How Amplifiers Work <ul><li>When people refer to "amplifiers," they're usually talking about stereo components or musical equipment. But this is only a small representation of the spectrum of audio amplifiers. There are actually amplifiers all around us. You'll find them in televisions , computers , portable CD players and most other devices that use a speaker to produce sound. </li></ul><ul><li>In this article, we'll see what amplifiers do and how they do it. Amplifiers can be very complex devices, with hundreds of tiny pieces, but the basic concept behind them is pretty simple. You can get a clear picture of how an amplifier works by examining the most basic components . </li></ul><ul><ul><li>A home stereo amplifier and receiver in one unit </li></ul></ul>
<ul><li>Sound is a fascinating phenomenon. When something vibrates in the atmosphere, it moves the air particles around it. Those air particles in turn move the air particles around them, carrying the pulse of the vibration through the air. Our ears pick up these fluctuations in air pressure and translate them into electrical signals the brain can process. </li></ul>
<ul><li>Electronic sound equipment works the same basic way. It represents sound as a varying electric current. Broadly speaking, there are three steps in this sort of sound reproduction: </li></ul><ul><li>Sound waves move a microphone diaphragm back and forth, and the microphone translates this movement into an electrical signal. The electrical signal fluctuates to represent the compressions and rarefactions of the sound wave. </li></ul><ul><li>A recorder encodes this electrical signal as a pattern in some sort of medium -- as magnetic impulses on tape , for example, or as grooves in a record. </li></ul><ul><li>A player (such as a tape deck) re-interprets this pattern as an electrical signal and uses this electricity to move a speaker cone back and forth. This re-creates the air-pressure fluctuations originally recorded by the microphone. </li></ul><ul><li>As you can see, all the major components in this system are essentially translators: They take the signal in one form and put it into another. In the end, the sound signal is translated back into its original form, a physical sound wave. In order to register all of the minute pressure fluctuations in a sound wave, the microphone diaphragm has to be extremely sensitive. This means it is very thin and moves only a short distance. Consequently, the microphone produces a fairly small electrical current. </li></ul><ul><li>This is fine for most of the stages in the process -- it's strong enough for use in the recorder, for example, and it is easily transmitted through wires. But the final step in the process -- pushing the speaker cone back and forth -- is more difficult. To do this, you need to boost the audio signal so it has a larger current while preserving the same pattern of charge fluctuation. </li></ul>
Pump it Up <ul><li>We saw that an amplifier's job is to take a weak audio signal and boost it to generate a signal that is powerful enough to drive a speaker. When you consider the amplifier as a whole, but the process inside the amplifier is a little more complex. In actuality, the amplifier generates a completely new output signal based on the input signal. You can understand these signals as two separate circuits. The output circuit is generated by the amplifier's power supply, which draws energy from a battery or power outlet. If the amplifier is powered by household alternating current , where the flow of charge changes directions, the power supply will convert it into direct current , where the charge always flows in the same direction. The power supply also smoothes out the current to generate an absolutely even, uninterrupted signal. The output circuit's load (the work it does) is moving the speaker cone. </li></ul>The basic concept of an amplifier: A smaller current is used to modify a larger current
<ul><li>The input circuit is the electrical audio signal recorded on tape or running in from a microphone. Its load is modifying the output circuit. It applies a varying resistance to the output circuit to re-create the voltage fluctuations of the original audio signal. </li></ul><ul><li>In most amplifiers, this load is too much work for the original audio signal. For this reason, the signal is first boosted by a pre-amplifier, which sends a stronger output signal to the power amplifier. The pre-amplifier works the same basic way as the amplifier: The input circuit applies varying resistance to an output circuit generated by the power supply. Some amplifier systems use several pre-amplifiers to gradually build up to a high-voltage output signal. </li></ul>Inside an amplifier, you'll see a mass of electronic components. The central components are the large transistors. The transistors generate a lot of heat, which is dissipated by the heat sink.
Electronic Elements <ul><li>The component at the heart of most amplifiers is the transistor. The main elements in a transistor are semiconductors , materials with varying ability to conduct electric current. Typically, a semiconductor is made of a poor conductor, such as silicon, that has had impurities ( atoms of another material) added to it. The process of adding impurities is called doping. </li></ul><ul><li>In pure silicon, all of the silicon atoms bond perfectly to their neighbors, leaving no free electrons to conduct electric current. In doped silicon, additional atoms change the balance, either adding free electrons or creating holes where electrons can go. Electrical charge moves when electrons move from hole to hole, so either one of these additions will make the material more conductive. </li></ul>The internal circuit of an amplifier
Bipolar-junction transistor <ul><li>. This sort of transistor consists of three semiconductor layers -- in this case, a p-type semiconductor sandwiched between two n-type semiconductors. This structure is best represented as a bar, as shown in the diagram below (the actual design of modern transistors is a little different). </li></ul>
<ul><li>The first n-type layer is called the emitter, the p-type layer is called the base and the second n-type layer is called the collector. The output circuit is connected to electrodes at the transistor's emitter and collector. The input circuit connects to the emitter and the base. </li></ul><ul><li>The free electrons in the n-type layers naturally want to fill the holes in the p-type layer. There are many more free electrons than holes, so the holes fill up very quickly. This creates depletion zones at the boundaries between n-type material and p-type material. In a depletion zone, the semiconductor material is returned to its original insulating state -- all the holes are filled, so there are no free electrons or empty spaces for electrons, and charge can't flow. When the depletion zones are thick, very little charge can move from the emitter to the collector, even though there is a strong voltage difference between the two electrodes. First internal view of an amplifier </li></ul>First internal view of an amplifier
Boosting the Voltage <ul><li>When depletion zones are thick, you can boost the voltage on the base electrode. The voltage at this electrode is directly controlled by the input current. When the input current is flowing, the base electrode has a relative positive charge, so it draws electrons toward it from the emitter. This frees up some of the holes, which shrinks the depletion zones. As the depletion zones are reduced, charge can move from the emitter to the collector more easily - the transistor becomes more conductive. The size of the depletion zones, and therefore the conductivity of the transistor, is determined by the voltage at the base electrode. In this way, the fluctuating input current at the base electrode varies the current output at the collector electrode and drives the speaker. </li></ul>
<ul><li>A single transistor like this represents one "stage" of an amplifier. A typical amplifier will have several boosting stages, with the final stage driving the speaker. </li></ul><ul><li>In a small amplifier -- the amplifier in a speaker phone, for example -- the final stage might produce only half a watt of power. In a home stereo amplifier, the final stage might produce hundreds of watts. The amplifiers used in outdoor concerts can produce thousands of watts. </li></ul><ul><li>The goal of a good amplifier is to cause as little distortion as possible. The final signal driving the speakers should mimic the original input signal as closely as possible, even though it has been boosted several times. </li></ul><ul><li>This basic approach can be used to amplify all kinds of things, not just audio signals. Anything that can be carried by an electrical current -- radio and video signals, for example -- can be amplified by similar means. Audio amplifiers seem to catch people's attention more than anything else, however. Sound enthusiasts are fascinated with variations in design that affect power rating, impedance and fidelity, among other specifications. </li></ul>
Amplifier <ul><li>Generally, an amplifier is any device that uses a small amount of energy to control a larger amount of energy. In popular use, the term today usually refers to an electronic amplifier , often as in audio applications. The relationship of the input to the output of an amplifier — usually expressed as a function of the input frequency — is called the transfer function of the amplifier, and the magnitude of the transfer function is termed the gain . </li></ul>General characteristics of amplifiers Most amplifiers can be characterized by a number of parameters . Gain The gain is the ratio of output power to input power, and is usually measured in decibels (dB). (When measured in decibels it is logarithmically related to the power ratio: G (dB) =10log(Pout/Pin)). Output dynamic range Output dynamic range is the range, usually given in dB, between the smallest and largest useful output levels. Since the lowest useful level is limited by output noise, this is quoted as the amplifier dynamic range.
<ul><li>Bandwidth and rise time </li></ul><ul><li>The bandwidth (BW) of an amplifier is usually defined as the difference between the lower and upper half power points . This is therefore also known as the −3 dB BW. Bandwidths for other response tolerances are sometimes quoted (−1 dB, −6 dB etc.). </li></ul><ul><li>As an example, a good audio amplifier will be essentially flat between twenty hertz to about twenty kilohertz (the range of normal human hearing), so the amplifier's usable frequency response needs to extend considerably beyond this (one or more octaves either side) and typically a good audio amplifier will have -3 dB points < 10 and > 65 kHz. </li></ul><ul><li>The rise time of an amplifier is the time taken for the output to change from 10% to 90% of its final level when driven by a step input. </li></ul><ul><li>Many amplifiers are ultimately slew rate limited (typically by the impedance of a drive current having to overcome capacitive effects at some point in the circuit), which may limit the full power bandwidth to frequencies well below the amplifiers frequency response when dealing with small signals </li></ul><ul><li>For a Gaussian response system (or a simple RC roll off), the rise time is approximated by: </li></ul><ul><li>Tr * BW = 0.35, where Tr is in seconds and BW is in Hz . </li></ul><ul><li>Settling time and aberrations </li></ul><ul><li>Time taken for output to settle to within a certain percentage of the final value (say 0.1%). This is usually specified for oscilloscope vertical amplifiers and high accuracy measurement systems. </li></ul>
<ul><li>Slew rate </li></ul><ul><li>Slew rate is the maximum rate of change of output variable, usually quoted in volts per second (or microsecond). </li></ul><ul><li>Noise </li></ul><ul><li>This is a measure of how much noise is introduced in the amplification process. Noise is an undesirable but inevitable product of the electronic devices and components. It is measured in either decibels or the peak output voltage produced by the amplifier when no signal is applied. </li></ul>Efficiency Efficiency is a measure of how much of the input power is usefully applied to the amplifier's output. Class A amplifiers are very inefficient, in the range of 10–20% with a max efficiency of 25%. Modern Class AB amps are commonly between 35–55% efficient with a theoretical maximum of 78.5%. Commercially available class D amplifiers have reported efficiencies as high as 97%. Linearity An ideal amplifier would be a totally linear device, but real amplifiers are only linear within certain practical limits. When the signal drive to the amplifier is increased, the output also increases until a point is reached where some part of the amplifier becomes saturated and cannot produce any more output; this is called clipping, and results in distortion
<ul><li>Some amplifiers are designed to handle this in a controlled way which causes a reduction in gain to take place instead of excessive distortion; the result is a compression effect, which (if the amplifier is an audio amplifier) will sound much less unpleasant to the ear. For these amplifiers, the 1dB compression point is defined as the input power (or output power) where the gain is 1dB less than the small signal gain. </li></ul><ul><li>Linearization is an emergent field, and there are many techniques, such us feedforward , predistortion , postdistortion, EER , LINC, CALLUM , cartesian feedback... in order to avoid the undesired effects of the non-linearities. </li></ul>
<ul><li>Electronic amplifiers </li></ul><ul><li>There are many types of electronic amplifiers for different applications. </li></ul><ul><li>One common type of amplifier is the electronic amplifier, commonly used in radio and television transmitters and receivers , high-fidelity ("hi-fi") stereo equipment, microcomputers and other electronic digital equipment, and guitar and other instrument amplifiers . Its critical components are active devices , such as vacuum tubes or transistors . </li></ul>Acoustic Amplifier Vacuum Tube Stereo Amplifier
Amplifier classes <ul><li>Class A </li></ul><ul><ul><li>Where efficiency is not a consideration, most small signal linear amplifiers are designed as Class A , which means that the output devices are always in the conduction region. Class A amplifiers are typically more linear and less complex than other types, but are very inefficient. This type of amplifier is most commonly used in small-signal stages or for low-power applications (such as driving headphones). </li></ul></ul><ul><li>Class B </li></ul><ul><ul><li>In Class B , there are two output devices (or sets of output devices), each of which conducts alternately for exactly 180 deg (or half cycle) of the input signal. </li></ul></ul>Amplifiers are commonly classified by the conductionangle (angle of flow) of the input signal through the amplifying device.
<ul><li>Class C </li></ul><ul><li>Popular for high power RF amplifiers, Class C is defined by conduction for less than 180° of the input signal. Linearity is not good, but this is of no significance for single frequency power amplifiers. The signal is restored to near sinusoidal shape by a tuned circuit, and efficiency is much higher than A, AB, or B classes of amplification. </li></ul><ul><li>Class AB </li></ul><ul><ul><li>Class AB amplifiers are a compromise between Class A and B, which improves small signal output linearity; conduction angles vary from 180 degrees upwards, selected by the amplifier designer. Usually found in low frequency amplifiers (such as audio and hi-fi) owing to their relatively high efficiency, or other designs where both linearity and efficiency are important (cell phones, cell towers, TV transmitters). </li></ul></ul>