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}.

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

## 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.

## 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}.

## FAQs

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

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

**When can CMRR be improved?**

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

**How can the CMRR of a differential amplifier be improved?**

emitter resistance increases 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