# [sdiy] 1-quadrant multiplier with 2164

David G Dixon dixon at mail.ubc.ca
Tue Jan 18 02:48:24 CET 2022

```Hi Rutger,

It is really easy to do what you want to do.

If I understand you correctly, for an input signal of 10Vpp centred on 0V,
you basically want the output signal to grow from a baseline of -5V, so that
when the VCA is off, the output is -5V DC, at 50% gain, the output is 5Vpp
from -5V to 0V, and at 100% the output is 10Vpp from -5V to +5V.

Let's assume that you have built the standard linearized VCA circuit and
that you have good 5V and -5V reference voltages available.  Presuming that
your input resistor into the amplifying 2164 VCA is 30k, and the feedback
resistor on the I-V converting opamp is also 30k, then all you have to do is
this:  Sum -5V into the 2164 VCA, in parallel with the input signal, through
a 30k resistor, and sum +5V into the I-V converter also through a 30k
resistor.  The +5V into the I-V converter will apply a constant bias of -5V
to the output signal.  The -5V into the VCA will counteract this bias in
proportion to the VCA's gain, such that, then the VCA is off, the bias is
-5V, and when the VCA is at unity gain, the bias is 0V.

Easy peasy, lemon squeezy.  Schematic attached.

Cheers,
Dave Dixon

_____

From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Rutger
Vlek via Synth-diy
Sent: Monday, January 17, 2022 1:04 PM
To: SDIY List
Subject: [sdiy] 1-quadrant multiplier with 2164

[CAUTION: Non-UBC Email]
Dear all,

I think i need a bit of wisdom from the 2164 guru's around here. The case is
as follows:
I'm in need of a 1-quadrant multiplier (VCA) that works on a bipolar signal.
The input would be a 10Vpp triangle wave centered around 0V. The output
should be the same with the VCA fully open, but... when closing I would like
to reduce the level with respect to the -5V power bound, so that a VCA
half-open would result in a triangle of half the amplitude, sitting between
-5V and 0V and a fully closed VCA would give -5V DC.

Of course I could level-shift both input and output of a 2164 bases VCA, but
I feel it could be simpler. Would it work to connect a -5V signal to the
ground pin of a 2164? And would that also require a shifted control voltage?

Regards,

Rutger
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