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Subject: Re: op-amp input protection using FETs
Date: Fri, 18 Oct 2002 05:06:04 -0700
X-Mailer: Mozilla 4.78 [en] (Win98; U)
Adam Seychell wrote:
> Can a pair of BSS139 depletion mode MOSFETs connected as per the
> diagram below be used as a bilateral current limiter for protecting
> the non inverting input of a op-amp ?
Protected against what????
Which op amp...they're not all equal?
> Am I correct in saying that;
> * During normal operation the current is zero and therefore the total
> resistance should be Rds+Rds+200 Ohms. This relatively low series
> resistance will not compromise low noise performance.
You're assuming that no noise (or offset) is generated in the fets.
Whether this is
a problem depends on your definition of low noise.
If the input current is always zero, you can't do anything. If the input
current charging input C at least.
Look at the relationship between Idss and the voltage you have to develop
resistor to reduce that. It's likely to be at least a few mA.
Why not just use one low Idss fet?
Shoving reverse current into your unspecified amplifier may well do more
to it's noise figure than the circuit you added.
Depending again on lots of unspecified requirements including the input
of your op amp, you can do some interesting things with a pair of clamp
and a tiny light bulb.
> | |
> ------- |
> connector | | | S D
> from o---+ +-+-- 200R--+-+ +-----> to non-inv
> outside D S | | |
> world | -------
> | |
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