[JoshK]

I am going to show my lack of understanding of how opamps work, haven't read up on them at all.  But I had a question with this schematic.  This schematic is from hypex's website.



I can see the differential pair and what it is doing, but what is the third opamp for?  Is it a gain stage?

[Dan Banquer]

Hi Josh;
   Sure looks like an amp that converts differential inputs to a single ended output. Check out some of the balanced to single ended drivers from Burr-Brown and Analog devices, I think you might see some similarities.
             d.b.

[Occam]

As Dan indicated, the above schematic converts balanced differential signals to single ended. That specific implementation is referred to as an 'instumentation amplifier', which have benefits associated with its topology.

The first 3 main links from a google search -
http://www.google.com/search?sourceid=navclient&aq=t&ie=UTF-8&rls=GGLR,GGLR:2006-06,GGLR:en&q=instrumentation+amplifier

wikipedial,national and allaboutcircuits are very informative

[ctviggen]

That's a very cool and useful circuit.

[JoshK]

Thanks, I think I get it now.  Thanks Paul for the links, I'll have to print them out and read them on the commute home. 

[ctviggen]

Preferably not while you're driving? 

[JoshK]

drive?  what's that?    NYC commute = public transit. 

[Audiovista]

Josh,

Some benefits of the very popular three op-amps instrumentation amplifier (from W. Jung's book "Op Amp Applications"):

- common-mode errors in the input amplifiers tend to be cancelled out by the third amp (I guess this answers your original question)
- overall gain is being set by only one resistor
- this is a differential amplifier and common-mode signals are amplified by a factor of 1, regardless of gain
- CMR of this amp (in theory) increases in proportion to the gain
- large common-mode signals can be handled at any gain

Analog Devices and Burr-Brown (Texas Instruments) make those in a nice tiny packages with very accurately trimmed internal resistors.

Boris