Differential Colpitts VCO

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hrkhari

Full Member level 4
Hi Guys:

If you go through the paper by Hajimiri:

R.Aparicio and A.Hajimiri,"A Noise-Shifting Differential Colpitts VCO", IEEE JSSC, vol.37, no.12, pp. 1728-1736, Dec 2002.

He had calculated the percentage of amplitude degradation as the ratio percentage of amplitude of a LC QVCO with coupling transistor, to the amplitude of a LC VCO without the coupling transistor.

I think, if one were to calculate the percentage of amplitude degradation it should be the ratio percentage of the difference of amplitude between the LC QVCO and LC VCO to the amplitude of the LC VCO.

Pls do comment on this definition.

Rgds

vfone

I don’t have the document that you are talking about, but usually as a rule of thumb the Loaded Q of the circuit, Q_loaded = (3/4)*Q_unloaded.
Is expected the same degradation for the output voltage with and without load (in your case the coupling transistor).
In the same time for overall output voltage/power has to be taken in consideration the mismatch loss that can appear between stages due to bad VSWR:
Mismatch_loss(dB) = 10*LOG(1-Γ^2)

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