Compensated Exp generators and VCO's

Rick Jansen sscprick at
Thu Jun 8 09:46:16 CEST 1995

In message <199506072347.QAA12594 at> you write:
> Some random comments on the VCO...  (I've been away for a while.)
> You don't need the opamp after the LM394.  In fact it makes things
> worse in that you have the LM394 collector current being turned into a
> voltage with the opamp and being turned back into the same current by
> the integrator.  In the process low notes get affected by any small
> offset voltages.  I like the simpler ARP and MOOG approach where you
> charge the cap directly with the exponentional generator output
> current. 

You're absolutely right. It's even worse: the polarity of the voltage 
in that schematic is wrong, the output of the exp generator is positive
while the sawtooth generator expects a negative. The reason I did convert 
the current to a voltage was that the discharge-time correction was based 
on voltage. There's a new schematic up there where indeed the collector 
current is fed directly into the cap. 

As with the moog filter I'm studying some books and try to combine 
things into something workable. Comments like these are the spice 
in the dish :-)


Rick Jansen
rick at
S&H's a module and s&h's looking good

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