--- title: "Transformer coupled class B, max efficiency, crossover" subject: "Electronics Devices and Circuit" chapter: 4 key: "semester_3/electronics_devices_and_circuit:transformer-class-b-efficiency-crossover" source: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/transformer-class-b-efficiency-crossover site: BCT Engineering --- # Transformer coupled class B, max efficiency, crossover Electronics Devices and Circuit · Chapter 4: Output Stages and Power Amplifiers - Asked in: 2078 Bhadra, 2+4+2 marks - Video: [Amplifier #4. Class B Push-Pull: Achieve Max Efficiency & Minimal Distortion - Full Circuit Analysis](https://www.youtube.com/watch?v=8xqBUOdg7pc) — Technologies Discussion - 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/transformer-class-b-efficiency-crossover - Whole chapter: https://www.bctengineer.com/md/semester_3/electronics_devices_and_circuit/chapter-4 ## Question Draw the circuit diagram of transformer coupled class B push-pull amplifier stage. And find its maximum efficiency. Define cross over distortion in class B amplifier. ## Short answer The circuit uses a centre‑tapped input transformer to drive two NPN transistors in push‑pull, with an output transformer combining half‑cycles. Maximum efficiency is π/4 = 78.5%. Crossover distortion is a dead zone at the zero crossing where neither transistor conducts for |Vbe| < 0.7 V, causing output flattening. ## Answer #### Circuit diagram ```text T1 (input) T2 (output) +--- B1 Q1 (NPN) C1 ---+ | E1 | o----+ || +--------+ | +--------+ || +----+ ) || ( | ( || ) | Vi ) || (-- CT -- GND GND VCC -- CT --( || ) [RL] ) || ( | ( || ) | o----+ || +--------+ | +--------+ || +----+ | E2 | +--- B2 Q2 (NPN) C2 ---+ ``` The centre-tapped input transformer T1 gives two drives of opposite phase to the bases of Q1 and Q2; both emitters are grounded. Q1 conducts on one half-cycle and Q2 on the other. The centre tap of the output transformer's primary is connected to VCC, and the collectors go to its two ends. The two half-cycle currents combine in the secondary to give the full waveform across RL. #### Maximum efficiency Let the peak collector current be Im and the peak voltage across one half of the primary be Vm = Im R'L, where R'L = (N1/N2)² RL is the load reflected into one half of the primary (N1 = turns on each half of the primary, N2 = secondary turns). - Output power: Po = Im² R'L/2 - Input power: Pi = VCC · (2Im/π) (two half-sine current pulses) η = Po/Pi = (π/4)(Im R'L/VCC) = (π/4)(Vm/VCC) The maximum swing is Vm = VCC, so **ηmax = π/4 = 78.5 %** #### Crossover distortion Crossover distortion is the distortion at the zero crossing of the output of a class B stage. For inputs between about −0.7 V and +0.7 V, neither transistor is forward biased, so the output stays at zero (a dead zone) and the waveform is flattened where it crosses zero. The effect is most visible at low signal levels. It is cured by biasing the transistors slightly above cut-off (class AB). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does the transformer-coupled class B stage produce the full output waveform? A. Q1 and Q2 conduct together all the time B. Only Q1 is used C. A capacitor stores each half-cycle D. Each transistor conducts on opposite half-cycles and the output transformer combines the currents Answer: D. The input transformer gives opposite-phase drives; the two half-currents combine in the output transformer secondary. (Key point: Push-pull operation) 2. What is the maximum efficiency and when does it occur? A. 50% at Vm = VCC B. 78.5% (25π %) at Vm = VCC C. 78.5% at Vm = VCC/2 D. 100% at Vm = 2VCC Answer: B. η = (π/4)(Vm/VCC), maximal when the swing Vm equals VCC. (Key point: Maximum efficiency) 3. What is crossover distortion? A. Distortion from too much gain B. Flattening at the zero crossing because neither transistor conducts for inputs within about 0.7 V C. Clipping at the supply voltage D. Noise from the transformer Answer: B. A dead zone around zero input gives zero output; it is cured by biasing slightly above cut-off (class AB). 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