Q.What is a differential amplifier? Draw four modes of differential amplifier?

A differential amplifier is a direct-coupled amplifier that amplifies the difference between two input signals; it is the main building block of an integrated op-amp.

[!ANSWER]

A differential (difference) amplifier amplifies the difference between two input voltages applied to its two inputs. Depending on how many inputs are driven and how the output is taken, it works in four modes: (1) dual input, balanced output, (2) dual input, unbalanced output, (3) single input, balanced output and (4) single input, unbalanced output.

[!TLDR]

A differential amplifier amplifies the difference between two input signals; it has four modes decided by the number of driven inputs (single or dual) and how the output is taken (balanced across the two collectors, or unbalanced from one collector to ground).

Differential amplifier: The differential amplifier (also called difference amplifier or emitter-coupled amplifier) is the main building block of IC amplifiers and the basic input stage of an integrated operational amplifier. It is a direct-coupled amplifier and, as the name implies, it amplifies the difference between the two input signals. It consists of two identical transistors $Q_1$ and $Q_2$ with equal collector resistors $R_{C1}=R_{C2}=R_C$, a common emitter resistor $R_E$, and two supplies $V_{CC}$ and $V_{EE}$. Inputs are applied to the two bases and outputs are taken from the collectors.

The four modes (decided by two choices — how many inputs are driven, and how the output is taken):

A balanced output is measured between the two collectors, while an unbalanced output is measured from a single collector with respect to ground. Each configuration should be drawn with the two identical transistors, the two collector resistors $R_C$, the common emitter resistor $R_E$, the $+V_{CC}$/$-V_{EE}$ supplies and the correct input/output connections, together with its block-diagram symbol. This is a standard 5-mark question in the Karnataka 2nd PUC Electronics question bank.

[!ANSWER]

A differential amplifier is a direct-coupled amplifier that amplifies the difference between two input signals and forms the input stage of an op-amp. Its four modes are: (1) dual input balanced output, (2) dual input unbalanced output, (3) single input balanced output, and (4) single input unbalanced output — 'balanced' meaning the output is taken between the two collectors and 'unbalanced' meaning it is taken from one collector to ground.