# [sdiy] Help understanding Serge 73 Oscillator

Dmitri SFC dankedout415 at gmail.com
Wed Feb 21 04:28:25 CET 2018

```Funny I think Serge possibly did patent that circuit as part of the Sonica.
http://www.keytarcentral.com/tag/sonica/

On Tue, Feb 20, 2018 at 2:30 PM, Ben Bradley <ben.pi.bradley at gmail.com>
wrote:

> I haven't seen this or any circuit using an LM3900 as an exponential
> converter before, but I recall using the LM3900 in circuits based on
> the datasheet circa 1980. But understanding the chip's operation (and
> the exponential nature of Ic vs. Vbe) I figured it out.
>
> The LM3900 inputs "respond" to current, rather than voltage as in
> regular op-amps. The output, though, is still a voltage. The input
> voltages pretty much stay at a diode drop above the negative or
> "ground" power input to the LM3900.
>
> If the + input has more current going into it than the - input does,
> the output goes toward the positive rail, otherwise it goes toward the
> negative rail. If it has a resistor in the feedback path as R17 in
> U1A, the output voltage will change to make the current into the -
> input the same as the current into the + input.
>
> The + input goes to the first two transistors that make up a current
> mirror, to change the direction of the current. This goes directly to
> the - input, so the inverted current from the + input is subracted
> from the - input. The third transistor and the gain stage handle the
> amplification and voltage output.
>
> U1a is wired as a voltage output at the connection of R17 and R19. It
> appears R19 doesn't "do" much here except limit the maximum current
> from U1a so it doesn't destroy the input of U1b.
>
> So the input of U1b is a voltage, in the range of about 0.4V to 0.7V
> or so. It's the SECOND transistor that does the exponential conversion
> (the first one just pulls current from the input voltage). It has a
> VOLTAGE input across the base and emitter, and this translates
> directly through the transistor's exponential function into a CURRENT
> output through the collector.  This current of course is matched by
> the - input from C3, which generates a ramp at the output and is reset
> by Q3 at a rate based on collector current.
>
> This is such a neat circuit, I have to ask - is it patented?
>
> On Tue, Feb 20, 2018 at 4:27 PM, 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.
> > _______________________________________________
> > Synth-diy mailing list
> > Synth-diy at synth-diy.org
> > http://synth-diy.org/mailman/listinfo/synth-diy
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy
>

--