[sdiy] Linear input on expo...confused

[Rutger Vlek]

Hi all,

I have a fairly standard expo converter here, quite similar to this one (https://plus.google.com/107231411209962227230/posts/C8iVtXKzvet?pid=5628330677174689314&oid=103342596752795218521). However, I needed a current source, rather than a sink. So I changed the circuit a bit and replaced the NPN pair with PNPs. Besides the expo input, I'd like to have a linear input as well, but this is where I'm stuck. I was expecting the principle to be the same as in the current sink expo example from the link, where a resistor to the inverting input of the opamp does the trick. But in my case it doesn't... Any ideas why?

Rutger