[sdiy] LM13700 single supply?

[Tim Stinchcombe]

> I am experimenting with the exponential VCO from Walter 
> Jung's IC Timer Cookbook 2nd ed., p. 239.  I want to use an 
> LM13700 in place of the CA3080, but he has this circuit set 
> up for single-supply. Can the LM13700 be used this way? 

Having been staring at the book for half an hour, my initial thought would
be no, it won't work with an LM13700. I don't fully understand the circuit
operation, but op amp A4 is actually supplying a *voltage* to pin 5 of the
3080, not a current as is usual - this is backed up by the sentence "Since
Q1 and the internal transistor at pin 5 of the 3080A have closely matched
Vbe/Ic characteristics...", so he seems to be relying on the interaction of
these two transistors to give the exponential response (and I'm also
guessing that in saying that transistors in the 3046 and 3080 are 'matched'
that he is implying that the chips are fabricated using the same
process...). The arrangement of transistors for the equivalent pins on an
LM13700 are quite different, so subbing that chip I think will mess up the
whole thing (generally the 3080 voltage at the Iabc pin is a diode drop
above ground; for the LM13700 it is *two* diode drops).

A quick simulation will probably clinch it one way or the other, but before
going to that extent I shall wait for input from other more experienced
members who maybe *can* fathom the circuit quicker than me, and if they
disagree I might then run a sim as a double-check!!

Tim
__________________________________________________________
Tim Stinchcombe 

Cheltenham, Glos, UK
email: tim102 at tstinchcombe.freeserve.co.uk