[sdiy] Expo converter ??

[René Schmitz]

At 23:41 17.01.03 -0500, you wrote:
>riddle me this:
>
>What are the advantages/disadvantages  of the NPN/PNP expo converter
>favored by ARP synths... vs the more well known NPN pair and opamp servo
>???
>
>For what parameters are the NPN / PNP matched ???
>
>All dissertations cheerfully entertained...
>
>H^) harry
>


A disadvantage is that it is more difficult to apply Rbe compensation.
(Moog-Hemsath, Rossum)
Also thermal matching is not quite as good as with the servo type. Usually
you have to resort to use descrete transistors glued together. 

But then again this circuit performs far better than one might expect. (At
least I was surprised.)
In the VCO 4069 it works nicely over more than 8 octaves. Even though I
didn't match the transistors. (I figured the effects of the dissimilar
transistor types together with the loose thermal coupling will excel those
introduced by a possible mismatch.)

Cheers,
 René





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uzs159 at uni-bonn.de
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