[sdiy] triangle core VCO comparators

[Olivier Gillet]

> How common is this *really* in the ordinary kind of op amp today?  The
> OP27 seems to have it.  Any others?

A few TI parts I've used, and which have them: OPA165x, OPA166x,
OPAx171, LME497x0, OPA277
A few parts which do not have them: TL07x, LM324 / LM358, OPAx134, OPAx192

I've found about it because there's a straightforward situation in
which they cause a problem: non inverting amp or buffer, with an
AC-coupled input (capacitor between signal input and V+ ; large
resistor between V+ and ground - typical guitar preamp). If the input
signal slews too fast for Vo and V- to follow, the voltage difference
between both inputs is large enough to cause one of the diodes to
conduct, and the resulting current charges the AC-coupling capacitor!
This causes an unwanted DC offset (which the capacitor at the input
was supposed to eliminate).

This issue drove me crazy - I've tried several op-amps I had and did
not understand why the circuit worked with some and not others - and I
could not locate the parameter that played a role since this is rarely
mentioned in the datasheets.

http://e2e.ti.com/cfs-file/__key/communityserver-discussions-components-files/14/input-diodes-for-ac-coupled-circuiits.pdf

On Sat, Feb 27, 2016 at 11:09 AM,  <mskala at ansuz.sooke.bc.ca> wrote:
> On Sat, 27 Feb 2016, rsdio at audiobanshee.com wrote:
>> I wouldn't call these common or ordinary. You'd think that low noise
>> would mean they'd have to remove the protection diodes, but I'm no
>> op-amp design expert. As I mentioned above, we ended up redesigning the
>
> I wonder if the changes in geometry needed to create low-noise input
> transistors tend to oppose their handling high voltages, so that the
> protection diodes become necessary.
> --
> Matthew Skala
> mskala at ansuz.sooke.bc.ca                 People before principles.
> http://ansuz.sooke.bc.ca/
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