SaM - Differential Amplifier

Memory Card

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• The definition of “Differential Amplifier” is:
  ==A Differential Amplifier is a type of amplifier that amplifies the voltage difference between two input signals while rejecting any common-mode signals (signals that are common to both inputs)==.
• To measure how effective a Differential Amplifier is at rejecting common-mode signals, we can calculate its CMRR (Common Mode Rejection Ratio).
• ==Here we can see some examples of Differential Amplifers==:
  • Circuit (Ideal Differential Amplifier)
    
  • Circuit (Differential Amplifier made with an O.A.)
    
  • Circuit (Differential Amplifier made with an O.A.)
    
  • Circuit (Instrumentational Amplifier):
    

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• When you find a single-input amplifier, the second input is supposed to be to a reference and fixed voltage.
• In general, since I’m speaking about amplifier, I suppose that ==it behaves linearly==.

So we have that:

Remember that:

• Real operational amplifier have this quality here, but the common mode gain is not zero.
  I accept it because perfect symmetry is not possible, but I have a differential gain which is actually much larger than the common mode gain.
• So for an Ideal Differential Amplifier we can write:$V_o=A_d\cdot (V^{+}-V^{-})$Instead for a Real DIfferential Amplifier:$V_o=A_1{V}^{+}-A_2{V}^{-}$

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• Online Source: Webpage
  
• Formula: $V_{out}=V_2\frac{R_3}{R_2+R_3}\frac{R_f+R_1}{R_1}-V_1\frac{R_f}{R_1}$
• If $R_1=R_2$ & $R_f=R_3$ (this is the specific differential amplifier used for an instrumentational amplifier):$V_{out}=V_2\frac{R_3}{R_2+R_3}\frac{R_3+R_2}{R_2}-V_1\frac{R_3}{R_2}=(V_2-V_1)\frac{R_3}{R_2}$

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NOTE: A differential amplifier output functions depends only on the difference: $V^{+}-V^{-}$. Here’s an example of a differential amplifier:
Where the output is: $V_{out}=2(V^{+}-V^{-})$ While here one that is NOT a differential amplifier:
Where the output is:$V_{out}=V^{+}-\frac{1}{2}{V}^{-}$