Linear (Class A/B) vs Switching (Class D) Amplifier
Linear amplifiers (Class A, AB, B) and switching amplifiers (Class D, E) represent two fundamentally different approaches to power amplification. Linear amplifiers are simple and low-distortion; switching amplifiers achieve high efficiency but require output filtering. The right choice depends on efficiency, audio quality, and operating frequency.
Linear Amplifier (Class A/AB)
A linear amplifier operates with the active device (transistor) in continuous conduction, following the input signal linearly. Class A conducts 360°; Class AB conducts slightly more than 180°. Both are inherently linear but dissipate significant power as heat.
Advantages
- Low distortion — THD < 0.01% common for Class AB audio amplifiers
- No switching — no EMI at switching frequency harmonics
- Simple output stage — no LC filter required
- Wide bandwidth — not limited by LC filter cutoff
Disadvantages
- Low efficiency: Class A ≤ 25%, Class AB ≤ 78%
- High heat dissipation — requires large heatsinks at high power
- Large physical size and weight compared to Class D at same power
- Higher cost per watt at power levels above ~100 W
When to use
Use linear amplifiers for precision RF, audio pre-amplifiers, studio amplifiers, and any application where low distortion and wide bandwidth matter more than efficiency.
Switching Amplifier (Class D)
A Class D amplifier switches the output transistors at high frequency (200 kHz–1 MHz) with a duty cycle proportional to the input signal. A low-pass LC filter reconstructs the audio. Efficiency reaches 90–98%.
Advantages
- High efficiency (90–98%) — minimal heat dissipation
- Compact and lightweight — small heatsinks, small inductors at high switching frequency
- Cost-effective at high power levels (100 W+)
- Now standard in consumer audio, automotive, and PA systems
Disadvantages
- Requires LC output filter — adds cost, size, and phase shift
- Switching frequency and harmonics require EMC filtering
- Higher THD than Class AB if not carefully designed (though modern designs achieve < 0.01%)
- Filter design critical for stable operation with reactive speaker loads
When to use
Use Class D for consumer audio, automotive amplifiers, PA systems, and battery-powered portable audio where efficiency and size are critical. Modern Class D ICs achieve performance rivaling Class AB.
Key Differences
- ▸Class AB efficiency ≤ 78%; Class D efficiency 90–98%
- ▸Class D requires LC output filter; Class AB does not
- ▸Class D generates switching harmonics (EMI); Class AB generates no switching noise
- ▸Class D is dominant in consumer electronics (phones, laptops, PA); Class AB in precision audio
- ▸At 100 W output, Class AB wastes ~50 W as heat; Class D wastes ~5–10 W
Summary
Class D has largely replaced Class AB in consumer and high-power audio applications due to its superior efficiency, smaller size, and lower cost. Class AB remains preferred for precision audiophile applications, RF power amplifiers, and instrumentation where low distortion is paramount. For battery-powered or high-power (>50 W) applications, Class D is almost always the better choice.
Frequently Asked Questions
Is Class D audio quality as good as Class AB?
Modern Class D ICs achieve THD+N below 0.003% and SNR above 110 dB — comparable to Class AB. The key is the output filter design, dead time optimization, and feedback topology. For most listening purposes, Class D is indistinguishable from Class AB.
What is Class A vs Class AB efficiency?
Class A has theoretical maximum efficiency of 25% (single-ended) or 50% (push-pull). Class AB improves this to 50–78% by operating near Class B (180° conduction) with a small bias current to eliminate crossover distortion. Class AB is the standard linear topology for audio power amplifiers.
What is a Class E amplifier?
Class E is a switching RF power amplifier topology that achieves > 90% efficiency at RF frequencies (10 MHz–GHz range) by shaping the transistor voltage and current waveforms to avoid simultaneous high V and high I. Used in RF power amplifiers, wireless power, and transmitters. Not suitable for audio.
Why does Class D amplifier need a low-pass filter?
The Class D output is a PWM signal at the switching frequency (200 kHz–1 MHz). Without filtering, the loudspeaker would receive the full switching waveform, causing high-frequency heating and potential damage. The LC low-pass filter reconstructs the audio signal below 20 kHz and blocks the switching frequency and harmonics.