SV: Re: [sdiy] simple s&h problem

[simon.oo.o at xs4all.nl]

Hi Karl,

Karl Ekdahl zei:
> i use the setup on rené's page,
> http://www.uni-bonn.de/~uzs159/sah.html (the one to
> the left). trying both the original setup and also
> removing r1 (that i don't really understand what its
> for) and the diode putting a resistor to -5v gate
> since my "sample" signal is 5v and my "not sample" is
> 0.

Note that:
1) Rene uses a 1nF capacitor, not a 1uF like you said. Are you sure about
the capacitor value you used?
2) Rene uses a bipolar FET, not a MOSFET. This avoids the gate to channel
charge transfer problem I mentioned, which becomes a serious issue with
small values of hold capacitor.

If you use the BS170 then you can remove R1 and the diode. Note that if
your sample signal is 5V then your input voltage should be, say, 1V at the
most, because the BS170 should have a gate-source voltage well above the
threshold voltage (max: 3V) to conduct. Also your input voltage should not
become more negative than -0.8V, otherwise the BS170 may conduct when the
control signal is 0V.

If you use a bipolar FET then R1 causes the gate source voltage to be 0
when the diode does not conduct. In that case, you need a fairly high
negative voltage (bigger than the pinchoff voltage of the FET) to make it
switch off.


Vriendelijke groet,

Simon Brouwer
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