I have seen previous similar answers about radio transmission, but looking at the following scheme I am not sure about the role C3 plays in the circuit. I guess it is needed to remove the DC signal from the transistor's base, but I cannot exactly figure out how it acts. Could you please help me?enter image description here


closed as off-topic by Leon Heller, laptop2d, Chris Stratton, Harry Svensson, Chupacabras Aug 1 '18 at 12:26

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  • \$\begingroup\$ Looks like a DC blocking cap, but I'm not sure so I'll let someone else answer. \$\endgroup\$ – ZuluDeltaNiner Jul 30 '18 at 13:22
  • \$\begingroup\$ It is a RF bypass capacitor. This oscillator feeds back its collector signal to its emitter. Base is grounded as far as oscillating frequency is concerned. \$\endgroup\$ – glen_geek Jul 30 '18 at 13:38
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    \$\begingroup\$ Radio jamming is illegal. Question should be closed. \$\endgroup\$ – Leon Heller Jul 30 '18 at 15:10
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    \$\begingroup\$ @LeonHeller we've had this discussion several times already: no, it shouldn't. It's not a good question (using a random circuit that came with no explanation), but it's not to be closed because it's illegal under most circumstances. \$\endgroup\$ – Marcus Müller Jul 30 '18 at 15:30
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    \$\begingroup\$ @Marco Radio jamming is illegal. Depends if you actually intend to do it or not. I don't think there's anything wrong with learning how a jammer operates even if you're not allowed to use it. Or you could connect it to your spectrum analyzer instead of an antenna to see what sort of signal it puts out. Point being, there's nothing inherently wrong with the question. \$\endgroup\$ – immibis Aug 1 '18 at 5:54


Internally to the BJT is a parasite called "Miller capacitance". It can be regarded as a capacitor between collector and base and is typically, for normal transistors, in the region of 10 pF. It reduces the gain of a transistor at high frequencies because it injects a signal current back into the base: -

enter image description here

The effect of C3

With C3 present, that injected current instead of flowing into the base, finds a lower impedance path down to ground and this extends the high frequency performance of a BJT circuit that uses the emitter as its input (as in the case of your oscillator because positive oscillatory feedback is fed to the emitter).

  • \$\begingroup\$ @Marco if you haven't taken the short tour may I recommend it to get a better understanding of how this site works. \$\endgroup\$ – Andy aka Jul 30 '18 at 13:57
  • \$\begingroup\$ Is there a name for this use of a capacitor? \$\endgroup\$ – immibis Aug 1 '18 at 5:55
  • \$\begingroup\$ @immibis I'm not aware of one other than RF decoupler. \$\endgroup\$ – Andy aka Aug 1 '18 at 8:50

In this circuit C3 does not block dc, it helps maintain a dc voltage at the transistor's base. It's part of the transistor's biasing arrangement.

Oh, and jamming a broadcast FM signal is illegal, at least in the U.S.


Energy is only stored in circulating-path networks. C3 and C4 complete one of the RF-energy-loops.


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