[sdiy] exponential vs linear vco cores

[Antti Huovilainen]

On Fri, 26 Feb 2010, cheater cheater wrote:

> thanks for the round up, that was a good bit of info as usual from you.
> How are the different V/I built?

Linear V/I converter uses opamp, transistor and resistor combination to 
force a current proportional to input voltage to a load connected to the 
transistor collector (the transistor isolates the load while the opamp 
makes the conversion more or less ideal by forcing a current I = V/R 
through the resistor and transistor emitter).

Exponential converter typically uses the I = Is*exp(Vbe/2Vt) relationship 
of a bipolar transistor. Usually a second transistor and opamp are added 
to stabilize the operating point. Vt and Is depend quite a lot on 
temperature so you still need additional compensation to get rid of 
temperature dependency. There are also some other approaches (such as the 
SSM2164) that use matched exponential amplifiers with one providing the 
output and other placed in opamp feedback loop to compensate for the 
tempco.

Antti

"No boom today. Boom tomorrow. There's always a boom tomorrow"
   -- Lt. Cmdr. Ivanova