Importance of matching in expo convertors?!

[Rene Schmitz]

At 09:44 01.09.99 +0200, you wrote:
>>For a really low parts count expo converter, check John Simontons
>>design for the Q control on the new state variable filter module.
>
>>http://www.paia.com/syntfsch.gif
>
>>If you stare at it long enough the temperature cancellation of the
>>second transistor starts to make sense. This circuit works too.
>
>What's the drawback with this circuit? There must be one, otherwise they
would have used it for the frequency CV too.
>
>/Jorgen

The reference current is not constant! It depends a little on Vbe of the
left transistor. Apart from that its the "normal" PNP expo convertor. 

Bye
 René



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