why we consider different Differential amp?

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020170

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**broken link removed**

above pictures explain about Common mode circuit and differential circuit of the differential amp.

differential amp have common mode and differential mode.

but when we analyze differential amp, we have to divide Diff. amp into two parts, Common mode and differential mode.

I don't understand difference common mode circuit and Differential mode circuit.

is it enough just to modify input signal? why we have to modify circuit fully to analyze common mode and differential mode?

I'll appreciate if you help me. thanks
 

in my opinion, common mode inputs usually aren't able to be eliminated by adjusting your inputs, since they are unadvoidable noises, which will be further amplified in case of a big gain of your diff amp. thus, a concern on the common mode rejection is necessary.
 

The common mode input is basically the average of the inputs at the + and - input terminals, in other words, it's the common input to both inputs. The differential input is the difference between the + and - terminals and is what you really care about. You want to have a high differential gain and a low common mode gain so that you can try to eliminate as much as the common mode input as possible.
 

i will try to explain as i understood,

lets first look at the inputs of the general cit. u will find them consists of two parts ac(signal u need to amplify) + dc(biasing voltage)+ noise

second: dc is common on both side.

third: any noise will affect both sides(v1,v2) by same value so the cit. see them as a common input(refelects in CM- move together up and down), is that signal which is similar on both sides, we need to reject this part as much as we can, we dont need the noise to appear at output. from analysis u will understand it more.

for the diff. mode : it represents the signals that when one up the second goes down, this signal is our desierd signal so we amplify it as much as we can.
 

I think that u can analyze the circuit without dividing it into parts, u will solve it by using exact analysis.
So u will assume that u have Vin1 and Vin2, and try to get the equation which will relate Vin1 and Vin2 with Vout1 and Vi=out2. In this equation if u put Vin1 = - Vin2 = Vid/2, u will get the differential gain, and if u put Vin1 = Vin2 = Vincm, then u will get the common mode gain.
If u compare ur exact solution to the approximated one (by dividing the circuit into two modes), u will find that both solutions are approximately equal. So we divide the circuit to let the analysis be faster.
 

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    020170

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besides above all, in zero IF receivers,we also adopt differential structure to cancel DC-offset.
 

common mode signal may staurate the follow circuit so we should check them.

And CMRR is a good index when your circuit in a noise environment. That will tell you how much noise you can reject.


Yibin.
 

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