DIY parts

[Ian Fritz]

Terry --

Thanks for the information on FET switching. I was wondering about
MOSFETs because several designs on the Web use them. I did note the
larger input capacitance and I also noted leakage-current numbers that
looked like they might be problematic on the low-frequency end. At the
risk of advertizing my incredible ignorance of switching transistors,
why is it that the gate capacitance discharges into the integrator
rather than just into the gate driving circuit? Is it because there's
capacitance from the gate to the drain as well as to the source?

I haven't been able to find a spec sheet on the KE4859. How does it
compare to the 2N4392, which seems fairly easy to find these days?

Best regards,

  Ian


terry michaels wrote:
> 
> I've got a few more thoughts on the KE4859.  Your suggestion of using a
> MOSFET is interesting.  Actually, I hadn't even thought of it.  You're
> right in that it typically has a low on resistance.  However, most MOSFET's
> have a much greater gate capacitance.  When pulsed to discharge the timing
> cap, this will dump a significant amount of charge into the integrator.
> The KE4859 has around 10 pf, which dumps a noticable amount of charge.
> That's one reason why the reset time on this VCO is longer than it should
> be, and there are brief glitches on the ends of the sawtooth.
> 
> When I redo this circuit, I'll look at adding a small cap to dump an
> opposite amount of charge to counter the gate capacitance.  I think it can
> be done very simply,  I'll keep you posted.
>