Why is a capacitor connected across the collector-emitter of the transistor? How does this capacitor prevent decay of LC oscillations?
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2\$\begingroup\$ Where's your schematic diagram? We're not clairvoyant. \$\endgroup\$– Dave TweedCommented Mar 29, 2014 at 22:02
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Some of us are clairvoyant.
With the base effectively grounded at RF, the transistor is obviously acting as a common-base amplifier, which is non-inverting. Therefore the collector-emitter capacitor provides positive feedback.
OK I cheated. You asked another question about the same circuit yesterday.
EDIT : The circuit is rather clever; it uses the same transistor for three separate purposes:
- at audio frequencies, it amplifies the microphone input signal as a Common Emitter amplifier (with relatively low gain, since the collector load at low frequencies is mostly the battery ESR). The Miller capacitance is too small to affect audio. However replacing C3 with a "proper" audio decoupling capacitor would reduce the load and therefore the audio gain.
- the amplified audio voltage modulates the miller capacitance, which is a few pf, and (in series with huge capacitors C1 and C3) acts in parallel with VC1 to frequency modulate the oscillator.
- A Common Base amplifier has non-inverting voltage gain but a current gain of 1. But crucially in this application, it has good high frequency performance because the miller capacitance is grounded (by C1) instead of providing catastrophic levels of negative feedback (at RF) as it would in Common Emitter mode. Thus the relatively small C2 provides enough current to develop an RF voltage at the emitter, and that is the input signal (to the emitter) which is amplified at the collector.
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1\$\begingroup\$ I have read that the capacitor takes a portion of the signal from the collector and feeds it back to the emitter to provide positive feedback. Can you just explain this process ? Does the capacitor allow high frequency collector current to reach the emitter and hence the positive feedback ? I know that positive feedback is what causes the oscillations to sustain. \$\endgroup\$ Commented Mar 30, 2014 at 9:03
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\$\begingroup\$ @Brian Drummond For item 2. How does amplified audio voltage modulate miller capacitance? As far as I know miller capacitance is a function of gain not the input signal. How this mechanism works so capacitance chance affects the resonance frequency? \$\endgroup\$– albinCommented Jun 11, 2016 at 16:19
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\$\begingroup\$ All reverse biassed diodes work as varicaps to some extent : the C-B junction is no different. The multiplication of the intrinsic capacitance by the miller effect is a function of gain, but the capacitance itself isn't constant - it's a function of the reverse voltage. Which ... doesn't vary very much. As the audio gain is < 1 (approx Rbattery/Re) the mic voltage itself is the main component of the modulating voltage. I should probably have named it collector-base capacitance rather than Miller capacitance to make it clear that the Miller effect isn't important here. \$\endgroup\$– user16324Commented Jun 11, 2016 at 16:53