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 A_d or decreasing A_c.

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Also Read: What is CMRR? | Common Mode Rejection Ratio

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Effect of R_E on CMRR

The expression for Differential gain A_d 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 R_E does not affect any negative feedback for the differential mode operation. Therefore the value of the A_d does not depend on the value of R_E.

The expression for Differential gain A_c 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 R_E introduces negative feedback and reduces the common mode gain A_c.

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

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

However, the dc biasing point (Q point) of the differential amplifier depends on the value of R_E since the expression for I_CQ and V_CEQ contains the value of R_E.

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

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Methods To Improve CMRR

As with a change in R_E 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:

Methods To Improve CMRR
1.	To use a constant current bias
2.	To use a current mirror circuit
3.	To use an active load.

The first two methods will effectively increase R_E without disturbing the biasing point whereas the last one will increase R_C to increase A_d.

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