A device which accepts an input signal and produces an output signal proportional to the input, is called an amplifier. An amplifier which amplifies the difference between.the two input signals is called differential amplifier.

**Introduction of Differential
Amplifier**

A device which accepts an input signal and produces an output signal proportional to the input, is called an amplifier. An amplifier which amplifies the difference between.the two input signals is called differential amplifier. The differential amplifier configuration is used in variety of analog circuits. The.differential amplifier is an essential and basic building block in modern IC amplifier .The Integrated Circuit (IC) technology is well known now a days, due to which the design of complex circuits become very simple. The IC version of operational amplifier is inexpensive, takes up less space and consumes less power. The.differential amplifier is the basic building block of such IC operational amplifier.

**Basics of Differential Amplifier**

The Differential
Amplifier amplifies the difference between two input voltage signal. Hence it
is also called as difference amplifier.

Consider
an ideal differential amplifier shown in the Fig. A

V_{1}
and V_{2} are the two input signals while Vo is the output. Each signal
is measured with respect to the ground.

In an
ideal differential amplifier, the output voltage Vo is proportional to the
difference between the two input signals. Hence we can write,

From
Equation 1 we can write,

where A_{D}
is the constant of proportionality. The A_{D} is the gain with which
differential amplifier amplifies the difference between two input signals. Thus
it is called differential gain of the differential amplifier.

Thus, Ad
= Differential gain

The
difference between the two inputs (V_{1} - V_{2}) is generally
called difference voltage and denoted as Vd.

Hence the
differential gain can be expressed as,

Generally
the differential gain is expressed in its decibel (dB) value as,

**Common Mode Gain Ac**

If we
apply two input voltages which are equal in all the respects to the
differential amplifier i.e. V1 = Vz then ideally the output voltage Vo = (V_{1}
- V_{2}) A_{d}, must be zero.But the output voltage of the
practical differential amplifier not only depends on thedifference voltage but
also depends on the average common level of the two inputs.

Such an
average level of the two input signals is called common mode signal denoted as
V_{C}

Practically,
the differential amplifier produces the output voltage proportional to such
common mode signal, also. The gain with which it amplifies the common mode
signal to produce the output is called common mode gain of the differential
amplifier A_{C.\}

Thus
there exists some finite output for V1 = V2 due to such common mode gain A_{C,}
in case of practical differential amplifiers.

So the
total output of any differential amplifier can be expressed as,

For an
ideal differential amplifier, the differential gain A_{d}, must be
infinite while the common mode gain must be zero.

But due
to mismatch in the internal circuitry, there is some output available for

V_{1}
= V_{2} and gain A_{C} is not practically zero. The value of
such common mode gain A_{C} very small while the value of the
differential gain A_{d} is always very large.

**Common Mode Rejection Ratio (CMRR)**

When the
same voltage is applied to both the inputs, the differential amplifier is said
to be operated in a common mode configuration. Many disturbance signals, noise
signal appear as a common input signal to both the input terminals of the
differential amplifier. Such a common signal should be rejected by the
differential amplifier. The ability of a differential amplifier to reject a
common mode signal is expressed by a ratio called common mode rejection ratio
denoted as CMRR. It is defined as the ratio of the differential voltage gain A_{d}
to common mode voltage gain A_{C}

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Electronic Circuits - BJT Amplifiers : Introduction of Differential Amplifier |

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