[sdiy] Buchla 259 integrator op amp question

[Donald Tillman]

On Feb 15, 2010, at 1:04 AM, Aaron Lanterman wrote:

> 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?
>

Hey Aaron,

No change to the analysis; it's fine.  Indeed, both of the input pins  
of IC 20 will be 7.5 volts.  That's a common-mode voltage and doesn't  
affect the output of IC20.

The negative input of IC20 is being fed by the current supplied by Q6/ 
Q8, and those guys need to run in to a positive voltage.  They  
wouldn't work at all if the collectors of Q6/Q8 were at zero, as Q5  
and Q7 would be biased inappropriately.

   -- Don


--
Don Tillman
Palo Alto, California
don at till.com
http://www.till.com