[sdiy] MOSFET leakage.

[ASSI]

On Sonntag, 18. Dezember 2005 12:18, René Schmitz wrote:
> I have experimented with BS108s as discharge switches in my VCO3, but
> didn't like the effects of the channel capacitance.

If you limit the overdrive over Vth on the gate this effect gets less 
severe. If you can spare another (loosely matched) MOSFET you can use 
the same arrangement as a simple MOSFET current mirror for the gate 
biasing.

> With a larger cap the gate to channel capacitance would have less
> impact. Also don't forget that one could neutralize the capacitance,
> by putting a second one in parallel, which gets a reset signal of
> opposite direction.

If it's really the gate to channel capacitance and/or (more likely) the 
Miller capacitance, try a cascode switch. This works even better 
together with the trick from above... If that doesn't do it, a 
differential integrator is called for, where the kickback charge can be 
made common mode.


Achim.
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