Circuit building blocks

Amplifier classes

A power amplifier makes a small signal source big enough to drive a speaker or an aerial. Its class says how long the output transistors are switched on during each wave, which trades sound quality against wasted heat.

Think of it as: Class A is a car with the engine always revving; class D is a dimmer flicking a light on and off too fast to see.

The variants

Class A

One transistor is biased half-on, so it conducts for the whole wave and copies it smoothly. The price: the same current flows even in silence, so it runs hot.

Conducts: 360° (all of the wave) · Best efficiency: 50 % (25 % with a plain resistor load) · Sound: Cleanest

Class B

Two transistors push and pull: Q1 (NPN) handles the positive half, Q2 (PNP) the negative half. Each is off half the time, so far less heat. But neither conducts until the input clears about 0.6 V, which leaves a gap where the halves meet.

Conducts: 180° per transistor · Best efficiency: 79 % (π/4) · Sound: Crossover glitch at every zero

Class AB

The same push-pull pair, but two diodes hold both bases about 0.6 V apart, so each transistor is already slightly on. The halves overlap, the gap disappears, and it wastes only a little more than class B.

Conducts: Just over 180° each · Efficiency: About 50 to 74 % · Sound: Clean

Class C

The transistor is biased off and only kicks for a short burst at each peak. On its own that would be a mess of pulses, but a tuned inductor-capacitor tank rings like a bell and turns each kick back into a sine at one frequency.

Conducts: Under 180° (here 120°) · Best efficiency: About 90 % · Sound: Not for audio: one frequency only

Class D

The MOSFETs never sit half-on: they switch fully on or off hundreds of thousands of times a second (PWM). The width of each pulse follows the input, and an LC filter averages the pulses back into the audio wave.

Switches at: 200 kHz to 1 MHz (slowed here) · Typical efficiency: Over 90 % · Sound: Clean after the filter

The classes side by side

Class	On for	Efficiency	Heat when silent	Used in
A	360°	≤ 50 %	High	Preamps, headphone and guitar amps
B	180° each	≤ 79 %	None	Rare alone (crossover glitch)
AB	Just over 180°	50–70 %	Low	Classic hi-fi, car amps
C	< 180°	≈ 90 %	None	Radio transmitters
D	Switching	> 90 %	Very low	Bluetooth speakers, phones, TVs

Other letters you may meet

Classes G and H are class AB amplifiers that switch to a higher supply rail only for loud peaks, which cuts the heat during normal listening.

Where you’ll use it

Driving speakers and headphones (class AB and D).
Small audio boards like the PAM8403 and MAX98357 are class D.
Radio transmitters (class C).
Op-amps use a small class AB push-pull stage at their output.

Common mistakes

No heatsink on a class A or AB output stage. They make heat even with no sound.
Leaving out the bias diodes in a push-pull stage and wondering why quiet music sounds rough (crossover distortion).
Long, unfiltered speaker wires on class D. The fast switching radiates noise into nearby radios.

Lessons that use it

Op-amps (3.3) end in a push-pull stage; MOSFET switching (3.2) and PWM (4.3) are what class D is built from.

Go deeper

\eta = \dfrac{\theta - \sin\theta}{4\,(\sin\tfrac{\theta}{2} - \tfrac{\theta}{2}\cos\tfrac{\theta}{2})}

Best-case efficiency for a conduction angle θ: 50 % at θ = 2π (class A), π/4 at θ = π (class B), approaching 100 % as θ shrinks (class C).

P_{\text{heat, A}} = U_{CC}\,I_Q \quad\text{(even with no signal)}

A class A stage burns its bias current all the time, so its heat is highest when silent.

P_{\text{loss, D}} \approx I^2 R_{DS(on)} + f_{sw}\,(E_{on}+E_{off})

Class D loses a little in the on-resistance and a little at every switch edge, which is why switching faster costs efficiency.