Replacement JFET for ASM-1 VCO

Rene Schmitz uzs159 at uni-bonn.de
Wed Nov 3 17:28:07 CET 1999


At 13:07 03.11.99 +0100, Martin Czech wrote:
>:::	>And the bipolar works better than all of them... Why do people like
>:::FETs so
>:::	>much...
>:::
>:::Leakage currents ?

That, and absence of saturation voltage. Discharges to 0V.
And one doesn't get past the limitation of the opamps (follower/integrator)
slewrate anyway, so to avoid overshoot and high frequency ringing one
should not make the discharge faster than dictated by the opamps slewrate. 

>Leakage is not dependend on device type (bipolar, jfet), but on individual
parameters. The jfet channel leakage can be considerable. At first sight the 
>gate leakage of jfets looks good, but wait til Ta=50C. See LM11 data sheet.
>

Erm, my FETs stay cold! But I'll remember that when I want to play in the
desert. :-)
At that temperature the jFET opamps will leak too. Their Tj will be higher
than Ta due to self heating.


Bye 
 René




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