[sdiy] expo circuit

[Dave Manley]

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