Importance of matching in expo convertors?!

[Haible Juergen]

	>>>For a really low parts count expo converter, check John Simontons
	>>>design for the Q control on the new state variable filter module.
	>>
	>>>http://www.paia.com/syntfsch.gif
	>>
	>>>If you stare at it long enough the temperature cancellation of
the
	>>>second transistor starts to make sense. This circuit works too.
	>>
	>>What's the drawback with this circuit? There must be one,
otherwise they
	>would have used it for the frequency CV too.
	>>
	>>/Jorgen
	>
	>The reference current is not constant! It depends a little on Vbe
of the
	>left transistor. Apart from that its the "normal" PNP expo
convertor. 
	>
	>Bye
	> René

Yes, no constant ref current. It's basically a differential pair working
only on one
branch of its transfer function, thus approximating the exp function. The
lower the
current, the better the exp approximation.
Advantage: only 2 transistors, and (unlike the ARP complementary pair
circuit)
you can get positive CV input for a current source, too, by choosing the
"right" base as input.
Disadvantage: You waste a lot of current. (Tail current much greater than
output current for decent expo approximation)
Cheap alternative (if you don't want to spend an opamp):
3-transistor-circuit
from EMS (see VCS-3 VCF), or 4-transistor circuit (actually 3 transistors
plus
zener diode): see VCS-3 VCOs.

All of these circuits just 1st order temperature compensation - tempco
resistor
required for precision applications.

The simple 2-transistor circuit is still my favorite one for non-critical
applications
(like replacing "log" or "reverse log" potentiometers with VCA circuits).

JH.