On Thu, 04 Dec 2008 18:33:54 -0700, Jim Thompson
wrote:


On Fri, 05 Dec 2008 01:24:53 +0000, Mike Monett wrote:

"RST Engineering \(jw\)" wrote:

It is a TL082. This thread started over in SEdesign and was ported
over here strictly to be able to upload a schematic. THe 358 we knew
wouldn't work and we were looking for a drop-in to replace it.

Jim

The TL082 has a wierd common mode spec. The minimum spec is +/- 11V for a
+/- 15V supply, but the numbers in the typical column are +15V, -12V, so
the range depends on the bias voltage on the input JFETs, J1 and J2.

I'd move the current sense circuit to the output of the LM317, and make
sure there is enough headroom to meet the minimum value of +11V at the
TL082 input pins. That would take some rework which might not be desirable.

An alternative might be to reduce the input common mode voltage in your
circuit by dropping the value of R102 and R109 to between 1k and 10k,
depending on your requirements. The goal is to bring the TL082 input to
something less than 11V, but not attenuate the desired signal by a large
amount. You could make up any gain loss in the second TL082.

Another method of extending the common mode range is shown in the article
"Remove Large Common-Mode Signals Using Standard Op Amps", at

http://electronicdesign.com/Articles...4832/4832.html

I hope this helps!

Best Regards,

Mike Monett

Common mode range of TL08x extends to the rails.

...Jim Thompson

You could show your students an analysis of how small errors in the
classic "differential amplifier" are disastrous.

The real world ain't AofE... the real world is nasty.

I penciled out a real world (nearly trivial) solution just now over
nachos... will post tomorrow ;-)

...Jim Thompson
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