Differential Amplifier

DIFFERENTIAL AMPLIFIER

The differential amplifier is used to amplify the difference between two input signals.

The differential amplifier may be implemented using BJTS or FETs and it is the commonly used building block in analog IC design.

The block diagram of an ideal differential amplifier is shown in Fig.3.6. It has two input terminals and one output terminal. The two input signals V₁ and V₂ are given as input and connected to the input terminals. V₁ and V₂ are measured with respect to ground terminal.

The output voltage V_O is proportional to the difference between two input signals V₁ and V₂.

i.e., V₁ ∞ (V₁ - V₂)

Common Mode Signal

Common mode refers to applying common input to both input terminals. A common signal to both the input terminal is called as common mode signal.

The output voltage for the common mode signal will be zero

V_O = V₁ - V₂

If V₁ = V₂ = V

⇒ V_O = 0

Practically the output will not be zero, it produces some minimum amount of voltage.

Differentially Mode Signal

The difference between the input signals is obtained as differential signal V_O.

The two input signals V₁ and V₂ are not same and it produces the difference between these two signals as output.

V_O = V₁ - V₂

The differential signal V_O is amplified by the differential amplifier. So it is called as differential amplifier.

Common Mode Gain (A_C)

The output of the differential amplifier can be modified as

V_O = A_d (V₁ - V₂) ...(2)

Where A_d - differential gain

Differential gain can be defined as the gain with which the differential amplifier amplifies the differential signal.

The output signal also depends in the average input voltage, called common mode signal V_C, given by

Common mode gain is defined as the gain with which a practical differential amplifier amplifies the common mode signal.

The output voltage due to common mode signal is given by.

V_O = A_C V_C

Where A_C - common mode gain, V_C - common mode signal

For an ideal differential amplifier, the common mode gain should be Zero.

Differential Mode Gain (A_d)

As mentioned earlier, the output is proportional to the differential gain, A_d and the differential voltage V_d. The output voltage V_O is given by

V_O = A_d V_d ...(3)

Where

A_d - differential gain

V_d - differential voltage

The differential gain can be written as

Differential gain A_d - V_O / V_d

In decibels, A_d can be expressed as

Common Mode Rejection Ratio (CMRR)

CMRR is defined as the ability of a differential amplifier to reject the common mode signal.

Need for Common Mode Rejection

The common mode signal is the signal which is present at both the input terminals of a differential amplifier. The best example of common mode signal is noise. Practically, the output will not be zero for the common mode signal. The differential signal produce a very small output voltage for common mode input signal. It should be capable of rejecting the common mode signal. Thus CMRR is required to determine how the common mode signal is rejected successfully. The CMRR is otherwise called as figure of merit of a differential amplifier. CMRR is defined as the ratio of differential gain A_d to the common mode gain A_c.

For an ideal differential amplifies CMRR should be infinity. In practical situations, CMRR should be as high as possible. In dB, it is expressed as

Features

The differential amplifier should have the following features

i. High differential gain

ii. Low common mode gain

iii. High Common Mode Rejection Ratio (CMRR)

iv. High input impedance

v. Low output mpedance

vi. High gain

vii. Large bandwidth