As an exercise Olin kindly gave me a circuit to try to analyze.  Here is what I can find in sub circuit R2, D3, C2, Q1.  But first I'm to focus on R2 and D3.

http://www.embedinc.com/products/eusb2/eusb2.pdf

D3 is a zener diode with a reverse breakdown voltage of 5.6 volts, sort of like a voltage sensitive switch.  

So R2 and D3 in this configuration functions as a stable voltage regulator to the base of Q1.

Am I close?

Lindy

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