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Could someone explain to me why the output of this oscillator is the way it is? With a DC input of 6V (no small signal), I get an oscillation between -2V and 7V, which was unexpected. Is this due to the inductor? Why is it uneven?

When I have the source modulate the amplitude of the oscillator (as shown in the picture), the envelope on top follows the source modulation, but the lower envelope is distorted. Is this because of my biasing? Also, I remember reading in my Communication Theory book that this gap between the envelopes can be decreased, which, if memory serves, would save power. How would that be achieved?

Any help with any of these questions would be appreciated.


1 Answer 1


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.

  • \$\begingroup\$ Thanks for the response Andy. I figured the extra voltage was from the LC circuit, but not sure how to calculate the min and max of Vout (over C3). I tried divorcing the bias from the rail, but it made no remarkable difference. I also tried different ratios of biasing resistors, but that only seemed to change the magnitude and not the asymmetry. \$\endgroup\$
    – MrUser
    Dec 8, 2014 at 23:52
  • \$\begingroup\$ If you prevented the circuit from oscillating by removing C4 and then looked at the output again during modulation, do you see the modulating signal on the output (obviously not the modulated carrier). \$\endgroup\$
    – Andy aka
    Dec 9, 2014 at 8:15
  • \$\begingroup\$ Yes. I see an attenuated and slightly phase-shifted version of the modulating signal at both the collector and at the output. \$\endgroup\$
    – MrUser
    Dec 19, 2014 at 14:51

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