Hey Im new to this forum, and this seems as an appropriate palce to post my question.
What I need is the following:

Some one to explain to me the way a Differential amplifier functions in Common mode and Differential mode. I have an idea but I need for someone to elaborate..

What the entry resistence (Rc) is for, what purpose does it acomplish

and

what function does the amp do to the signal aplied to its terminals

I hope someone can help me out here.
I need to get this paper done in a few hours and Im not very sure on the subject.

Ok well , awaiting a reply..

p.s. Sorry if you see some gramatical errors, english isnt my first language..

 

Hi,

Very briefly, a difference amplifier amplifies only voltage signals that are different at the input terminals. For instance, applying 2 volts to both inputs results in no output - the 2 volts is common to the inputs.

For the best ability to not alter the input signal, the input resistance should be very high. A low input resistance might lower the signal voltage through loading.

For more information, look up instrumentation amplifiers. They are designed to be high input resistance differential amplifiers. Burr-Brown makes a line of instrumentation amps.

 

Its a while since I have looked into Differential amplifiers, but I don't recall the concept of "entry resistance" - is this the same as input resistance? If so, the difference amplifier has a relatively low value of input resistance, recall ideally we want infinite input resistance to the op amp. Since difference amplifiers are generally implemented in intstrumentation systems where the are used to amplify small signals, the low input resistance can be problematic. This is where the instrumentation amplifier noted by beenthere is a superior choice.

You may be interested in looking at:

Differential Amplifers

and

Instrumentation Amplifiers

for more details.

Dave