Explain CMRR IN AN OP-AMP

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AEC Question

nirjan · Answer

Well, CMRR (Common Mode Rejection ratio) is the measure of how well an Op-Amp rejects the common mode signals.Common mode configuration is when, We have same voltage at both the terminals. An op-amp is required not to amplify the input rather it should be rejecting the input. Differential mode configuration is when there are different input at the terminals.If the gain of the Op-amp in common mode is Ac and Differential mode is Ad. CMRR is just the ratio of common mode gain differential mode gain.Given by, CMRR = Ac/Ad, to express in dB,CMRR = 20log10(Ac/Ad) dB.Let's move onto the summing amplifier. The circuit will look like,Aim of the circuit is to achieve the sum of v1,v2,v3 in amplified form at output terminal.Now as the Op-amp has a very large input impedance, there will no current going into the Op-amp.Therefore, i = i1+i2+i3 is what flowing at the output side. Also there is a virtual short between the points A and B, there for voltage at these two nodes will be equal. B point is at ground there the point A will also be at 0 volts. Considering these lets dive into analysis.Equating the currents at input and output side,To achieve summing we will be setting,Then the result will be,Now let's move onto the differential amplifier circuit.The circuit looks like,Same current rule applies and the virtual short makes Voltage at point A (VA) will be equal to voltage at point B(VB).Now hold on! what's VB should be?VB is w the voltage drop across R3 resistor, which is,Let's get into current analysis,The current i1 is what is also flowing at the output side because of hing input impedance of the Op-amp, equating the current of input and output side, Solving algebraically,So, output voltage becomes,Now to achieve the differential operation we should do the following,Now the output becomes,Differential operation achieved!!Let's move onto the problem,Looking the circuit, it's just a summing amplifier and we just put the required value given in the problem into the equation we got,where,Putting these values,Output will be -40V.

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Explain CMRR IN AN OP-AMP

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