--- title: "Classification of amplifiers by operating region" subject: "Electronics Devices and Circuit" chapter: 4 key: "semester_3/electronics_devices_and_circuit:amplifier-classification-operating-region" source: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/amplifier-classification-operating-region site: BCT Engineering --- # Classification of amplifiers by operating region Electronics Devices and Circuit · Chapter 4: Output Stages and Power Amplifiers - Asked in: 2081 Baisakh, 4 marks - Video: [Transistor Amplifiers - Class A, AB, B, & C Circuits](https://www.youtube.com/watch?v=0e_OUyGCaBs) — The Organic Chemistry Tutor - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/amplifier-classification-operating-region - Whole chapter: https://www.bctengineer.com/md/semester_3/electronics_devices_and_circuit/chapter-4 ## Question Explain the classification of amplifiers according to 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. ## Answer 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. ```text Class A Class B Class AB Class C ~~~~~~~ /\ /\ /\ /\ ^ ^ full wave half wave >half short pulses ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the basis for classifying power amplifiers as A, B, AB and C? A. The supply voltage used B. The load resistance C. The portion of the input cycle for which the device conducts (conduction angle) D. The number of stages Answer: C. Classes are defined by conduction angle, which depends on the Q point set by bias. (Key point: Basis of classification) 2. Which describes class B? A. Conducts 360 degrees, biased at centre of load line B. Conducts less than 180 degrees, biased below cut-off C. Conducts 180 degrees, biased at cut-off, needs two devices in push-pull D. Conducts 180 degrees, biased in saturation Answer: C. Class B is biased at cut-off so each transistor conducts half a cycle; two are needed for the full wave. (Key point: Class B properties) 3. Why is class A efficiency low even though distortion is lowest? A. It conducts only briefly B. It draws constant dc current regardless of signal C. It needs a tuned load D. It has crossover distortion Answer: B. The Q point is mid-load-line so dc current flows all the time, wasting power (max 25% or 50%). (Key point: Class A trade-off) 4. Why must a class C amplifier use a tuned LC load? A. To increase dc current B. To raise conduction angle C. To bias the transistor D. To restore a sine wave from short heavily distorted current pulses Answer: D. Class C conducts for less than half a cycle, and the tuned tank restores a sinusoid; used in RF power amplifiers. 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