[sdiy] Gilbert Cells and VCAs with Predistortion

[Don Tillman]

   > Date: Sat, 02 Jul 2005 16:20:58 -0500
   > From: Ryan Williams <destrukto at cox.net>
   > 
   > I saw a circuit and description of the multiplier in the book
   > 'Analysis and Design of Analog Integrated Circuits' that used two
   > diode connected transistors and a differential current source for
   > the predistortion.
   > 
   > http://www.sdiy.org/destrukto/temp/4npn_circuit.png
   >
   > next I noticed that the diodes on the LM13700 are the same as the diodes 
   > in my circuit and that this would be an easy way to get the same results 
   > without an npn array. so the lm13700 and important points of the math 
   > are shown here:
   > http://www.sdiy.org/destrukto/temp/lm13700_circuit.png

Hi Ryan,

This is actually an old technique; check out the data sheet for the
Motorola MC1494 Four Quadrant Multiplier.  

Motorola implemented it with far fewer parts by using a diff amp with
an emitter resistor for the balanced current sources.

Without having done a proper analysis, I believe that this circuit is
effectively the same as the LM13700 diodes.  ("Huh?  The LM13700
diodes are fed from a current source while the MC1494 diodes are fed
from a voltage source.  How can they be the same?"  It's no problem if
the common mode signal bounces up and down a little bit; it's feeding
a diff amp anyway.)

This is very different from the CA3280 approach, which uses the input
diodes as a dynamic resistor.

   > So my questions. Have I made a mistake, is this not as good as I
   > thing it is?

I think it will be roughly the performance of the CA3280 linearizer.

   > How could I go about measuring the performance of these circuits
   > (other than looking at the scope and saying 'that looks good'). I
   > want to actually have some numbers that verify my thoughts.

Drive the VCA circuit with a triangle wave, build an additional
circuit to take the difference between the VCA output and the triangle
wave VCA input, hand adjust the levels to cancel, and display the
difference voltage vs. the triangle on a 'scope in XY mode.

  -- Don

-- 
Don Tillman
Palo Alto, California
don at till.com
http://www.till.com