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I'm currently building an FM Microphone kit and I'd like to understand its operation before physically putting it together. The documentation only provides an overview of some aspects and provides no calculations or insight into the selection of component values.

  1. Am I correct in identifying R1 and the condenser microphone (hereafter Mic,) and R2 and R3 as voltage dividers?
  2. Given an input of 9V, R2∩R3 will output 1.59V?
  3. For R1∩Mic to be in voltage equilibrium with R2∩R3, the impedance of the Mic at rest must be 59KΩ?
  4. Why is a polarised type used for C1? (a tantalum cap was provided in the kit if that helps)
  5. What are C2 and C3 doing? Do they form some sort of lowpass filter?
  6. To keep the LC tank oscillating, how do you calculate what power to inject each cycle? (I know it has to be in phase)

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    \$\begingroup\$ Comment on a few of Andy's good points: (3) is as he says because of C1. The DC voltage at each side of C1 can be different and the AC component is passed by C1. Tantalum can cause some nasty surprises where more than minimal energy is available.- go s/c with even very moderate overvoltage. Once 1 uF was high for a non-polar cap but now ceramics at 1 uF are small cheap and commonplace. (6) "Enough" :-). While this is amenable to calculation around 2 to 10 pF is typical and what works well is liable to be what is used. \$\endgroup\$ – Russell McMahon Dec 29 '15 at 11:13
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    \$\begingroup\$ You could do worse than adoring snow leopards :-) \$\endgroup\$ – Russell McMahon Dec 29 '15 at 11:13
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    \$\begingroup\$ This circuit was discussed couple weeks ago. It discussed how the transistor influenced the frequency by it's changing Cbc. Found it: electronics.stackexchange.com/questions/204854/… \$\endgroup\$ – jippie Dec 29 '15 at 14:18
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  1. You are correct
  2. You are correct although it might be a tad lower because of a little bit of base current but the important point is that the BJT will be biased.
  3. No this isn't important - the output impedance of the microphone circuit will be approximately 27K (R1 acts as drain resistor for the internal JFET in the mike)
  4. C1 doesn't need to be polarized - I would pick a ceramic capacitor and avoid an electrolytic because it's unclear which side will have the more-positive voltage
  5. C2 and C3 hardly do anything at audio frequencies but, at 100MHz they ensure that the base is strongly de-coupled - this improves the modulation process and may in fact prevent it if they were not there.
  6. C4 injects the positive feedback power into the emitter and the collector will naturally be in phase and produce a higher AC voltage thus ensuring oscillation. There's a lot more to how this works and how the centre frequency of the tank will be slightly shifted from theoretical but this appears too much to discuss at this point.
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