- 2081 Baisakh · 4 marks
Classification of amplifiers by operating region
Short answer
Amplifiers are classified by conduction angle: Class A conducts 360° (max efficiency 25–50%), Class B 180° (78.5%), Class AB between 180° and 360° (50–78.5%), and Class C less than 180° (>90% with tuned load). The Q‑point location determines the conduction angle.
Explain the classification of amplifiers according to operating region.
Power amplifiers are classified by the portion of the input cycle for which the active device conducts. This depends on the Q point (bias) and is described by the conduction angle.
| Class | Conduction angle | Q point | Max efficiency |
|---|---|---|---|
| A | 360° | middle of the load line | 25 % (series-fed), 50 % (transformer-coupled) |
| B | 180° | at cut-off | 78.5 % |
| AB | between 180° and 360° | just above cut-off | between 50 % and 78.5 % |
| C | less than 180° | below cut-off | above 90 % (tuned load) |
Class A
The transistor is biased at the centre of the load line and conducts for the whole cycle. Output is a faithful copy of input with the lowest distortion, but a constant dc current is drawn, so efficiency is low. Used in small-signal and low-power audio stages.
Class B
Biased at cut-off, so each transistor conducts for half a cycle. Two devices in push-pull are required to reproduce the full wave. Efficiency is high, but crossover distortion appears.
Class AB
A small forward bias is given so that both devices conduct for slightly more than half a cycle. Crossover distortion is removed while efficiency stays high. It is the most common choice for audio power stages.
Class C
Biased below cut-off, so current flows for less than half of the cycle (short pulses). The output is heavily distorted, so it must feed a tuned LC load (which restores a sine wave). It is used in RF power amplifiers.
Class A Class B Class AB Class C
~~~~~~~ /\ /\ /\ /\ ^ ^
full wave half wave >half short pulses
✓ Answer checked by Claude Opus 5.5 (high effort)
Related questions
- JFET ID and VDS (VP -5.5 V, IDSS 10 mA), pinch-off check · Electronics Devices and Circuit, chapter 3 · asked 3×
- JFET ID and VDS (VP -3.5 V, IDSS 10 mA), pinch-off check · Electronics Devices and Circuit, chapter 3 · asked 3×
- JFET ID and VDS (VP -5.5 V, IDSS 12 mA) · Electronics Devices and Circuit, chapter 3 · asked 3×
- Depletion MOSFET Q-points (IDSS 10 mA, VP -4 V) · Electronics Devices and Circuit, chapter 3
- Torque-slip curve: starting, maximum and operating region · Electrical Machines I, chapter 5
- Torque-slip curve; linear operating region · Electrical Machine, chapter 5
- Torque-slip characteristic and linear operating region · Electrical Machines I, chapter 5
- MIT classification system · Soil Mechanics, chapter 3 · asked 2×