# Gain Magnitudes Equal: Rejecting Common Mode Signals

• shawrix
In summary, it is necessary for the gain magnitudes of inverting and non-inverting ends to be equal in magnitudes to be able to reject the common mode signal. Also can someone explain the ratio obtained of voltage divider in difference amplifier.
shawrix
Why is it necessary for the gain magnitudes of inverting and non- inverting ends to be equal in magnitudes to be able to reject the common mode signal?

Also can someone explain the ratio obtained of voltage divider in difference amplifier.

R4/R4+R3[1+R2/R1]= R2/R1 Where,

R1 input resistance, R2 feedback resistor, R3 and R4 are voltage dividers used to attenuate the positive gain.

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This seems very much a homework-type question.

Vout = A+*V+ - A-*V-

For the resistor ratios, draw the circuit, establish the effect of the "virtual earth", derive the applicable equations, and manipulate the algebra.

because your sine waves have to be out of phase 180 degrees in order to reject the common mode noise, if the gain is different it will clip the sum. maybe someone else could elaborate on this.

Intuitively (and trivially): The common mode signal is an identical signal applied to both the + and - inputs, so obviously if the gains are different the inputs will not cancel each other.

But, NOxy is correct, you should grind through the equations and see why this is true. I will say I dislike the term "virtual earth". I'd rather say "just assume that the + and - inputs will be at equal voltages". That will always be true in an ideal amplifier if the output is not saturated.

There is nothing to explain about the resistor ratio equation (if what you wrote was correct). You just need to work through it for a couple of examples to understand it forever. Nothing I can explain will replace working it out yourself. It's only hard the first time.

1 person
An ideal OpAmp is a differential amplifier. If you are working on that basis and then you find it will amplify A+B, the resulting output will not be what you want. It would mean, at the trivial level, that you would need to get the input bias voltage 'exact'.

I fully endorse meBig Guy's advice that:
Nothing I can explain will replace working it out yourself. It's only hard the first time.

1 person
Thanks..

I'm late to the party...

so will just reinforce the advice to bite the bullet and do the algebra.

Solve the circuits for voltage at inverting and non-inverting inputs of the IC.
You'll get an equation for each input.
Set those voltages equal,
because that's how operational amplifiers work - they force their inputs equal,
and it is the job of the designer to surround the amplifier with a network that let's it do that.

If it cannot make them equal, it can't "operate".

So Vinverting = f1(a lot of resistors),
and Vnoninverting = f2(a lot of resistors)

Set the two functions equal one another,

old jim

## 1. What is the purpose of rejecting common mode signals?

The purpose of rejecting common mode signals is to eliminate unwanted noise or interference in a signal. Common mode signals are those that are present in both the positive and negative inputs of an amplifier, and by rejecting them, we can achieve a more accurate and precise measurement of the desired signal.

## 2. How does gain magnitude equalization help in rejecting common mode signals?

Gain magnitude equalization ensures that the gain of the amplifier is the same for both the positive and negative inputs. This means that any common mode signals will be amplified equally and cancel each other out, resulting in their rejection.

## 3. Can a single-ended amplifier reject common mode signals?

No, a single-ended amplifier is unable to reject common mode signals because it only amplifies one input and does not have a reference to compare it with. Differential amplifiers with equal gain magnitudes for both inputs are required for rejecting common mode signals.

## 4. What types of signals can be considered as common mode?

Any noise or interference that is present in both inputs of a differential amplifier can be considered as a common mode signal. These can include power line noise, electromagnetic interference, or any other unwanted signals that are picked up by the circuit.

## 5. Is it necessary to reject common mode signals in all types of measurements?

No, it is not necessary to reject common mode signals in all types of measurements. It is most crucial in situations where the desired signal is small and needs to be accurately measured without interference. In other cases, such as high power circuits, common mode rejection may not be as critical.

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