# What are the different methods to improve CMRR in Differential Amplifiers?

There are different methods to improve CMRR in differential amplifiers, we will discuss the possible methods to improve CMRR.

CMRR is the figure of merit of a differential amplifier. Its value should be as high as possible. Because CMRR is the ratio of differential voltage fain Ad and common mode voltage fain Ac, we can improve CMRR by either increasing Ad or decreasing Ac.

## Effect of RE on CMRR

The expression for Differential gain Ad can be given by,

\boxed{\mathbf{A_d = \frac{-h_{fe} R_c}{(R_s + h_{ie})}}}

We have learned that for a dual input balanced output differential amplifier, the emitter resistor RE does not affect any negative feedback for the differential mode operation. Therefore the value of the Ad does not depend on the value of RE.

The expression for Differential gain Ac can be given by,

\boxed{\mathbf{A_c = \frac{-h_{fe} R_c}{(R_s + h_{ie}) + [(1 + h_{fe})2R_E]}}}

However, in the common mode operation, the value of resistance RE introduces negative feedback and reduces the common mode gain Ac.

The higher the value of RE, more is the negative feedback, and the less common mode gains Ac. Thus with an increase in RE, Ac decreases, and CMRR increases.

So basically to increase the CMRR, RE should be increased.

However, the dc biasing point (Q point) of the differential amplifier depends on the value of RE since the expression for ICQ and VCEQ contains the value of RE.

Hence with a change in RE to improve CMRR, the Q point will get shifted. This is not desirable. Therefore we need to use some other methods to improve CMRR.

## Methods To Improve CMRR

As with a change in RE to improve CMRR, the Q point will get shifted. This is not desirable. Therefore we need to use some other methods to improve CMRR. These methods are:

The first two methods will effectively increase RE without disturbing the biasing point whereas the last one will increase RC to increase Ad.

## FAQs

How can I improve the CMRR of my op-amp?

ByÂ lowering the output impedance and increasing the input impedance of the op-amp.

When can CMRR be improved?

High resistance REÂ will reduce the common mode gain thus improving the CMRR

How can the CMRR of a differential amplifier be improved?

emitter resistance increasesÂ then the value of CMRR increases.

What happens if CMRR is high?

A higher CMRR value indicates a better ability to reject common-mode noise

Hello friends, my name is Trupal Bhavsar, I am the Writer and Founder of this blog. I am Electronics Engineer(2014 pass out), Currently working as Junior Telecom Officer(B.S.N.L.) also I do Project Development, PCB designing and Teaching of Electronics Subjects.

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