home-made vca

[gstopp at fibermux.com]

     You know I just proto'ed the Analog Devices circuit up and it works 
     fine! Tony - check your wires! Sorry to break it to you like this... 
     don't feel too bad, I hook stuff up wrong all the time.:)
     
     Anyway I used a TL082 for the two input op-amps, half of another for 
     the output diff amp, two MAT-01's for the matched pairs, and a couple 
     2N2222A's where they're supposed to be. You may notice that there are 
     a bunch of 30K resistors in the app note circuit, which I didn't have 
     so I used 33K's. I left out all the caps except for the input cap.
     
     Anyway frequency response with a 47uF input cap is from below 1 Hz to 
     above 300 KHz (remember, I left the caps out). No phase shift, no 
     distortion, sine, square, sawtooth, or triangle waves. Inputs over 
     around 6V p-p will start to distort.
     
     Max gain is around 2. CV range is from between -4 and -3 volts 
     (minimum gain) to just above ground (maximum gain). I shifted this 
     range up to be between ground and +5 by adding a 27K resistor from V- 
     to the 4.7K/330 ohm junction on the control voltage input. However to 
     get a 100K input impedance here you'd need to add a buffer (hey 
     there's an extra one in the output TL082...).
     
     Next thing I'll try is using some garden variety NPN's instead of the 
     matched pairs, and trying to match them to get the same performance.
     
     Although this VCA is not perfect for all purposes (it is not DC 
     coupled for one thing), it may be real useful for audio only. I'll 
     report on further findings.
     
     - Gene
     gstopp at fibermux.com
     


______________________________ Reply Separator _________________________________
Subject: Re: Re[2]: home-made vca
Author:  Tony Clark <clark at andrews.edu> at ccrelayout
Date:    2/24/97 11:06 AM
     
   Okay, I discovered what my main problem was, I forgot the input 
decoupling capacitor!  Doh!  Now it seems to work fine...sort of.
   Gain is about 1.5 now at max CV (which is roughly 1V).  Phase shift is 
pretty decent at 10kHz and above, but is pretty bad on the low end.  
Frequency response is horrible.
   Now a couple of interesting things, my circuit won't pass a square 
wave to save its life.  It works very well for sine and triangle waves 
(there's slight rounding on the peaks), but a square wave produces a 
great surfable curve on the output peaks.  No doubt I must still be 
missing something from the MAT-04 papers.
   Anyway, this is a very noise free circuit.  I'll work on the other 
problem and report when I have some better results.