The circuit looks very simple, but there is actually a lot going on. A small change in voltage creates enough change in the emitter to base junction's gap to cause capacitance variation needed for FM. The varactor effect, which is used in the circuit to achieve frequency modulation is dependent upon voltage as opposed to current. The microphone power is insignificant and cannot provide the modulation levels needed for usable level changes in the oscillator's output amplitude. The voltage from the microphone and current (E X I = Power) both are low amplitude compared to the current and voltage of the oscillator. Unlike FET's, bipolar junction transistors depend upon current changes for their gain. My guess is it will be 1% or less as the audio power level from the microphone is much, much less than the RF power level. There is a very small AM modulation component. Most of the early devices that used this circuit were sold as kits, a market that was and still is very cost sensitive. Darlington amps were built from discretes so in addition to two transistors and half a dozen resistors plus caps, the comparatively low cost dynamic microphone won out in the intro years. Even the fading germanium transistors were still around $2.50 each ($13.00 today). In the late 60's and early 70's when these were gaining traction the cost of transistors was significantly more than today. The crystal microphone was low cost but was hampered by it sensitivity to humidity so many of these devices were sold with dynamic microphones. At the time (1970) electret microphones were budget busters, often 10 X more than the dynamic microphone. Dynamic microphones produce significantly more voltage than many crystal and electret microphones. It is both device (transistor) dependent and also audio source dependent.ĭynamic microphones were the 800 pound gorilla in the late 1960s and early 70's when these appeared. Since the base emitter junction capacitance can vary from production run to production run and from manufacturer to manufacturer it is best to use the same manufacturer that the original author used. The circuits displayed are depending on the very small capacitance change the low level mike audio can create to minimize distortion. However if distortion less than 5% is the goal then you need a circuit that is much more stable than this design. The frequency is tuned by compressing or spreading the windings of the coil to vary its self inductance and distributed capacitance. My guess is the microphone is a crystal mike with low output. As you are driving the base emitter junction with audio frequencies little of the audio is shunted to ground by the 0.01 uF cap given the audio low frequency and the caps relatively high reactance. The junction capacitance will vary just enough to cause reactance modulation of the oscillator. The designer depended on the base emitter junction to act sort of like a varactor diode (voltage variable capacitance diode). L1 in parallel with the 36 pF cap determine the oscillator frequency. The current from the collector to the emitter is in phase. The 6.8 pF feeds a small amount of the rf to the emitter to sustain oscillation. The cap divider makes me think the design is based on a Colpitts topology. The base is grounded at RF by the 0.01 uF cap. The device is used in a Common Base Mode.
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