Help on op-amp (?) circuit

[rr]

OpAmps and comparators are really two different animals. They can
sometimes be interchanged, but not always. It pays to read the
spec sheets for nuances.

The lm324/lm358 opamp works OK as a comparator. But the bi-fet
devices, such as lf347/tl082 will phase reverse on the output if
one of the inputs gets too close to the rail. Other opamps have
a limited differential input range, and often require lots of
head-room for the p/s rails.

Comparators are typically faster than an opamp (relative cost),
and go from rail to rail on the inputs.

The only caution I have on the lm339 (quad), lm393 (dual) comparator,
is to diode clamp the inputs for protection. Go any lower than
300 mv below ground on the input, and poooofff, no input stage...

Easy for noise to do this.

BobR.


Simon Quested wrote:
> 
> Hi Bruce and All
> 
> > Use a comparotar, something like a 339, I think is the number.  Op-Amps
> > can get twitchy, for enables. Bruce
> 
> So what's the difference between the 2 devices, I have never understood this.
> You can use an op amp for a comparator but not visa versa ?
> 
> Cheers
> 
> Simon
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