[sdiy] Gilbert Cells and VCAs with Predistortion

[Ryan Williams]

hi all,

My internet was down for a few days and I had nothing else to do except 
study some circuits. I was looking at the data sheet for the HFA3101 
(Gilbert Cell) and my analysis left me wondering what is the best way to 
predistort the input so that the temperature and tanh effects cancel.  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.

next I applied this predistortion circuit to a simple VCA that needs 
only 4 matched and thermally connected NPN transistors. the circuit is 
here (untested and maybe a bit difficult to read):
http://www.sdiy.org/destrukto/temp/4npn_circuit.png
http://www.sdiy.org/destrukto/temp/transient.png
This circuit should handle about 20Vp-p input signals but the voltage to 
diff. current converter (adapted from MAT-04 data sheet) does not work 
at DC.

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

that last image shows both the tanh and the Vt temperature variable 
being completely removed.  I assume that I have a flaw somewhere because 
I've never seen anyone use the LM13700 with a 'real' current input like 
this. It seems to me that this method is much better than using 
resistors on the inputs with voltages connected.

So my questions. Have I made a mistake, is this not as good as I thing 
it is? also, I want to build these circuits, especially the 4npn VCA 
because I expect it could turn out to be a good replacement for VCA 
circuits when all the OTAs are gone.  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.

-ryan williams

btw, the hfa3101 looks like it would make a deccent 4-quadrant 
multiplier but the price with the rest of the needed circuitry looks 
like it would be about the same as or more than an AD633.