[sdiy] Buchla 259 integrator op amp question

[Jerry Gray-Eskue]

The LM311 has a ground pin (1) that supplies the output reference. In the
schematic pin 1 is at 0v.

Check out the 10.0 Schematic Diagram in the data sheet to see how pin 1 is
set up.

http://www.national.com/ds/LM/LM111.pdf

- Jerry

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of Aaron Lanterman
Sent: Monday, February 15, 2010 3:04 AM
To: synth diy
Subject: [sdiy] Buchla 259 integrator op amp question


Greetings,

I've puzzled over this for years:

http://rubidium.dyndns.org/~magnus/synths/companies/buchla/Buchla_2590_2_200
.jpg

IC20 and C20 (0.0047 poly) form the integrator for the triangle core.

My analysis says that the output of IC22, the comparitor, is a square wave
alternating between ground and 4.39 volts (I don't get the 4.29 marked in
the diagram but I doubt it's that precise anyway). These voltages correspond
to voltages at the output of IC20 of -2.608 volts and 2.5453 volts, which is
a nice 5.15 volt peak to peak triangle wave.

I did all this assuming the + terminal of IC20 was ground.

But - there's a 7.5 volts put on the + terminal of IC20. By golden op amp
rules, this puts a 7.5 volts on the - terminal of IC20. What is that 7.5
volts doing there? Should it change my analysis any? Does the fact that
there's just a cap in the feedback loop mean that the current input to the
integrator can happily sit a 7.5 volts while the output wiggles around 0
volts?

I think I may have asked this before but I've forgotten what the answer is.

It's relevant since I'm lecturing on it today, and I usually handwave when I
get to that part. ;)

- Aaron
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