Which configuration is better for oscillator - CG or CS?

[vagarwal]

oscillator design

looking at the S parameters of a transistor/FET device, can we tell which configuration would be better for oscillator design - CG or CS?

 

[v4eda]

Re: oscillator design

My dear friend :arrow:
the application dictates the choice of the active device (BJT, FET, HEMT ) and also the choice of the topology (whether Common Gate /Common source) and you just can't really choose the topology by just looking at the S-pars; of course you can find out the oscillatory tendency of the device by looking at the mag of S11/S22.
:wink: Generally Common Gate topology is preferred for fixed frequency oscillators where the required tuning range is not much; the regenerative inductor and the Cgs of the acitve device fix the range of the negative resistance and hence reduce the BW.
whereas the Common Source topology provides a larger negative resistance bandwidth (with Cgs being absorbed in the feedback capacitance) is used in VTOs and frequency synthesisers requiring a larger tuning range;
Hope this answers your query.....

bye