[sdiy] MOSFET instead JFET?

[René Schmitz]

At 13:49 13.02.02 +0100, >>>marjan<<< wrote:
>
>
>I was looking at mosfet datasheet (BS170), it's Ron=5 ohm, nice it is,
>has anyone tried it instead jfet in VCO (for cap discharging), maybe
>just
>few changes to original ccts?
>Is there something crucial that prevents us from using mosfets in this
>way 
>that I don't see?
>

Hi Marjan,

I have experimented with that, using BUZ71 and BS108 as discharge switches. 
(See VCO 1&2 at my site.)
Works sort of ok in schemes which have the cap to +15V, I haven't been
successful 
in integrator based circuits.

The JFET or diode discharge schemes work better IMO, because there is no
point 
makeing the cap discharge faster than the opamps slewrate.
Any input signal faster than the slewrate leads to spikes and rippleing
oscillations.
So you should match the opamp speed to the discharge speed. 

Btw, Ron is the limiting figure only for the last couple of volts (where
the FET is 
in its linear region), but Idss determines how fast the cap can be
discharged when at 
the begin of the discharge cycle. If one looks at the discharge on a scope
it starts 
linear and ends in an exponential. 

Cheers,
 René


-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159