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?
Electronic – AM (Amplitude Modulation) Colpitts
amoscillator
Related Solutions
The op-amp output is tied to ground and, there is no positive feedback anyway so it wouldn't work. Even if you switched the LC arrangement to give positive feedback there is no non-linear circuit to give gain control - the circuit, when "wired correctly" will alternate between full clipping and hardly anything.
Maybe you were trying to do this one: -
It works because there is positive feedback - the amp inverts (180º shift) and the L and C conspire to produce another 180º at resonance. I can't vouch for it working and I don't think the capacitor C2 will do anything except make it appear like a colpitts oscillator. I found it by googling "op-amp colpitts".
This is a better circuit and you can swap the crystal and series 75pF for an inductor methinks: -
It works better because there are diodes that limit the amplitude and make the circuit stable. It's basically a small variation on the standard emitter follower colpitts circuit.
L1 and C3 form (or can form) a highly resosnt series pass tuned circuit and you can expect voltage amplification of a sinewave (at the collector) to the output across C3. Normal stuff for tuned circuits.
The problem with the asymmetry is that you are also feeding the modulating signal through L1 as well and so you are seeing, on the "o-scope" the addition of a perfectly normal modulated waveform and the modulation signal.
There may be other artefacts too - it may not be a perfectly symmetrical modulation due to the bias onto the base receiving the modulated signal. Ideally you need to be only modulating the collector to obtain a more symmetrical output signal. Try isolating the base bias from V1 and see what happens then.
Best Answer
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.