[sdiy] Help understanding Serge 73 Oscillator

Ben Bradley ben.pi.bradley at gmail.com
Tue Feb 20 23:30:27 CET 2018

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.
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