I understand that the attenuation will stop the circuit from oscillating, I'm looking for ideas on how to get around this problem.I would say it needs an output buffer to isolate the sensor from directly influencing the oscillator. It may be a university project but in real life nobody would directly load an oscillator, at least a Colpitts one, without expecting it to influence frequency, amplitude or even stop it running.
Brian.
Q1 is missing DC bias, so it's completely expectable to see no oscillations.
Simulation circuit is using UIC option, usually sufficient as initial kick start. Also the description in post #1 sounds like dying-out oscillation, oscillation condition not fulfilled. We might check the circuit if we have the LTspice files.Kicker?
Simulation circuit is using UIC option, usually sufficient as initial kick start. Also the description in post #1 sounds like dying-out oscillation, oscillation condition not fulfilled. We might check the circuit if we have the LTspice files.
How did you calculate the parameters so that the circuit kept oscillating? In my case, I cannot change the values of the sensor components. This is my main "problem", I understand that when placing the sensor the Berkhausen criteria are no longer met, but I can't see which components of my Colpitts/differential amplifier I change to compensate for this.As guessed, the loop gain is too low to maintain oscillations. A quick check that the topology can basically work as oscillator:
View attachment 175475
In the case of my first circuit, with the differential pair, where should I add the components that compensate for the loss in dB and the phase correction? At the output of the circuit (Vout)? Or in the feedback loop that goes to the sensor?I didn't calculate anything, just arbitrarily modified component values to show that the topology can oscillate.
You apparently added the filter to a working oscillator without thinking about necessary circuit modifications. The filter has a voltage gain of -14 dB if driven by a low impedance source respectivel -20 dB power gain with matched impedance. Additional gain loss is caused by loading the original oscillator circuit.
Plotting the filter transfer function separately shows 180° phase shift in resonance, respectively the oscillator should use an inverting amplifier. A single CE stage seems to provide insufficient voltage gain, thus I tried a cascode circuit.
View attachment 175503
As stated above, about 180° degree phase shift of the sensor in resonance suggests an inverting amplifier, similar to a crystal oscillator. An inverting amplifier could be used with an inverting tank circuit, e.g. tapped inductor.In the case of my first circuit, with the differential pair, where should I add the components that compensate for the loss in dB and the phase correction? At the output of the circuit (Vout)? Or in the feedback loop that goes to the sensor?
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