What is a differential amplifier?

Hello.

"The differential amplifier is one of the most important circuits in
electronics and an understanding of its operation is vital."
See the complete description and diagrams in:
See the complete description and diagrams in:
"Appendix 1: The Differential Amplifier"
http://alpha400.ee.unsw.edu.au/elec3006/appendix1.pdf

"A differential amplifier is a basic building block seen as the first
stage of an operational amplifier. Its importance to the success of
the op-amp is paramount."
See the complete description and diagrams in:
"LABORATORY 9 THE DIFFERENTIAL AMPLIFIER"
http://www.engga.uwo.ca/ece241b/LAB-9%20-%20DIFF%20AMPLIFIER.pdf

Note that these two documents are in PDF format, so the Adobe Acrobat
Reader is required. If you don't
have that, visit Adobe's web site:   
http://www.adobe.com/products/acrobat/readstep2.html 


More information:

"DIFFERENTIAL AMPLIFIERS
A differential amplifier has two possible inputs and two possible
outputs. This arrangement means that the differential amplifier can be
used in a variety of ways. Before examining the three basic
configurations that are possible with a differential amplifier, you
need to be familiar with the basic circuitry of a differential
amplifier."
source: "Special Amplifiers," hosted by tpub.com
http://www.tpub.com/neets/book8/32.htm

 "One of the most basic Analog IC [Integrated Circuit] is the
differential amplifier. The differential amplifier requires two
transistors of equal physical characteristics. The two transistors
must match like identical twins."
source: "Basic Electronics: Integrated Circuit Amplifying Components
Differential Amplifiers"
http://home.att.net/~jailai/beet4_38.htm#p1


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I hope this helps.

As I am a mechanical engineer, electronics are not my best suit. 
Thank you for your help.

Maybe this will help, since I'm an electrical engineer...Most
amplifiers are what is known as "single ended" or an absolute amplier.
 They amplify the signal with respect to ground.  This is fine for a
large number of applications where the signal is close to ground in
relation to the change you might be able to see.  For example, if a
source signal varied from 0 to .5 volts and you wanted to amplify it
by a factor of 10 to get a output of 0 - 5 volts.

However, many times, you might have a signal whose variation is
relatively small as compared to its difference to ground.  An example
of this would be a signal that varied from 7 to 7.5 volts, where you
were only interested in the variation of the signal, and you wanted
that fairly accurately.  The term differential refers to measuring the
difference between two inputs, as opposed to one compared with ground.

While it may seem that the difference is semantic, it is not. 
Differential amplifiers are designed to specifically ignore, or
regect, anything but the difference signal.  This property is known as
the amplifier's CMRR, or common mode rejection ratio, and is quite
often over 100dB (a whole lot).  A real life example of where a
differential amplifier is needed is is in measuring current shunt. 
Typically, in measuring a current in a circuit, like the current
consumed in a laptop, a very small but precisely known resistor is
place in the circuit path, so that the current can be measured by
V=IR.  Now, the resistance in wires, etc., between that shunt and
ground may in fact be several times greater than the resistance of the
shunt itself.  Plus, while the shunt would be designed so that its
resistance wouldn't vary by much over temperature, that wouldn't be
the case for the offset.  In this case, a differential amplifier of
high accuracy known as an instrumentation amplifier is used.

Hope that helps.