[sdiy] expo circuit

[Dave Manley]

Dave Manley wrote:
> I was looking at JH's String Filter schematics 
> (http://www.jhaible.heim.at/string_filter/jh_string_filter_sch.pdf)
> and the resonance expo circuit with the transistor (Q4) connected as a 
> reverse biased diode seemed similar to something I'd seen before 
> (lower right on the schematic).  A little digging shows it is very 
> similar to the synthi expo (as seen in the schematics on the Hinton 
> site: http://www.hinton-instruments.co.uk/ems/vcs3osc1.gif).
>
> So...what exactly is the function of Q1 and Q4? Does it give better 
> conformance?
>
> -Dave
>
To partially answer my own question - according to my copy of the IC 
Array Cookbook,
the reverse biased junction acts as a zener diode (~7V). Q1 together 
with Q4 and the pullup
resistor form a constant current source for the diff pair.

Is this a fairly straightforward and robust expo or are there subtleties 
to watch out for?

-Dave