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Re: JFET Model

 

You are mixing several effects in your question. A Clapp oscillator is a special case of a Colpitts with an added decoupling capacitor between the main LC tank resonator and the grid/base/gate element, depending of course on which type of active device is used in the circuit. Although there are other effects, the main purpose of decoupling with a very small C is to minimize tempco effects on resonant frequency, particularly useful if the LC tank is electrically tuned by the use of a varacap (varacter) for some or all of the tank C.
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Beyond that all solid state junctions, even those of relatively linear devices such as JFETs display some dC/dV effects, and so in general changing applied biases and b+ (+vcc) voltages will cause some shift in operation frequency for any oscillator circuit. The only real difference is the degree of change, which by careful design and topology can be made quite small. In other cases the effects of "miller" and junction capacitance changes can be used to desired effect to improve circuit performance.
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In the case of JFETs specifically there are devices in the JFET family such as the U310 series which have very low internode capacitances, so low that oscillators with?slightly over 1 Gigaherz center frequencies can be realized. Indeed I have personally designed varacter tuned versions of such an oscillator using miniature solid shield coaxial line as the "LC tank" with extremely low noise and very good temp stability for use in reciever and transmitter circuitry, and used LTspice years later to model the circuit and see how well the predictions of performance matched the actual lab results I had recorded during bench and temperature testing. LTspice did quite well, although as is always the case some departure from real world results will be expected since some models are ideal.
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Cordially -?RC.


On Tuesday, December 24, 2013 2:45 PM, Andy wrote:
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"I am not sure, but it is pretty likely that the JFET spice model does not include the voltage dependency of Cdg."

Actually, I believe the SPICE models for most transistors, including JFETs, are quite
complete
?and include most effects almost anyone would care about. ?(With a few exceptions.) ?I am almost certain this includes capacitance vs. voltage, vs. temperature, etc., which I think is supposed to be modeled rather accurately. ?Of course it depends on the right parameter values being present in the .MODEL statement; but even if those parameters are wrong, or omitted, LTspice still should show the dependency.

On the other hand, I would guess that the majority of vacuum tube / valve SPICE models do not. ?Those models are behavioral macro-models, built-up out of things like ideal VCVS and VCCS sources, and might not account for major dependencies on e.g. the supply voltage.

Regards,
Andy
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