Power Amplifiers Class A

A Class A power amplifier conducts current through its output transistor for the full 360 degrees of every input cycle. The LM386 audio amplifier IC operates in Class A, and it is why your phone speaker runs warm after playing music for an hour.

Core Concept

Class A operation means the transistor is biased to a quiescent point (Q-point) in the middle of its load line. The collector current never falls to zero. Both positive and negative halves of the input signal ride on top of this DC bias current.

Because the transistor conducts continuously, it dissipates power even with no input signal. At best, a Class A amplifier with a resistive load can convert 25% of the DC supply power into useful AC output power. The rest becomes heat. A transformer-coupled Class A stage can reach 50% efficiency in theory, but practical circuits rarely exceed 40%.

Despite poor efficiency, Class A amplifiers produce very low crossover distortion because there is no transition region where the transistor switches on or off. High-end audio amplifiers like those built around the 2N3055 use Class A for this reason.

Key Equations

Maximum efficiency (resistive load): η_max = 25%

Maximum efficiency (transformer coupled): η_max = 50%

DC power supplied: PDC = VCC × ICQ where ICQ is the quiescent collector current.

AC output power (peak-to-peak swing Vpp, load RL): Pac = Vpp² / (8 × RL)

Transistor dissipation: Pdiss = PDC - Pac Maximum transistor dissipation occurs at zero signal input.

Q-point for maximum swing: ICQ = VCC / (2 × (RC + RE))

Given:
  VCC = 12V
  RC = 100 Ω (load)
  RE = 47 Ω
  ICQ = 40 mA (quiescent current)

Why this formula:
  Class A efficiency = Pac / PDC; max output swing
  occurs when Q-point is centred on the load line.

Formula:
  PDC = VCC × ICQ
  Pac_max = (VCC²) / (8 × RC)   [resistive load, centred Q]
  η = Pac_max / PDC × 100%

Substitution:
  PDC = 12 × 0.040 = 0.48 W

  Max Vswing peak = ICQ × RC = 0.040 × 100 = 4V
  Pac_max = (4)² / (2 × 100) = 16 / 200 = 0.08 W

Calculation:
  η = 0.08 / 0.48 × 100% = 16.7%

  Transistor dissipation at max signal:
  Pdiss = 0.48 - 0.08 = 0.40 W

Final Answer:
  PDC = 480 mW
  Pac_max = 80 mW
  Efficiency = 16.7% (well below the 25% theoretical maximum)
  Transistor must dissipate 400 mW even at full output

Exam Tip: The maximum theoretical efficiency of a Class A amplifier is 25% for a resistive collector load and 50% for a transformer-coupled load. GATE questions often give a circuit and ask for efficiency or transistor dissipation. Remember: maximum dissipation in the transistor happens at zero input signal, not at maximum signal. Students frequently get this backwards and write that peak signal causes peak transistor heating.

Key Properties

• Conduction angle is 360 degrees. The transistor conducts for the entire input cycle without interruption.
• Maximum efficiency is 25% with a resistive load. Transformer coupling raises the limit to 50%.
• Lowest distortion of all amplifier classes because there is no crossover distortion or clipping at small signals.
• Maximum transistor power dissipation occurs at zero AC input, when all DC power is wasted as heat.
• The Q-point is set near the centre of the AC load line: ICQ ≈ VCC / (2RL) for maximum undistorted swing.
• Used in small-signal pre-amplifier stages, high-fidelity audio output stages, and RF driver stages where linearity matters more than efficiency.
• A BD139 (NPN, 80V, 1.5A) or 2N3055 (NPN, 60V, 15A) are typical Class A output transistors in audio circuits.

Quick Revision

• Conduction angle: 360° (transistor always ON).
• Max efficiency: 25% (resistive), 50% (transformer coupled).
• Q-point centred on load line for maximum undistorted swing.
• PDC = VCC × ICQ; Pac = Vp² / (2RL).
• Pdiss is maximum at zero signal input, not at full signal.
• Low distortion, very low efficiency. Opposite of Class D.
• Increasing the quiescent current improves linearity but lowers efficiency.
• Exam trap: writing maximum transistor dissipation occurs at maximum signal power. It actually occurs at zero signal, because with no AC output, all PDC is dissipated in the transistor.