--- title: "Crossover distortion and quasi-complementary class AB" subject: "Electronics Devices and Circuit" chapter: 4 key: "semester_3/electronics_devices_and_circuit:crossover-quasi-complementary-class-ab" source: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/crossover-quasi-complementary-class-ab site: BCT Engineering --- # Crossover distortion and quasi-complementary class AB Electronics Devices and Circuit · Chapter 4: Output Stages and Power Amplifiers - Asked in: 2075 Chaitra, 7 marks; 2068 Baisakh, 7 marks (asked 2 times) - 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/crossover-quasi-complementary-class-ab - Whole chapter: https://www.bctengineer.com/md/semester_3/electronics_devices_and_circuit/chapter-4 ## Question Define crossover distortion in class B amplifier. Draw quasi-complementary symmetry class AB amplifier. And explain how crossover distortion is eliminated in class AB amplifier. ## Short answer Crossover distortion is a flattened dead zone near zero crossing because neither output transistor conducts for |vi| < 0.7 V. Quasi-complementary stage uses an NPN Darlington above and an NPN power device with PNP driver (feedback pair) below. Class AB eliminates distortion by diode biasing that keeps both transistors slightly on, ensuring smooth handover. ## Answer #### Crossover distortion In a class B push-pull amplifier, each transistor conducts only when its base-emitter voltage exceeds about 0.6 to 0.7 V. For a small input, between about −0.7 V and +0.7 V, neither transistor conducts, so the output stays at zero near the zero-crossing. The output waveform is therefore flattened (a dead zone) at each crossing between the positive and negative half-cycles. This harmonic distortion is called **crossover distortion**. It is most serious for small signals. #### Quasi-complementary symmetry class AB amplifier When complementary power transistors (a matched NPN and PNP) are not available, both output transistors are NPN. The upper half is an NPN Darlington. The lower half uses a PNP driver and an NPN power device in a feedback-pair (Sziklai) arrangement, which behaves like a single PNP transistor. ```text +VCC ---+------------------+------------------+ | | | [R1] C C | | | +--------------B Q1 (NPN driver) | | E --------------B Q2 (NPN power) D1 E D2 | Vi o-||--+ [Re] D3 | | +--------o Vo | | | | [Re] [RL] | | | | +------------------+ GND | E C +--------------B Q3 (PNP driver) | | C --------------B Q4 (NPN power) [R2] E | | -VCC ---+-------------------------------------+ ``` (Upper half: NPN Darlington Q1–Q2. Lower half: the PNP driver Q3 drives the base of the NPN power transistor Q4; Q4's collector is joined to Q3's emitter, forming the feedback pair, and Q4's emitter goes to −VCC. Exact wiring varies slightly between textbooks.) #### How class AB removes crossover distortion 1. The diodes D1–D3 (or a VBE multiplier) between the two driver bases drop about 2 V, one diode drop for each base-emitter junction in the path (two in the upper Darlington, one for Q3). This is just enough to forward bias the output stage. 2. At zero input both output halves carry a small quiescent current (class AB). The conduction angle of each is slightly more than 180°. 3. When the input goes positive, the upper transistor increases its current while the lower one smoothly reduces its current to zero (and vice versa). The two currents add through the crossover with no gap. 4. The diodes are mounted close to the output transistors, so the bias tracks temperature and prevents thermal runaway. The small Re resistors add stability. The output is then a smooth sine wave. The cost is a small idle current and an efficiency a little lower than that of ideal class B. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does crossover distortion occur in a class B stage? A. Neither transistor conducts while the input is below about 0.7 V, giving a dead zone B. The supply voltage is too high C. Both transistors conduct together D. The load is too small Answer: A. Each transistor needs about 0.6 to 0.7 V base-emitter to conduct, so near zero crossing the output is flat. (Key point: Cause of crossover distortion) 2. In a quasi-complementary stage, how is the lower half built? A. Two PNP power transistors B. A PNP driver with an NPN power transistor as a feedback pair acting like a PNP C. A single NPN transistor D. A zener diode and a resistor Answer: B. When matched complementary power devices are unavailable, a PNP driver plus NPN power device in a feedback pair behaves like a PNP. (Key point: Quasi-complementary arrangement) 3. What is the purpose of diodes D1 and D2 between the driver bases? A. To clip the signal B. To act as the load C. To drop about 1.2 to 1.4 V so the output junctions are just forward biased D. To supply the signal Answer: C. Their forward drop biases the output base-emitter junctions at the edge of conduction, so both transistors carry a small quiescent current. (Key point: Role of diodes) 4. In class AB, what happens as the input goes positive through the crossover? A. Both transistors switch off B. The upper transistor turns off before the lower turns on C. The upper current increases while the lower smoothly falls to zero, so the currents add with no gap D. Only the lower transistor conducts Answer: D. Both conduct slightly at the crossing, so the handover is smooth and the dead zone is removed. (Key point: Smooth handover) ## Related in Electronics Devices and Circuit - [Quasi complementary-symmetry class AB amplifier](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Aquasi-complementary-class-ab) — Electronics Devices and Circuit, chapter 4 - [Complementary-symmetry class AB with Darlington pair](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Acomplementary-symmetry-class-ab) — Electronics Devices and Circuit, chapter 4, asked 6× - [Crossover distortion and its elimination](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Acrossover-distortion) — Electronics Devices and Circuit, chapter 4, asked 6×