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Schematic

I have three questions:

  1. Is it common emitter or common collector (emitter follower) or something else? How do you recognize it?

  2. What is the function/usage of R1 and C1? Are they for biasing the transistor?

  3. The most important question is: What is the function of C3?

I asked the third question because if it is common emitter, we can take the output from the collector Vc and there is no need for C3. If it is a common collector (emitter follower) we can take the output from the emitter Ve and also there is no need for C3. I don't know why collector voltage and emitter voltage are connected by C3?

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  1. It's common-base. You can recognize it because the base is bypassed with large value capacitor to a supply rail or ground (it's at AC ground). Large is in relation to the operation frequency, which in this case appears to be about 100MHz.

  2. C1 keeps the base grounded for RF, R1 provides bias when the input is open (or capacitively coupled).

  3. L1 and C2 + C3 + Ccb are a resonant circuit. The output is taken from the collector. C3 provides feedback to the emitter, around half the voltage at the collector appears at the emitter. Without C3 it won't oscillate, or at least not very well. C2 allows the oscillation frequency to be determined separately from the amount of feedback.

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    \$\begingroup\$ Don't you just hate it when you give a good answer and no one gives you any credit including the OP. +1 from me \$\endgroup\$ Sep 11, 2014 at 13:16

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