[sdiy] Help understanding Serge 73 Oscillator

houshu at muj.biglobe.ne.jp houshu at muj.biglobe.ne.jp
Wed Feb 21 15:47:03 CET 2018

Hello Guy,

Japanese explanation is available at:

I don't want to translate the whole article.  Sorry.

Input current mirror of U1b has exponential response.
Negative input terminal of U1a drifts with temperature, 
which compensates for the drift of the exponential response.

Saitama, JAPAN.

> 2018/02/21 6:27 Guy McCusker <guy.mccusker at gmail.com> wrote:
> Hello list!
> I'm having a hard time figuring out how the exponential response is
> generated in the Serge 73 oscillator design. The schematic is here:
> http://electro-music.com/wiki/pmwiki.php?n=Schematics.OldSergeVCOUsingLM3900
> Like a lot of Serge designs there is heavy use of LM3900s which I do
> not understand properly. It seems to me that the first stage (U1a) is
> a more or less regular summing stage, the second (U1b) is an
> integrator, and the third (U1c) is a comparator to reset the ramp via
> the transistor across the integrating cap. So far so straightforward.
> But how (if at all) is an exponential CV response generated? The notes
> on the schematic hint that somehow the transistors within the LM3900s
> are being pressed into service as an expo converter but I can't wrap
> my tiny brain around it.
> Can anyone help please?
> Cheers,
> Guy.
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