Differential amplifier is a basic building block of an op-amp.
The function of a differential amplifier is to amplify the difference between two input signals.

The two transistors Q₁ and Q₂ have identical characteristics.

The resistances of the circuits are equal, i.e. R_E1 = R_E2, R_C1 = R_C2 and the magnitude of +V_CC is equal to the magnitude of –V_EE.

These voltages are measured with respect to ground.

To make a differential amplifier, the two circuits are connected as shown in figure.
The two +V_CC and –V_EE supply terminals are made common because they are same.
The two emitters are also connected and the parallel combination of R_E1 and R_E2 is replaced by a resistance R_E.
The two input signals v₁ & v₂ are applied at the base of Q₁ and at the base of Q₂.
The output voltage is taken between two collectors.
The collector resistances are equal and therefore denoted by R_C = R_C1 = R_C2.
Ideally, the output voltage is zero when the two inputs are equal.
When v₁ is greater then v₂ the output voltage with the polarity shown appears.
When v₁ is less than v₂, the output voltage has the opposite polarity.
The four differential amplifier configurations are following:
- Dual input, balanced output differential amplifier.
- Dual input, unbalanced output differential amplifier.
- Single input balanced output differential amplifier.
- Single input unbalanced output differential amplifier.

These above configurations are shown in figure above, and are defined by number of input signals used and the way an output voltage is measured.
If use two input signals, the configuration is said to be dual input, otherwise it is a single input configuration.
On the other hand, if the output voltage is measured between two collectors, it is referred to as a balanced output because both the collectors are at the same dc potential w.r.t. ground.
If the output is measured at one of the collectors w.r.t. ground, the configuration is called an unbalanced output.
A multistage amplifier with a desired gain can be obtained using direct connection between successive stages of differential amplifiers.
The advantage of direct coupling is that it removes the lower cut off frequency imposed by the coupling capacitors, and they are therefore, capable of amplifying dc as well as ac input signals.

Dual Input, Balanced Output Differential Amplifier:

The circuit is shown in 1st figure, v₁ and v₂ are the two inputs, applied to the bases of Q₁ and Q₂ transistors.
The output voltage is measured between the two collectors C₁ and C₂ , which are at same dc potentials.

The internal resistances of the input signals are denoted by R_S because R_S1= R_S2.
Since both emitter biased sections of the different amplifier are symmetrical in all respects, therefore, the operating point for only one section need to be determined.
The same values of I_CQ and V_CEQ can be used for second transistor Q₂.
Applying KVL to the base emitter loop of the transistor Q₁.

The value of R_E sets up the emitter current in transistors Q₁ and Q₂ for a given value of V_EE.
The emitter current in Q₁ and Q₂ are independent of collector resistance R_C.
The voltage at the emitter of Q₁ is approximately equal to -V_BE if the voltage drop across R is negligible.
Knowing the value of I_C the voltage at the collector V_C is given by:
V_C =V_CC – I_C R_C
and V_CE = V_C – V_E
= V_CC – I_C R_C + V_BE
V_CE = V_CC + V_BE – I_CR_C
From the two equations V_CEQ and I_CQ can be determined.
This dc analysis applicable for all types of differential amplifier.

Since the two dc emitter currents are equal.
Therefore, resistance r’_e1 and r’_e2 are also equal and designated by r’_e .
This voltage across each collector resistance is shown 180° out of phase with respect to the input voltages v₁ and v₂.
This is same as in CE configuration. The polarity of the output voltage is shown in Figure.
The collector C₂ is assumed to be more positive with respect to collector C₁ even though both are negative with respect to to ground
Applying KVL in two loops 1 & 2:

= R_C (i_e1 – i_e2)

A differential amplifier amplifies the difference between two input signals.
Defining the difference of input signals as v_d = v₁ – v₂ the voltage gain of the dual input balanced output differential amplifier can be given by

Differential Input Resistance:

Differential input resistance is defined as the equivalent resistance that would be measured at either input terminal with the other terminal grounded.
This means that the input resistance R_i1 seen from the input signal source v₁ is determined with the signal source v₂ set at zero.
Similarly, the input signal v₁ is set at zero to determine the input resistance R_i2 seen from the input signal source v₂.
Resistance R_S1 and R_S2 are ignored because they are very small.

Output resistance is defined as the equivalent resistance that would be measured at output terminal with respect to ground.
Therefore, the output resistance R_O1 measured between collector C₁ and ground is equal to that of the collector resistance R_C.
Similarly the output resistance R_O2 measured at C₂ with respect to ground is equal to that of the collector resistor R_C.
R_O1 = R_O2 = R_C

The current gain of the differential amplifier is undefined.
Like CE amplifier the differential amplifier is a small signal amplifier.
It is generally used as a voltage amplifier and not as current or power amplifier.