> An op-amp is only an op-amp if it has predominantly negative =A0feedback
> that =A0gives =A0a closed loop =A0gain a tiny fraction of open loop gain.
> Otherwise it's just an IC "gain block".

> To me and Op Amp is NOT a TL 072 or 741. Those are high gain
> differential input amplifiers that let you build an =A0op-amp.

> But what ever you call it, Alice and HD ought to spot you have drawn a
> Schmitt trigger, not a Linear Amplifier of accurately defined gain (
> Operational Amplifier). I have an old valve based op-amp schematic some
> place...

For extra points ... :-).
Turn a Scmitt trigger into a linear amplifier, almost :-).

The use of CMOS "digital" inverters as linear amplifiers is well
known. eg place a 1M resistor from input to output with a 100k input
resistor from a lowish impedance source and you have a ~~~=3D 10x linear
amplifier, subject to some real world stuff.

BUT take an eg 74C14 / CD40106 / ... or similar Schmitt trigger
inverter and use the same circuit. At audio frequencies linear AC gain
appears to happen. Observe output with an oscilloscope and be suitable
impressed. The gate produces PWM at as high a frequency as it can
manage given stray capacitance and this is usually high enough a
frequency to allow "audio" to be produced as is. Add a sniff of RC
output filter to get rid of the switching noise. Add a few pF at the
input to bring the frequency down into the gate's intended switching
range. Quite a fun result, although not something you'd usually expect
to do with a Schmitt inverter.



           Russell

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