What is this diode doing?

[cuelist]

I can't figure out what the diode in this circuit is doing.

As far as I can see it's a typical Sallen & Key 2:nd order highpass filter. Since opamp is acting as follower ("-" tied to output) there should be no difference between "+" input and output.

Anyone?

[/b]

 

[clintrubber]

It'll be in the realm of 'does nothing unless something could get broken under abuse and then it comes into action'.

It could be thought of as preventing that + & - inputs get too much away from each other.
As for instance those two anti-// diodes internally in a '5532, see for instance this file, the pic on page three:
http://www.semiconductors.philips.com/acrobat_download/datasheets/NE_SE5532_A_SA5532_3.pdf

FWIW, that 33178 has proitection diodes as well, but to pos-supply:
http://www.onsemi.com/pub/Collateral/MC33178-D.PDF (see page 1)

 

[TedF]

It's got to be just a mistake..... maybe a 'cut and paste' error; a bit of a precision rectifier got left in.

 

[AMZ-FX]

It could be a bit of obfuscation :grin: "oops, we accidentally made a mistake in the schematic"

-Jack

 

[bcarso]

[quote author="AMZ-FX"]It could be a bit of obfuscation :grin: "oops, we accidentally made a mistake in the schematic"

-Jack[/quote]

haha Jack.

This reminds me of an old wheeze attributed to Frank Kelly, when he was with UREI before the Harman acquisition:

"Really?? Wow---that's the third time today I've never heard of that before!"

 

[aurt]

Offhand, I'd say the backstroke. :wink:

 

[Svart]

some kind of strange feedback clipper?

 

[clintrubber]

[quote author="AMZ-FX"]It could be a bit of obfuscation :grin: "oops, we accidentally made a mistake in the schematic"

-Jack[/quote]
There's a certain circuit that comes to mind now... :wink:

http://home.hetnet.nl/~chickennerdpig/FILES/Pultec/driver_02_60.jpg
(redrawn schematic of the near class A driver of the largest M$n%ey EQ-box)


Also it reminded me of the geofex-article about funny confusing parts (actually real parts on the PCB) to disamuse tracers. Couldn't find it back right now, but it was about on purpose wrongly labelled parts and nothing-doing components between irrelevant circuit-nodes.

Hey, a colleague added a few totally irrelevant transistors to his IC-layout once - exactly for this purpose - and indeed were they faithfully copied in a 'version' by another manufacturer. It had a somewhat different layout but had cloned the circuit topology exactly :twisted:

 

[Crusty2]

There was a story in another forum about a medical equipment company who had a continuing problem with a cloning competitor. So finally, on a new design, they decided to have a custom chip manufactured to thwart further cloning activity.

The topper was that they had the custom chip labeled with a standard part number from a common TTL gate. Apparently, the cloner was quite confused when the pc boards they invested so much time and money into were completely non-functional. Ha!

 

[clintrubber]

[quote author="Crusty2"]
The topper was that they had the custom chip labeled with a standard part number from a common TTL gate. Apparently, the cloner was quite confused when the pc boards they invested so much time and money into were completely non-functional. Ha![/quote]
Well done ! :thumb:

 

[Matt Syson]

Hi
I suppose the diode could either reduce differential voltage spikes on the input if the output were shorted or possibly to help recover from gross overload in the event of latchup of the opamp. Other parts of the design may give a clue.
Matt S