How would I amplitude modulate the Colpitts oscillator directly?
Can this be done by making Vcc the audio signal and DC offsetting the audio signal in order to get the Colpitts to fundamentally oscillate?
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\$\begingroup\$ Found out using simulations that making Vcc the audio signal with a DC offset of 12V gets what I want. Not sure if this is a "good" AM wave since the peak and trough isn't centred at 0V. \$\endgroup\$– Guy LeeCommented Jun 27, 2017 at 2:56
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\$\begingroup\$ @ Guy Lee Insert a DC_block High Pass Filter, with F3dB maybe 10% of Fcarrier (far above the audio modulation). Your signal will become centered. Your oscillator will have some FM, because of varying capacitance in the collector-base junction. \$\endgroup\$– analogsystemsrfCommented Jun 27, 2017 at 4:12
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\$\begingroup\$ @analogsystemsrf How would stray FM in the oscillator affect demodulating the signal? Would this mean that my audio signal may get frequency shifted? \$\endgroup\$– Guy LeeCommented Jun 28, 2017 at 5:41
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You can't push a colpitts oscillator (or hartley or clapp) too far because you will either get an overly distorted sine wave or you'll kill oscillations completely. Even if distortion wasn't too bad (i.e. you didn't push things too much) there would be an associated frequency modulation due to the changing bias conditions brought about by amplitude changes.
The underlying mechanism here is the so-called "miller" capacitance between base and collector - basically the depletion layer in the PN junction in that part of the BJT is modulated by the voltage across collector and base. In fact any oscillator of this type produces a cyclic distortion that is related to the change in capacitance due to the actual oscillation voltage appearing between base and collector.
So, my advice is add an amplitude modulator to the output of the colpitts and this can be easily done with a diode (and the appropriate DC control levels) plus an output filter resonant at the carrier frequency. Here's a very simple AM circuit idea you can experiment with: -
The blue waveform is the modulated carrier and the red signal is a triangle wave modulation signal. You can get quite respectable results with virtually a really small handful of components.
