Re: some questions about high precesion constant current source
- From: bill.sloman@xxxxxxxx
- Date: 29 Aug 2005 06:41:58 -0700
Have you got a a 10R resistor in series with the gate of the MOSFET,
mounted fairly close to it?
MOSFETs have a nasty habit of oscillating at a few hundred MHz - fast
enough that a cheap oscilliscope won't see it - and this can invalidate
a DC analysis of the circuit. In particular, the inputs of the op amp
will tend to follow the peak of the AC envelope of the signal they are
looking at at, rather than the average, and this can look like an
out-of-spec input offset voltage.
A "gate stopper" somewhere between ten and a few hundred ohms usually
kills this sort of oscillation. At Cambridge Instruments, we had enough
trouble with oscillating MOSFETs that the 10R gate resistor was
compulsory.
It wasn't always necessary, and sometimes 10R wasn't enough, but at
least we always had space on the printed circuit board to fit a gate
stopper resistor if we needed one.
-------------
Bill Sloman, Nijmegen
.
- References:
- some questions about high precesion constant current source
- From: eehinjor
- some questions about high precesion constant current source
- Prev by Date: Re: OT: Living in AZ
- Next by Date: Re: The 1952 Radiotron Designers Handbook
- Previous by thread: some questions about high precesion constant current source
- Next by thread: Re: some questions about high precesion constant current source
- Index(es):
Relevant Pages
|
