Re: gEDA-user: opamp slew rate limiting

[John Doty <jpd@xxxxxxxxx>]

On Nov 10, 2009, at 10:58 AM, carzrgr8@xxxxxxxxxxxxx wrote:

> This particular opamp is opa2132 from TI, and has slew rate of  
> about 20 V/us.

FET input. Makes a difference.

An opamp hits its slew rate when its differential *input* stage is  
driven to its limit. For a bipolar input without emitter resistors,  
that'll happen at a few times kT/q, say roughly 100 mV differential.

But for a FET input, it takes a considerably larger differential  
input voltage to to drive the input stage to its limit. This is  
independent of *output* saturation.

For a unity-gain stable opamp, there's a relationship between the  
maximum slew rate and the differential input voltage needed to make  
it happen. For a given gain-bandwidth product, the higher the slew  
rate, the larger the voltage (Horowitz and Hill explain this well).  
That's why FET inputs tend to increase slew rate, and why some  
bipolar opamps use emitter resistors in the input stage to accomplish  
the same effect. So, if your signals are small, a "high slew rate"  
opamp may not be what you want.

But as others have noted, it seems a comparator is what you want.  
Comparators are not constrained by closed-loop stability  
requirements, so they escape the limitations of opamps here.

John Doty              Noqsi Aerospace, Ltd.
http://www.noqsi.com/
jpd@xxxxxxxxx




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