AW: Re[2]: home-made vca

[gstopp at fibermux.com]

     You need to see the schematic! The input goes to a pair of OP-41 
     op-amps, which have ordinary 2N2222 transistors in their feedback 
     loops, which differentially drive the tied emitters of two 
     differential pairs (made from the four matched transistors in the 
     MAT-04). The output is taken differentially from one leg each of the 
     differential pairs, by an OP-27 op-amp.
     
     It's called the "Voltage Controlled Attenuator" and it's App Note 
     AN-105. The control voltage range is unity a zero volts, with gain 
     dropping to -50dB by CV = -3 volts.
     
     - Gene


______________________________ Reply Separator _________________________________
Subject: AW: Re[2]: home-made vca
Author:  Haible Juergen <Juergen.Haible at nbgm.siemens.de> at ccrelayout
Date:    2/21/97 2:03 AM


>      The Analog Devices app note for the MAT-04 quad matched NPN hints that 
>
>      the biggest effect of transistor mis-matching in VCA designs is 
>      distortion.
     
Hmm, but even with perfectly matched transistors, you got this principal 
nonlinearity - was it a tanh(x) or something like this ...
     
The specs for their example circuit are as follows: 
>
>      Frequency response - 20Hz-20KHz flat 
>      Noise level - 110dB below full output 
>      Distortion - under 0.03%
     
Does this apply to a simple differential pair input structure, or are 
you talking about a linearized circuit?
     
JH.