Amplifier DistortionIf 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. InFourier analysis, any distorted periodic waveform can bebroken down into frequency components. Thesecomponents are harmonics of the fundamental frequency. 1
HarmonicsHarmonics 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 the2nd and 4th are called even harmonics. 2
Harmonic DistortionAccording to Fourieranalysis, if a signal is notpurely sinusoidal, then itcontains harmonics. 3
Harmonic Distortion CalculationsHarmonic 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 harmonicThe total harmonic distortion (THD) is determined by: % THD D 2 D 2 D 2 100 2 3 3 4
Class C AmplifiersA class C amplifier conducts for lessthan 180. In order to produce a fullsine wave output, the class C uses atuned circuit (LC tank) to providethe full AC sine wave.Class C amplifiers are usedextensively in radio communicationscircuits. 5
In Class C, the amplifying device is deliberately notoperated linearly. Instead, it is operated as a switch inorder to reduce resistance loss. The anode conductionangle in Class C operation is usually made as short as ispossible. In effect, the tank circuit makes the RF outputsine wave--like a bell that is struck at a constant rate by ahammer.
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 thetransistor 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 toground. A magnetic field builds up in the coil depending on the magnitude of the current. At the same time thevoltage over the capacitor discharge through the resistor making another current flow I2 (blue) also through thetransistor. 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 thetransistor 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 flowthrough the capacitor and into the resistor (antenna).
Class D AmplifierA class D amplifier amplifiespulses, and requires a pulsedinput.There are many circuits thatcan convert a sinusoidalwaveform to a pulse, as wellas circuits that convert apulse to a sine wave. Thiscircuit has applications indigital circuitry. 8
The term "class D" is sometimes misunderstood asmeaning a "digital" amplifier. While some class D ampsmay indeed be controlled by digital circuits, the powerstage deals with voltage and current as a function of time.The smallest amount of noise, timing uncertainty, voltageripple or any other non-ideality immediately results in anirreversible change of the output signal. A digital circuit alsouses physics to operate, but those same errors will onlylead to incorrect results when they become so large that asignal representing a digit is distorted beyond recognition
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
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 amplifiers maximum power, and around 50% at low power levels.
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
• 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).
Class T amplifier• Is an audio amplifier IC design. Rather than being a separate "class" of amplifier, Class T is a registered trademark for Tripaths 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 Tripaths 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.