This document discusses different types of amplifier classes and their characteristics. It describes how class C amplifiers use a tuned circuit to generate a full sine wave output from a non-linear output and how class D amplifiers require a pulsed input signal. It also summarizes the advantages of class D amplifiers, including increased efficiency and reduced size, and lists some common uses like home theater and powered speakers.

1. Amplifier Distortion
If the output of an amplifier is not a complete AC sine wave,
then it is distorting the output. The amplifier is non-linear.
This distortion can be analyzed using Fourier analysis. In
Fourier analysis, any distorted periodic waveform can be
broken down into frequency components. These
components are harmonics of the fundamental frequency.
1

2. Harmonics
Harmonics are integer multiples of a fundamental frequency.
If the fundamental frequency is 5kHz:
1st harmonic 1 x 5kHz
2nd harmonic 2 x 5kHz
3rd harmonic 3 x 5kHz
4th harmonic 4 x 5kHz
etc.
Note that the 1st and 3rd harmonics are called odd harmonics and the
2nd and 4th are called even harmonics.
2

3. Harmonic Distortion
According to Fourier
analysis, if a signal is not
purely sinusoidal, then it
contains harmonics.
3

4. Harmonic Distortion Calculations
Harmonic distortion (D) can be calculated:
An
% nth harmonicdistortion  %Dn   100
A1
where
An is the amplitude of the fundamental frequency
An is the amplitude of the highest harmonic
The total harmonic distortion (THD) is determined by:
% THD  D 2  D 2  D 2    100
2 3 3
4

5. Class C Amplifiers
A class C amplifier conducts for less
than 180. In order to produce a full
sine wave output, the class C uses a
tuned circuit (LC tank) to provide
the full AC sine wave.
Class C amplifiers are used
extensively in radio communications
circuits.
5

6. In Class C, the amplifying device is deliberately not
operated linearly. Instead, it is operated as a switch in
order to reduce resistance loss. The anode conduction
angle in Class C operation is usually made as short as is
possible. In effect, the tank circuit makes the RF output
sine wave--like a bell that is struck at a constant rate by a
hammer.

7. During the positive period of the input signal (On stage)
During the positive period of the input signal the transistor will conduct (On-state). You can imagine that the
transistor is a switch which connects the emitter with the collector.
What will happened now is that the current I1 (red) flow through the coil and then into the transistor and down to
ground. A magnetic field builds up in the coil depending on the magnitude of the current. At the same time the
voltage over the capacitor discharge through the resistor making another current flow I2 (blue) also through the
transistor. The I2 current passes the resistor (antenna) which radiate the energy.
During the negative period of the input signal (Off stage)
During the negative period of the input signal the transistor will not conduct (Off-state). You can imagine that the
transistor is an open switch. No current can pass through the collector to the emitter.
The magnetic filed which was build up in the coil will now collapse and generate a current I1 (red) which will flow
through the capacitor and into the resistor (antenna).

8. Class D Amplifier
A class D amplifier amplifies
pulses, and requires a pulsed
input.
There are many circuits that
can convert a sinusoidal
waveform to a pulse, as well
as circuits that convert a
pulse to a sine wave. This
circuit has applications in
digital circuitry.
8

9. The term "class D" is sometimes misunderstood as
meaning a "digital" amplifier. While some class D amps
may indeed be controlled by digital circuits, the power
stage deals with voltage and current as a function of time.
The smallest amount of noise, timing uncertainty, voltage
ripple or any other non-ideality immediately results in an
irreversible change of the output signal. A digital circuit also
uses physics to operate, but those same errors will only
lead to incorrect results when they become so large that a
signal representing a digit is distorted beyond recognition

10. Topologies
• There are basically two Class-D topologies - half-bridge (2 output devices are
used) and full-bridge (4 output devices).
• Each one has its own advantages. For example, half-bridge is obviously simpler
and has more flexibility as a half-bridge amplifier can be bridged as with classical
topologies. If it is not correctly designed and driven, can suffer from "bus
pumping" phenomena (transfer current to the power supply that can make it
increase its voltage producing situations dangerous to the amplifier, supply and
speaker).
• Full bridge requires output devices rated for half the voltage as an half bridge
amplifier of the same power, but it is more complicated. Figures 5a and 5b show
both topologies conceptually. Obviously, many components such as decoupling
capacitors, etc are not shown

11. Half bridge Class-D topology

12. Full bridge Class-D topology

13. Advantages
• Despite the complexity involved, a properly designed class D
amplifier offers the following benefits:
– Reduction in size and weight of the amplifier,
– Reduced power waste as heat dissipation and hence smaller (or
no) heat sinks,
– Reduction in cost due to smaller heat sink and compact circuitry,
– Very high power conversion efficiency, usually above 90%
above one quarter of the amplifier's maximum power, and around
50% at low power levels.

14. Uses
• Home Theatre systems. In particular the economical "home theatre
in a box" systems are almost universally equipped with class D
amplifiers. On account of modest performance requirements and
straightforward design, direct conversion from digital audio to
PWM without feedback is most common.
• Mobile phones. The internal loudspeaker is driven by up to 1 W.
Class D is used to preserve battery lifetime.
• Powered speakers
• High-end audio is generally conservative with regards to adopting
new technologies but class D amplifiers have made an appearance

15. • Active subwoofers
• Sound Reinforcement and Live Sound. The weight reduction makes
class D amplifiers more transportable. The Crest Audio CD3000, for
example, is a class D power amplifier that is rated at 1500 W per
channel, yet it weighs only 21 kg (46 lb).
• Bass amplifiers Again, an area where portability is important.
Example: Yamaha BBT500H bass amplifier which is rated at
500 W, and yet it weighs less than 5 kg (11 lb).

16. Class T amplifier
• Is an audio amplifier IC design. Rather than being a separate "class" of
amplifier, Class T is a registered trademark for Tripath's amplifier technologies.
• It is an implementation of Class D amplifiers, but is claimed to improve the
control scheme to create more efficient and higher quality audio amplification.
• The control signals in Class T amplifiers may be computed using digital signal
processing or fully analog techniques. Currently available implementations use a
loop similar to a higher order Delta-Sigma (ΔΣ) (or sigma-delta) modulator, with
an internal digital clock to control the sampled comparator.
• Despite Tripath's claimed increased performance at a price similar to class D
technology, financial difficulties caused Tripath to file for Chapter 11 bankruptcy
protection on 8 February 2007.