--- title: "Why class A amplifier is cooler with load" subject: "Electronics Devices and Circuit" chapter: 4 key: "semester_3/electronics_devices_and_circuit:class-a-cooler-with-load" source: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/class-a-cooler-with-load site: BCT Engineering --- # Why class A amplifier is cooler with load Electronics Devices and Circuit · Chapter 4: Output Stages and Power Amplifiers - Asked in: 2073 Chaitra, 6 marks - 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/class-a-cooler-with-load - Whole chapter: https://www.bctengineer.com/md/semester_3/electronics_devices_and_circuit/chapter-4 ## Question Explain why class A amplifier is cooler with load than without load. ## Short answer Class A amplifier's dc input power is constant. When no load or no signal, all power heats the transistor. With a load and ac signal, some power goes to the load as Po, so the transistor dissipates less heat, running cooler. ## Answer A class A amplifier draws a constant dc power Pi = VCC · ICQ from the supply, whether or not a signal is present. This power is shared between the transistor, the load and any resistors in the circuit. #### Without a signal and without a load (no ac power delivered) Po = 0, so all the input power that is not lost as dc in resistors is dissipated in the transistor. In a transformer-coupled stage (negligible dc resistance in the primary): PD = Pi − Po = Pi (maximum) The transistor is at its hottest. In a class A stage the dissipation is maximum under no-signal conditions. #### With a load and a signal Part of the input power is converted to useful ac power Po delivered to the load, so the heat in the transistor falls: PD = Pi − Po (minus any dc power lost in resistors such as RC) **Example 1 (transformer-coupled):** VCC = 12 V, ICQ = 60 mA, so Pi = 0.72 W. With no signal, PD = 0.72 W. At full swing (50 % efficiency) Po = 0.36 W, so PD falls to 0.36 W, half the no-signal value. **Example 2 (series-fed):** VCC = 12 V, RC = 100 Ω, Q point at the centre: ICQ = 60 mA, VCEQ = 6 V, Pi = 0.72 W. Here RC is the load, and it also takes dc power ICQ² RC = 0.36 W, so with no signal the transistor dissipates VCEQ · ICQ = 0.36 W. At full swing RC also receives Po = 0.18 W of ac power (25 % efficiency), so the transistor dissipation falls to 0.36 − 0.18 = 0.18 W. Explanation in points: 1. Input power is constant (Pi = VCC ICQ), independent of the signal. 2. With the load connected, the signal power Po leaves the transistor and goes to the load. 3. So the heat in the transistor is lower, and it runs cooler. 4. Without a load (or without a signal) Po = 0, so the transistor dissipates the most (PD = Pi in a transformer-coupled stage): it is at its hottest. This is why a class A amplifier must be designed for the no-signal worst case, and the heat sink is chosen for the no-signal dissipation PD = VCEQ · ICQ. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does the input power of a class A amplifier change with signal? A. It rises with signal B. It falls with signal C. It is zero without signal D. It stays constant, Pi = VCC ICQ Answer: D. The dc bias current is fixed, so the supply delivers constant power whether or not a signal is present. (Key point: Constant input power) 2. Which expression gives the transistor dissipation? A. PD = Pi - Po B. PD = Pi + Po C. PD = Po D. PD = Pi x Po Answer: A. Power not delivered to the load is dissipated in the transistor. (Key point: Transistor dissipation) 3. Why is the transistor cooler with a load and signal than without? A. The supply voltage drops B. The load reduces ICQ to zero C. Part of the constant input power leaves as ac power in the load, so less heat remains in the transistor D. The transistor is switched off Answer: C. With Po greater than zero, PD = Pi - Po is smaller; with no signal or load, PD = Pi, the maximum. (Key point: Why cooler with load) 4. For which condition is a class A stage and its heat sink designed? A. Full signal B. No signal, where PD = Pi C. Half signal D. Short-circuited input Answer: B. Dissipation is highest with no signal, so the design must cover PD = Pi. 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