The structure from your temp.jpg is the ideal drawing of an OA buffer.
At the output of the OA you'll have + Vref +/- offset. The supply must be
( for this configuration ) at least +/-5V . In practice is often use a
resistor in negative feedback loop and the same resistor between + input
and reference. Dimensioning of that resistor take care about the input
biasing current in +/- input of OA.
Asuming the output OA impedance is near 0, the "line" will be keept at
+Vref +/- offset + Vd where Vd is the potential across diode and have the
expression:
Vd = k*Vt*(ln I - ln I sat )
where: k = 1.5
       VT = T/11000  T in kelvin degree
       I = flowing diode current
       Isat = saturation diode current

Last relation doesn't say anything else that Vd is temperature dependent.
I hope now is clear like a source...

best, Vasile


On Tue, 26 Feb 2002, Benjamin Bromilow wrote:

> Hi all,
>
> Does this circuit make sense to anyone? Someone very kindly drew it up for
> me and e-mailed it but it doesn't make sense to me! It is supposed to take a
> 2.5v ref, buffer it and limit the input line (going horizontally) to around
> this level.
> However, in my mind, there appears to be some problems. Perhaps the second
> comma is critical!  :o)..... Firstly, the op-amp doesn't appear to be in
> voltage follower mode which is my understanding of a buffer mode.
> Additionally, I'm not sure where any excess voltage is supposed to be
> "disposed" of other than through the op-amp output stage. Is this okay?
> Am I being dense here or am I missing something??
>
> Ben
>

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