Differential Amplifier: Working, Circuit Diagram, Formula and Configurations

What is a Differential Amplifier?

A differential amplifier is an electronic amplifier that amplifies the difference between two input voltages while rejecting any voltage that is common to both inputs. It is also known as a difference amplifier or op-amp subtractor. A differential amplifier is an analog circuit with:

• Two input terminals
• One output terminal

The output voltage is ideally proportional to the difference between the two input voltages.

Differential Amplifier Formula

The output voltage of a differential amplifier is given by:

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Where:

• V_o = Output voltage
• A_d = Differential gain
• V₁ = Input voltage at the inverting terminal
• V₂ = Input voltage at the non-inverting terminal

BJT Differential Amplifier Circuit

A BJT differential amplifier consists of:

• Two Bipolar Junction Transistors (BJTs) — Q₁ and Q₂
• Two collector resistors — R_C1 and R_C2
• One common emitter resistor — R_E
• Positive power supply — V_CC
• Negative power supply — V_EE

The input signals are applied to the bases of the transistors, and the output is obtained from their collector terminals.

Working of Differential Amplifier

Suppose a sinusoidal signal is applied to transistor Q₁.

As the input voltage at Q₁ increases:

• The transistor conducts more heavily
• The collector current I_C1 increases
• The voltage drop across collector resistor R_C1 increases
• The output voltage V_O1 decreases

At the same time:

• The emitter current increases
• The voltage drop across emitter resistor R_E increases
• The emitter terminals move toward a positive potential
• The base-emitter voltage of Q₂ decreases

As a result:

• The collector current I_C2 decreases
• The voltage drop across R_C2 decreases
• The output voltage V_O2 increases

Thus, the signal appearing at the collector of Q₂ is amplified and phase-shifted by 180° compared to the signal at Q₁.

Differential Op-Amp

A differential amplifier can also be implemented using an operational amplifier (Op-Amp). In this configuration, input signals are applied to both the inverting and non-inverting terminals of the op-amp. The output voltage produced is proportional to the difference between the two input voltages. Therefore, the differential amplifier acts as a subtractor circuit by subtracting one input signal from the other.

Construction of Differential Amplifier

A differential amplifier can be constructed using:

• Bipolar Junction Transistors (BJTs)
• Field Effect Transistors (FETs)
• Operational Amplifiers (Op-Amps)

Circuit Features of Differential Amplifier

The differential amplifier circuit contains:

• Two input terminals
• Two output terminals
• Two matched transistors (Q₁ and Q₂)
• Common emitter resistor R_E
• Common positive supply V_CC
• Common negative supply V_EE

Ideally, both transistors should have identical characteristics for proper differential operation.

Configurations of Differential Amplifier

There are four main configurations of a differential amplifier:

1. Dual Input Balanced Output

In this configuration:

• Two input signals are applied
• Output is taken from both transistors

2. Dual Input Unbalanced Output

In this configuration:

• Two input signals are applied
• Output is taken from only one transistor

3. Single Input Balanced Output

In this configuration:

• Only one input signal is applied
• Outputs are taken from both transistors

4. Single Input Unbalanced Output

In this configuration:

• Only one input signal is applied
• Output is taken from only one transistor

Applications of Differential Amplifier

• Operational amplifier input stages
• Noise reduction circuits
• Signal conditioning systems
• Instrumentation amplifiers
• Communication systems
• Analog signal processing

Advantages of Differential Amplifier

• Excellent noise rejection capability
• High voltage gain
• Improved signal stability
• Can amplify very small signals accurately

Conclusion

A differential amplifier is one of the most important analog circuits used in electronics. It amplifies the difference between two signals while rejecting common unwanted signals and noise. Differential amplifiers are widely used in operational amplifiers, communication systems, and signal processing applications.