you received it on 95MHz and 108MHz - it may well be transmitting on a couple of spaces in between that a digitally tuned FM receiver isn't able to perfectly tune to. FM receivers in the UK tune every 200kHz and conceivably there could be a couple of spots that they jump over.

You put a monstruous 1000uF cap across the coil - that cap at the sort of frequencies being transmitted probably looked like an inductor in series with several ohms and maybe moved things about a bit but, due to so many harmonics being generated you were still bound to find that an FM receiver could still pick-up the transmission. Here's a graph showing the impedance of an electrolytic capacitor changing to inductance above a certain frequency: -

enter image description here

How long is a piece of string or how much inductance has a piece of wire - wire has inductance and getting something like 0.1uH is trivial - 10 cm of wire has around 100nH = 0.1uH.


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