---
title: "Differential amplifier using BJT"
subject: "Basic Electronics Engineering"
chapter: 4
key: "semester_2/basic_electronics_engineering:diff-amp-bjt-circuit"
source: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/diff-amp-bjt-circuit
site: BCT Engineering
---

# Differential amplifier using BJT

Basic Electronics Engineering · Chapter 4: The Operational Amplifier and Oscillator

- Asked in: 2074 Bhadra, 2 marks; 2067 Chaitra, 2 marks (asked 2 times)
- Video: [BJT:  Differential Amplifier Explained](https://www.youtube.com/watch?v=yXJADXuWtlI) — ALL ABOUT ELECTRONICS
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/diff-amp-bjt-circuit
- Whole chapter: https://www.bctengineer.com/md/semester_2/basic_electronics_engineering/chapter-4

## Question

Draw the circuit diagram of differential amplifier using BJT (and indicate its input and output voltage waveforms).

## Short answer

A BJT differential amplifier uses two matched transistors with a common emitter resistor (or current source). A differential input produces amplified outputs at the collectors: one in phase, one out of phase. The output voltage is proportional to (v1 – v2) with high common-mode rejection.

## Answer

A **differential amplifier** amplifies the difference between two input signals and rejects the signal common to both.

#### Circuit (BJT, dual input, dual output)
```text
                 +VCC
          +-------+-------+
          |               |
        [RC]            [RC]
          |               |
 vo1 o----+               +----o vo2
          |               |
         C|               |C
 v1 o----B| Q1        Q2  |B----o v2
         E|               |E
          |               |
          +-------+-------+
                  |
                [RE]  (or constant current source)
                  |
                -VEE
```
Two matched transistors Q1 and Q2 share a common emitter resistor RE (or a constant current source). The inputs v1 and v2 are applied to the bases; the outputs vo1 and vo2 are taken from the collectors.

#### Input and output waveforms
For a differential input, v1 = +vd/2 and v2 = −vd/2 (sine waves 180° out of phase):
- vo1 is an amplified sine wave 180° out of phase with v1.
- vo2 is an amplified sine wave 180° out of phase with v2, and hence in phase with v1.
- The differential output taken between the collectors, vo = vo2 − vo1 = Ad vd, has twice the amplitude of either single collector output.

```text
 v1:      /\    /\         input 1
         /  \  /  \
        /    \/    \

 v2:    \    /\    /       input 2 (180° out of phase with v1)
         \  /  \  /
          \/    \/

 vo1:   \    /\    /       amplified, 180° out of phase with v1
         \  /  \  /
          \/    \/

 vo2:     /\    /\         amplified, in phase with v1
         /  \  /  \
        /    \/    \
```
Output voltage: vo = Ad (v1 − v2) + Ac (v1 + v2)/2, and CMRR = Ad/Ac.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is common to Q1 and Q2 in the BJT differential amplifier?
   A. Emitter resistor RE or constant current source
   B. Base terminal
   C. Collector terminal
   D. Input signal
   Answer: A. Two matched transistors share RE or a current source, with inputs at bases and outputs at collectors. (Key point: Circuit structure)

2. For differential input v1 = +vd/2 and v2 = -vd/2, how is vo1 related to v1?
   A. In phase
   B. 180° out of phase
   C. 90° out of phase
   D. Equal to zero
   Answer: B. Each common-emitter-like side inverts, so vo1 is amplified and 180° out of phase with v1. (Key point: Phase of outputs)

3. What is the differential output vo1 - vo2?
   A. Ac vd
   B. Ad vd
   C. Zero
   D. vd/2
   Answer: B. The differential output equals the differential gain times the differential input; CMRR = Ad/Ac. (Key point: Output voltage)

## Related in Basic Electronics Engineering

- [BJT differential amplifier collector waveforms](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Adiff-amp-bjt-waveform) — Basic Electronics Engineering, chapter 4
- [Output waveforms of two diode circuits](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Adiode-circuits-output-waveforms) — Basic Electronics Engineering, chapter 2
- [Transconductance and voltage gain of BJT](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Avoltage-gain-transconductance-bjt) — Basic Electronics Engineering, chapter 3, asked 2×
- [CE configuration input/output characteristics](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Ace-npn-characteristics) — Basic Electronics Engineering, chapter 3, asked 3×
- [BJT bipolar, MOSFET unipolar; BJT diff amp](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Abjt-bipolar-mosfet-unipolar-diff-amp) — Basic Electronics Engineering, chapter 3, asked 2×

## Related in other subjects

- [CE amplifier input, output resistance and gain (RE1, RE2)](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Ace-partial-bypass-numeric) — Electronics Devices and Circuit, chapter 2
- [Output offset voltage of 741 inverting amplifier](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Aoutput-offset-741-inverting) — Advanced Electronics, chapter 2
- [Common base amplifier circuit and gains](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Acb-amplifier-analysis) — Electronics Devices and Circuit, chapter 2
- [CE amplifier input resistance and overall gain](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Ace-input-resistance-gain-680k) — Electronics Devices and Circuit, chapter 2