[sdiy] the obscure output of a 2164

David G Dixon dixon at mail.ubc.ca
Thu Mar 9 07:53:23 CET 2017


Hi Busby,
 
Here's my advice:
 
1) Learn how to read the datasheet.  It's chock full of useful info.
 
2) The 2164 is a current amplifier.  It takes a current input (the input pin
is a virtual ground -- if you don't know what that is, then you have no
business doing electronics, so quit now) and amplifies it to a current
output.  The gain factor is a function of the voltage at the VC pin.  0V is
unity gain (current out equals current in).  The current approximately
doubles for each 200mV decrease at the VC pin (negative control voltages
give high gains, positive voltages give low gains).
 
3) The rating of the 2164 is about -100dB full scale.  This is five decades
(20dB/decade -- again, if you are confused about dB and decades, it might be
best if you took up another hobby).  The datasheet says that the gain factor
is -33mV/dB, which is -200mV/6dB which is -200mV/octave (an octave means a
doubling of the current).  That means that the minimum gain of the device is
reached when the VC voltage is about 3300mV, or 3.3V.  Hence, a control
voltage of 3.3V more or less completely turns off the VCA.  The maximum
useful gain of the 2164 (where it just starts to distort) is about +20dB
(about one decade of gain), which is reached at a VC voltage of about
-660mV.  In general, I try to avoid using the 2164 at gains higher than
unity.  Hence, the useful VC control voltage range of 2164 is roughly -3.3V
to +0.7V.
 
4) The 2164 VCA must terminate at virtual ground.  That is why all the
circuits you see have the 2164 feeding into the negative input of an opamp
(and that opamp better have some negative feedback).  If a 2164 is not fed
to virtual ground, it goes absolutely apeshit (I know this from bitter
experience).  This is the single most inconvenient aspect of using 2164s.
The only other thing they can be fed into is another 2164 VCA (because the
input is virtual ground).  This quality of being able to stack 2164 VCAs in
series can come in quite handy.  However, be careful about using 2164s in
parallel, as they will not retain independent control (I learned this the
hard way as well).
 
5) The maximum current you ever want to feed into a 2164 VCA is about 330uA.
This is why the input resistor is always 30k, because this will give 330uA
at an input voltage of 10V.  It can go higher, but it will distort.  Bottom
line, just always use 30k resistors (or higher) into the 2164 VCA.
 
6) Also note that the VC pin has an input impedance of 5k.  This can
complicate designs if you are not aware of it.  If you look at the schematic
in the datasheet, it shows an internal 5k resistor to ground at the VC pin.
Take this into account if you are dividing voltages into this pin.
 
7) Make absolutely sure that you have all the power and ground pins
connected to their respective power rails and ground before powering up the
2164, as failure to do so will fry the chip, guaranteed, particularly
failure to connect the negative rail.
 
I can't really think of anything else about 2164 at the moment.  It's a
bloody useful chip, and has quite literally put my kids through college.
 
Cheers,
Dr. Sketch-n-Etch


  _____  

From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Busby
Bergson
Sent: Wednesday, March 08, 2017 10:20 PM
To: synth-diy at synth-diy.org
Subject: [sdiy] the obscure output of a 2164


hi list- 

my eyes glaze over a bit with datasheets - and experts have better advice,
anyway :
i'm seeing a lot of designs with the output of a 2164 directly patched to an
op-amp....an inverting amplifier arrangement, with a feedback loop, usually
somewhere between 30k and 300k..... i have no idea how to calculate the gain
of this, though, because i'm unclear on exactly what's coming out of the
2164.. any pointers?

-bb

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