[sdiy] weird PNP/NPN expo questions

[Ian Fritz]

At 07:59 AM 7/18/2009, Dan Snazelle wrote:

>I am working on a project that right now is using a expo made out of a PNP 
>and an NPN
>(the NPN collector is the output into the VCO)
>
>basically the opamp summer (with tempco, pitch control and expo FM)
>  goes into the base of the the PNP. the PNP emitter goes through a 100k 
> into +V and also into the BASE
>of the NPN.
>
>there is also a 220 ohm resistor going grom the collector of the PNP into 
>the -v rail.
>
>the NPN emitter goes to ground and the collector connects to the VCO/lfo
>
>so....my question is this
>
>1. is this setup a curent SINK or a current SOURCE?

The PNP is an emitter follower whose purpose is temperature compensation of 
the NPN exponentiating transistor.  The NPN is a current sink, since 
current must flow into its collector.

This is an obsolete method.  Why not just use a standard design?

Ian