Class A Power Amplifier

Design a single-ended Class A power amplifier using an emitter follower output stage. Class A operates with the transistor conducting for the full cycle - low distortion but limited efficiency (max ~25% with resistive load, ~50% with inductive/transformer coupling).

Power Requirements

Supply voltage (Vcc): V
Load impedance: Ω
Quiescent current (Iq): A
Low frequency (-3dB): Hz

Transistor Selection

Transistor:
β (hFE):
Ic max: A
Pd max: W
θjc: °C/W

Thermal Environment

Ambient temperature: °C
Max junction temp: °C
Heatsink (θsa): °C/W

Hover over labels for explanations.

Schematic

Component Values

R1 (bias):
R2 (bias):
Re (emitter):
Cin:
Cout:

Operating Point

Quiescent current (Ic):
Emitter voltage (Ve):
Collector-Emitter voltage (Vce):
AC load (Re ∥ Rload):

Input Requirements

This is an emitter follower (voltage gain ≈ 1). The input signal must be approximately equal to the desired output voltage. A preamp or driver stage is typically needed.

Input voltage (peak):
Input voltage (RMS):
Input impedance:

Power Analysis

Max output power:
Output voltage (RMS):
Output current (RMS):
DC input power:
Efficiency at full power:

Thermal Analysis

Thermal limit:
Dissipation (idle):
Dissipation (full power):
Junction temp (idle):

Load Line

The DC load line (solid) shows the path with no signal. The AC load line (dashed) shows the operating path when driving the speaker load through the coupling capacitor.

Output Waveforms

Design Notes

About this topology

This calculator designs an Emitter Follower (common collector) Class A amplifier with capacitor-coupled output. The emitter resistor (Re) sets the DC bias point, while the load (speaker) is AC-coupled through Cout.

Why Capacitor-Coupled Output?

Pros
  • No DC current through the speaker (protects voice coil)
  • Load impedance doesn't affect bias point
  • Can drive any load impedance without redesigning bias
Cons
  • Large electrolytic capacitor required for bass response
  • Low frequency roll-off (high-pass filter effect)
  • Re wastes power even when driving the load

Direct-Coupled Alternative

Without Cout, the load replaces Re entirely:

Pros
  • No coupling capacitor needed
  • Full bandwidth down to DC
  • Slightly better efficiency (no Re power loss)
Cons
  • DC current flows through speaker (can damage it)
  • Bias point depends on load impedance
  • Changing speakers requires recalculating bias

Emitter Follower vs Common Emitter

Emitter Follower (this design) Common Emitter
Voltage gain ≈ 1 (no gain) High (10-100+)
Current gain High (β) High (β)
Output impedance Low (good for driving speakers) High (needs buffer for speakers)
Input impedance High Medium
Distortion Very low (100% negative feedback) Higher (less inherent feedback)
Typical use Output/buffer stage Voltage amplifier/preamp stage

Bottom line: The emitter follower is ideal as an output stage because of its low output impedance and low distortion, but it requires a preamp to provide voltage gain. A complete amplifier often combines both: a common emitter stage for voltage gain feeding an emitter follower for current gain and speaker drive.

Why is Class A inefficient?

In Class A, the transistor conducts continuously. The quiescent current must be at least as large as the peak output current to avoid cutoff distortion.

Power Budget

PDC = Vcc × Ic (constant, regardless of signal)
Pout = Vout(rms)² / Rload
Pdissipated = PDC - Pout

Maximum Efficiency

For an ideal emitter follower with resistive emitter load:

ηmax = 25% (when Vswing = Vcc/2)

With transformer or inductor coupling (no DC drop across load):

ηmax = 50%

Worst Case Dissipation

Class A dissipates maximum power at idle (no signal), not at full power. This is the opposite of Class B/AB amplifiers.