Classy Audio Amplifiers, Part 4

Steve Taranovich
Jan 8 · 3 min read
Harman Crown Audio XLS DriveCore 2 series two-channel power amplifiers using a proprietary Class D amplifier design

Here is our final section on audio amplifiers. Before I sum up the remaining classes of audio amps, I feel it is warranted that I give you a few more details on Class D amplifiers than I had in Part 3 of this series; Class D is a really interesting technical concept for audio.

Class D

Most other classes of audio amps have higher output power dissipation than Class D. Class D amps have lower power dissipated at their output which means less heat produced, better board footprint and lower cost; they are excellent for battery operation.

This topology has an output stage that produces a voltage pulse train. Their output transistors have no current in them when they are not switching. When they do switch, there is low VDS upon conducting and hence lower power dissipation.

Audio signals are converted into pulses via a modulator, followed by a low-pass filter to keep EMI low and not drive excess high frequency energy into the speakers. Figure 1.

Figure 1: The differential switching output stage and low-pass filter of a Class D audio amplifier (Image from Analog Devices)

Substituting GaN transistors in Figure 1 provides for lower losses. Thus, feedback is reduced, which lowers Total Intermodulation Distortion (T-IMD) and Damping Factor (DF) — — the ratio between the nominal load impedance (typically 8W), as well as enabling lower source impedance of the amplifier, increasing sound quality. See this article by EPC.

Class C, E and F

Class C is only used for RF (radio frequency) applications using a tuned (inductor/capacitor (LC) ‘tank’) circuit to minimize waveform distortion. It operates over a limited frequency range in which the tank circuit is resonant. Output device conduction time is less than 180°. These amplifiers are common below 100 kHz.

Classes E and F, similar to Class C, use RF amplifier topologies that rely on LC tank circuits. Class E amps are used in the VHF and microwave frequency ranges. The difference between Class-E and Class-C amplifiers is that the active device is used as a switch with Class-E, rather than operating in the linear portion of its transfer characteristic.

Class-F amplifiers are, offshoots of Class AB, are similar to Class-E amplifiers, but use a more complex load network. This network improves the impedance match between the load and the switch. Class-F eliminates the input signal’s even harmonics; thus, the switching signal is close to being a square wave. This improves efficiency because the switch is either saturated or turned off.

Class G

This amplifier is very common for high-power amplifiers used in sound reinforcement applications. They are often very powerful (2kW or more in some cases), but more efficient than Class-AB. At low power, a Class-G amp operates from low voltage supply rails, minimizing output transistor dissipation. When necessary, the signal draws current from the high voltage supply rails, using a second set of transistors to provide the signal peaks.

Class H

Class H uses rail modulation. When in low demand operation, it uses a lower rail voltage than the Class A/B designs — this reduces power dissipation. When high power is needed, this amp dynamically increases the rail voltage to handle high amplitude transients.

Happy listening!

Written for @SupplyFrame

Supplyframe

Discussing the business of hardware and hardware manufacturing.

Steve Taranovich

Written by

BEEE NYU, MSEE Brooklyn Polytech, Eta Kappa Nu Honor Society, IEEE Educational Activities Chairman, Electronics Design Engineer 40 years, Tech writer 9 years

Supplyframe

Discussing the business of hardware and hardware manufacturing.

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