[sdiy] Weird op amp features & rules of thumb

[Scott Nordlund]

I have noticed this in other op amps as well, at least the 324 (not
that I'd ever use one in an audio application) and 4558, probably
others as well.  It's a consequence of the differential input topology,
and necessarily not specific to JFET input op amps.  Some kinds of
input stages are better suited to wide-ranging inputs, I believe some
will even work with voltages exceeding the supply rails by a diode drop
or two (was it folded cascode?  I don't remember...).

This usually isn't a problem for inverting amplifiers, integrators,
etc, as you noted- when they're overdriven, it's something further down
the chain.  Voltage followers are susceptible, though. 


> From: lanterma at ece.gatech.edu
> To: synth-diy at dropmix.xs4all.nl
> Date: Sat, 29 Nov 2008 03:04:56 -0500
> Subject: [sdiy] Weird op amp features & rules of thumb
> 

> * If the voltage difference on the input pins exceeds a magic number,  
> the TL07x/TL08x do a really nasty phase inversion where the + acts  
> like - and the - acts like +. For typical negative feedback mode  
> circuits where the pins try to stay at the same voltage, this probably  
> isn't a problem, but if it's being used as a comparator (or in a  
> voltage-starved feedback mode like the Buchla timbre circuits, in  
> which the op amp can't keep the voltages the same), this could yield  
> all sorts of vicious nastyness. So blindly plugging in a TL07x/08x for  
> another op amp may be dangerous and should be carefully considered.

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