what is differential amplifier

A differential amplifier is a type of electronic amplifier that multiplies the difference between two inputs by some constant factor (the differential gain).

Theory

Many electronic devices use differential amplifiers internally. The output of an ideal differential amplifier is given by:

Where and are the input voltages and A_d is the differential gain.
In practice, however, the gain is not quite equal for the two inputs. This means, for instance, that if and are equal, the output will not be zero, as it would be in the ideal case. A more realistic expression for the output of a differential amplifier thus includes a second term.

A_c is called the common-mode gain of the amplifier.
As differential amplifiers are often used when it is desired to null out noise or bias-voltages that appear at both inputs, a low common-mode gain is usually considered good.

The common-mode rejection ratio, usually defined as the ratio between differential-mode gain and common-mode gain, indicates the ability of the amplifier to accurately cancel voltages that are common to both inputs. Common-mode rejection ratio (CMRR):

In a perfectly symmetrical differential amplifier, A_c is zero and the CMRR is infinite. Note that a differential amplifier is a more general form of amplifier than one with a single input; by grounding one input of a differential amplifier, a single-ended amplifier results. An operational amplifier, or op-amp, is a differential amplifier with very high differential-mode gain, very high input impedances, and a low output impedance. Some kinds of differential amplifier usually include several simpler differential amplifiers. For example, an instrumentation amplifier, a fully differential amplifier, an instrument amplifier, or an isolation amplifier are often built from several op-amps.

Differential amplifiers are found in many systems that utilise negative feedback, where one input is used for the input signal, the other for the feedback signal. A common application is for the control of motors or servos, as well as for signal amplification applications. In discrete electronics, a common arrangement for implementing a differential amplifier is the long-tailed pair, which is also usually found as the differential element in most op-amp integrated circuits. A differential amplifier is used as the input stage emitter coupled logic gates.