[sdiy] Any LM331 experts around?

[Ian Fritz]

Mystery revealed.

I built the high-speed version *without* the  2N2222 circuit.  It had a 
pretty severe T drift, about -400 ppm/K.  More than 10x the 
spec.  Apparently the spec is only for the 10 kHz application.  The 2N2222 
circuit is for temperature compensation -- it puts extra current into the 
timing cap as T increases.  So, technically, I guess it's a modification 
for high freq operation, as they say, but really it's a tempco 
correction.  I got the final drift to under 10 ppm/K.

Tracking of the full VCO (including the expo converter) is within 0.05% 
over 11 octaves. The currents at the low end are very small, since the max 
current is 0.1 mA.   Probably the low end could be fixed with a small 
amount of offset trim.

This is easily my best VCO ever.  What a fabulous chip!

   Ian

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I'm looking at the LM331 data sheet and can't figure out what the 
transistor in Fig. 4 is for.  They just say that Fig. 3 has been modified 
for high freq operation.  Any expertise or ideas?