Class D Power Amplifier

This article intends to cover basic concepts of switching mode power amplifier, and the design of class D power amplifier.

Simple Switching Amplifier

Schematic of switching amplifier

Current and voltage waveform

where we have

As for the fundamental components

Mathematica code for Fourier series

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f[x_] := Piecewise[{{0, -a <= x <= a}, {1, x < -a}, {1, x > a}}]
FourierCosCoefficient[f[x], x, 1]

Hence, we can calculate the efficiency as

To compare with the class A PA, we write the output power as

We can plot the efficiency and PUF vs. conduction angle

Efficiency and PUF vs conduction angle

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Eff[a_] := 2 Sin[a]^2/a/(Pi - a);
PUF[a_] := 8 Sin[a]^2/Pi/(Pi - a);
Plot[{Eff[a], PUF[a]}, {a, 0, Pi},
PlotLegends -> "Expressions"]

The peak power occurs at a conduction angle of around 113 degrees. Though the structure dissipate no heat, the peak efficiency is about 81%, as some of the power is wasted in harmonic components.

Switching Amplifier with Harmonic Short

To remove harmonic components, we can place a harmonic short across the load.

Switching amplifier with harmonic short

In this case, the voltage waveform must assume a sinusoidal form.

Waveform of the switching amplifier with harmonic short

Repeat the analysis, we have

It can be plotted as

Efficiency and PUF vs conduction angle of switching amplifier with harmonic short

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Eff[a_] := Sin[a]/a;
PUF[a_] := 4 Sin[a]/Pi;
Plot[{Eff[a], PUF[a]}, {a, 0, Pi},
PlotLegends -> "Expressions"]

The peak power occurs at a conduction angle of 90 degrees with an efficiency of 63 %.

Class D Power Amplifier

Class D power amplifier

The key point is that the current in the LCR branch is constrained to remain sinusoidal. With a sufficiently large bypass capacitor, the voltage $v_{sw}$ would remain $V_{dc}$ when the switch is turned to A, and remain zero when turned to B.

Waveform of class D power amplifier

We have

It can achieve theoretically 100 % efficiency with 1 dB higher power than class A amplifier.

A realization

Schematic

Simulated result

Reference

  1. S. C. Cripps. RF Power Amplifier for Wireless Communications. Artech House, 2014.
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