Amplifier

The term amplifier as used in this article can mean either a circuit (or stage) using a single active device or a complete system such as a packaged audio hi-fi amplifier.

An electronic amplifier is a device for increasing the current, voltage or power of a signal. It does this by taking power from a power supply and controlling the output to match the input signal shape but with a larger amplitude. An idealized amplifier can be said to be "a piece of wire with gain", as the output is an exact replica of the input, but larger.

Classification of amplifier stages and systems

Different designs of amplifiers are used for different types of applications and signals. We can broadly divide amplifiers into three categories—

• small signal amplifiers,
• low frequency power amplifiers and
• RF power amplifiers.

Each of these calls for a slightly different design approach, mainly because of the physical limitations of the components used to implement the amplifier, and the efficiencies that can be realised.

There are many alternative classifications that address different aspects of amplifier designs, and they all have some effect on the design parameters and objectives of the circuit. Amplifier design is always a compromise of numerous factors, such as cost, amount of power consumed, devices that have real-world imperfections, and the need to match the amplifier to the input signal as well as the output load.

Classification of amplifier stages by common terminal

One set of these classifications include terms referring to “common terminal “ connections, where the design is described by the terminal of the active device that is tied closest to ground. Examples include terms such as common emitter, common plate, or common drain, and these names also reflect the type of active device used to amplify the signal. For instance, common emitter refers to an amplifier with a bipolar transistor as the active device, while common plate would be for a vacuum tube amp, while a common drain amp would signify the use of MOSFET or JFET devices. Designs exist for almost any terminal of any active device to be held to ground in an amplifier, for different reasons that are reflected in each use. See also: common collector, common base.

Inverting or non-inverting

Another way to classify amps is the phase relationship of the input signal to the output signal. An inverting amplifier produces an output that is 180 degrees out of phase of the input signal, or a mirror image of it if viewed on an oscilloscope. A non-inverting amplifier maintains equal phase relationship between the input and output waveforms. An emitter follower is a type of this amplifier, indicating that the signal at the emitter of a transistor is following (matching phases) with the input signal.

This description can apply to a single stage or a complete system.

Function

Other amps may be classified by their function or output characteristics. These functional descriptions usually apply to complete amplifier systems or sub systems and rarely to individual stages.

• A servo amp indicates an integrated feedback loop to actively control the output at some desired level. A DC servo indicates use down to DC levels, where the normal fluctuations of an audio or RF signal do not occur. These are often used in mechanical actuators, or devices such as DC motors that must maintain a constant speed or torque. An AC servo amp can do this for some ac motors.

• A linear amp denotes that it has a precise amplification factor over a wide range of frequencies, and is often used to boost signals for relay in communications systems. A non-linear amp is made to amplify only a specific narrow or tuned frequency, to the exclusion of all other frequencies.

• A RF amp refers to an amp designed for use in the radio frequency range of the electromagnetic spectrum, and is often specially made to feed antenna systems.

• An audio amp is specially designed and made for use in reproducing audio frequencies, with special considerations made for driving speakers. These often have multiple amps grouped together as separate or bridgeable channels to accommodate different audio reproduction systems.

• A special type of low power amp with almost ideal characteristics is used in instruments and for signal processing, among many other varied uses. These are known as operational amplifiers, or op-amps. This is because this type of amplifier is used in circuits that perform mathematical algorithmic functions, or "operations" on input signals to obtain specific types of output signals.

Voltage current or power amplification

Amplifiers can be designed to increase signal voltage (voltage amp), current (buffer amp), or both (power amp), of an electronic signal. Electronic amplifiers can operate off either single sided supplies (either + or – voltage “rail”, or “bus”, and ground), or double-sided or balanced supplies (+ and – supply rails, and ground).

The different methods of supplying power result in many different methods of Bias (electrical engineering). Bias is the method by which the active devices are set up to operate properly, or by which the DC component of the output signal is set to the midpoint between the maximum voltages available from the power supply. Most amplifiers use sets of devices that are matched in specifications except for polarity. These are called complementary pairs. Class A amplifiers generally use only one device, unless the power supplies are set to provide both positive and negative supplies, in which case a dual device symmetrical design may be used. Class C amps, by definition, use a single polarity supply.

Amplifiers are also often designed to have multiple stages hooked in series to increase gain. Each stage of these designs is often a different type of amp to suit the needs of each stage. For instance, the first stage might be a Class A stage, feeding a class AB push-pull second stage, which then drives a class G final output stage, taking advantage for the strengths of each type, while minimizing the weaknesses.

There also exist special “stacked” transistors, called Darlington pairs, which have two specially matched transistors in a single case. Transistors or other active devices are also often hooked in parallel, or “strapped”, in order to multiply the amount of current that the final output stage can deliver to the load.

Interstage coupling method

Audio amplifiers are sometimes classified by the coupling method of the signal at the input, output, or between stages. Different types of these include capacitive, inductive (transformer), and direct coupled amps. Each method has its advantages and compromises.

Angle of flow or conduction angle

The letter system of amplifier classification assigns a letter to different designs of electronic amplifiers. These designs are classified according to the relationship between the input wave form and the output wave form, as well as the amount of time that the active components used to amplify a signal are conducting electricity. This time is measured in degrees of duration of sine wave test signal applied to the input of an amplifier, with 360 degrees representing one full cycle.

Implementation

Amplifiers can be implemented using transistors of various types, or vacuum tubes (valves). Other more exotic forms of amplifier are also possible using different types of devices. Such exotic amplifiers are often used for microwave or other extremely high frequency signals.

Amplifier classes

Amplifier circuits are classified as A, B, AB and C for analog designs, and class D and E for switching designs. For the analog classes, each class defines what proportion of the input signal cycle (called the angle of flow) is used to actually switch on the amplifying device:

Class A

100% of the input signal is used (conduction angle a = 360° or 2π)

Class AB

more than 50% but less than 100% is used. (181° to 359°, π < a < 2π)

• Class AB1 applies to tube or transistor amplifiers in class AB where the grid or base is more negatively biased than it is in class A.

• Class AB2 applies to tube or transistor amplifiers in class AB where the grid or base is often more negatively biased than in AB1, and the input signal is often larger. When the drive is high enough to make the grid or the base more positive, the grid or base current will increase. It is possible depending on the level of the signal input for the amplifier to move from class AB1 to AB2.

Class B

50% of the input signal is used (a = 180° or π)

Class C

less than 50% is used (0° to 179°, a < π)

This can be most easily understood using the diagrams in each section below. For the sake of illustration, a bipolar junction transistor is shown as the amplifying device, but in practice this could be a MOSFET or vacuum tube device. In an analog amplifier, the signal is applied to the input terminal of the device (base, gate or grid), and this causes a proportional output drive current to flow out of the output terminal. The output drive current is obtained from the power supply. The voltage signal shown is thus a larger version of the input, but has been changed in sign (inverted) by the amplification. Other arrangements of amplifying device are possible, but that given (common emitter, common source or common cathode) is the easiest to understand and employ in practice. If the amplifying element is linear, then the output will be faithful copy of the input, only larger and inverted. In practice, transistors are not linear, and the output will only approximate the input. Non-linearity is the origin of distortion within an amplifier. Which class of amplifier (A, B, AB or C) depends on how the amplifying device is biased—in the diagrams the bias circuits are omitted for clarity.

Any real amplifier is an imperfect realization of an ideal amplifier. One important limitation of a real amplifier is that the output it can generate is ultimately limited by the power available from the power supply. An amplifier can saturate and clip the output if the input signal becomes too large for the amplifier to reproduce.

Class A

Class A amplifiers amplify over the whole of the input cycle such that the output signal is an exact scaled-up replica of the input with no clipping. Class A amplifiers are the usual means of implementing small-signal amplifiers. They are not very efficient—a theoretical maximum of 25% is obtainable, but for small signals, this waste of power is still extremely small, and can be easily tolerated. It is only when we need to create output powers with appreciable levels of voltage and current does Class A become problematic. In a Class A circuit, the amplifying element is biased such that the device is always conducting to some extent, and is operated over the most linear portion of its characteristic curve (known as its transfer function or transconductance curve). Because the device is always conducting, even if there is no input at all, power is wasted. This is the reason for its inefficiency.

If we wish to produce large output powers from a Class A circuit, the power wastage will become significant. For every watt delivered to the load, the amplifier itself will, at best, waste another watt. For large powers this will call for a large power supply and large heat sink to carry away the waste heat. Class A designs have largely been superseded for audio power amplifiers, though some audiophiles believe that Class A gives the best sound quality, due to it being operated in as linear a manner as possible. In addition, some aficionados prefer vacuum tube (or "valve") designs over transistors, for a number of reasons: Tubes are more commonly used in class A designs, which have an asymmetrical transfer function. This means that distortion of a sine wave creates both odd- and even-numbered harmonics. They claim that this sounds more "musical" than the purely odd harmonics produced by a symmetrical push-pull amplifier. Though good amplifier design can reduce all harmonics to inaudibility in a sound reproduction system, the differences in harmonic content are essential to the sound of intentional electric guitar distortion. Another is that valves use many more electrons at once than a transistor, and so statistical effects lead to a "smoother" approximation of the true waveform—see shot noise for more on this. Field-effect transistors have similar characteristics to valves, so these are found more often in high quality amplifiers than bipolar transistors. Historically, valve amplifiers often used a Class A power amplifier simply because valves are large and expensive; Many Class A design uses only a single device. Transistors are much cheaper, and so more elaborate designs that give greater efficiency but use more parts are still cost effective. A classic application for a pair of class A devices is the long-tailed pair, which is exceptionally linear, and forms the basis of many more complex circuits, including many audio amplifiers and almost all op-amps.

Class B and AB

Class B amplifiers only amplify half of the input wave cycle. As such they create a large amount of distortion, but their efficiency is greatly improved. This is because the amplifying element is switched off altogether half of the time, and so cannot dissipate power. A single Class B element is rarely found in practice, though it can be used in RF power amplifiers where the distortion is unimportant. However Class C is more commonly used for this.

A practical circuit using Class B elements is the complementary pair or "push-pull" arrangement. Here, complementary devices are used to each amplify the opposite halves of the input signal, which is then recombined at the output. This arrangement gives excellent efficiency, but can suffer from the drawback that there is a small glitch at the "joins" between the two halves of the signal. This is called crossover distortion. A solution to this is to bias the devices just on, rather than off altogether when they are not in use. This is called Class AB operation. Each device is operated in a non-linear region which is only linear over half the waveform, but still conducts a small amount on the other half. Such a circuit behaves as a class A amplifier in the region where both devices are in the linear region, however the circuit cannot strictly be called class A if the signal passes outside this region, since beyond that point only one device will remain in its linear region and the transients typical of class B operation will occur. The result is that when the two halves are combined, the crossover is greatly minimised or eliminated altogether.

Class B or AB push-pull circuits are the most common form of design found in audio power amplifiers. Class AB is widely considered a good compromise for audio amplifiers, since much of the time the music is quiet enough that the signal stays in the "class A" region, where it is reproduced with good fidelity, and by definition if passing out of this region, is large enough that the distortion products typical of class B are relatively small. Class B and AB amplifiers are sometimes used for RF linear amplifiers as well.

Negative feedback

Feedback feeds the difference of the input and part of the output back to the input in a way that cancels out part of the input. The main effect is to reduce the overall gain of the system. However the unwanted signals introduced by the amplifier are also fed back. Since they are not part of the original input, they are added to the input in opposite phase, subtracting them from the input.

Careful design of each stage of an open loop (non-feedback) amplifier can achieve about 1% distortion. With negative feedback, 0.001% is typical. Noise, even crossover distortion can be practically eliminated. Feedback was originally invented so that replacing a burnt-out vacuum tube would not change an amplifier's performance (manufacturing realities require that tubes and transistors with the same part number will have close but not identical gain). Negative feedback also compensates for changing temperatures, and degrading or non-linear components. While amplifying devices can be treated as linear over some portion of their characteristic curve, they are inherently non-linear; their physics dictates that they operate using a square law. The result of non-linearity is distortion.

The application dictates how much distortion a design can tolerate. For hi-fi audio applications, instrumentation amplifiers and the like, distortion must be minimal, often better than 1%.

While feedback seems like a universal fix for all the problems of an amplifier, many believe that negative feedback is a bad thing. Since it uses a loop, it takes a finite time to react to an input signal, and for this short period the amplifier is "out of control." A musical transient whose timing is of the same order as this period will be grossly distorted, even though the amplifier will show incredibly good distortion performance on steady-state signals. Proponents of feedback refute this, saying that the feedback "delay" is of such a short order that it represents a frequency vastly outside the bandwidth of the system, and such effects are not only inaudible, but not even present, as the amplifier will not respond to such high frequency signals.

The argument has caused controversy for many years, and has led to all sorts of interesting designs—such as feedforward amplifiers (e.g. digital signals on many cell-site base-station transmitters are precompensated for the radio amplifier's distortion). The fact remains that the majority of modern amplifiers use considerable amounts of feedback, though the best audiophile designs seek to minimise this as much as possible.

The concept of feedback is used in operational amplifiers to precisely define gain, bandwidth and other parameters.

A practical circuit

For the purposes of illustration, this practical amplifier circuit is described. It could be the basis for a moderate-power audio amplifier. It features a typical (though substantially simplified) design as found in modern amplifiers, with a class AB push-pull output stage, and uses some overall negative feedback. Bipolar transistors are shown, but this design would also be realisable with FETs or valves.

The input signal is coupled through capacitor C1 to the base of transistor Q1. The capacitor allows the AC signal to pass, but blocks the DC bias voltage established by resistors R1 and R2 so that any preceding circuit is not affected by it. Q1 and Q2 form a differential amplifier (an amplifier that multiplies the difference between two inputs by some constant), in an arrangement known as a long-tailed pair. This arrangement is used to conveniently allow the use of negative feedback, which is fed from the output to Q2 via R7 and R8. The negative feedback into the difference amplifier allows the amplifier to compare the input to the actual output. This way the amplifier "knows" the actual output and uses that knowledge to adjust the gain. The amplified signal from Q1 is directly fed to the second stage, Q3, which provides further amplification of the signal, and the DC bias for the output stages, Q4 and Q5. R6 provides the load for Q3. So far, all of the amplifier is operating in Class A. The output pair are arranged in Class AB push-pull, also called a complementary pair. They provide the majority of the current amplification and directly drive the load, connected via d.c. blocking capacitor C2. The diodes D1 and D2 provide a small amount of constant voltage bias for the output pair, just biasing them into the conducting state so that crossover distortion is minimised. This design is simple, but a good basis for a practical design because it automatically stabilises its operating point, since feedback internally operates from DC up through the audio range and beyond. Further circuit elements would probably be found in a real design that would roll off the frequency response above the needed range to prevent the possibility of unwanted oscillation. Also, the use of fixed diode bias as shown here can cause problems if the diodes are not both electrically and thermally matched to the output transistors—if the output transistors turn on too much, they can easily overheat and destroy themselves, as the full current from the power supply is not limited at this stage. A common solution to help stabilise the output devices is to include some emitter resistors, typically an ohm or so. Calculating the values of the circuit's resistors and capacitors is done based on the components employed and the intended use of the amp.

For the basics of radio frequency amplifers using valves, see Valved RF amplifiers.

Class C

Class C amplifiers conduct less than 50% of the input signal and the distortion at the output is high, but efficiencies of up to 90% can be reached. Some applications can tolerate the distortion, such as audio bullhorns. A much more common application for Class C amplifiers is in RF transmitters, where the distortion can be vastly reduced by using tuned loads on the amplifier stage. The input signal is used to roughly switch the amplifying device on and off, which causes pulses of current to flow through a tuned circuit. The tuned circuit will only resonate at particular frequencies, and so the unwanted frequencies are dramatically suppressed, and the wanted full signal (sine wave) will be abstracted by the tuned load. Provided the transmitter is not required to operate over a very wide band of frequencies, this arrangement works extremely well. Other residual harmonics can be removed using a filter.

Class D

Main article: PWM amplifier

A class D amplifier is a power amplifier where all power devices are operated in on/off mode. Output stages such as those used in pulse generators are examples of class D amplifiers. Mostly though, the term applies to devices intended to reproduce signals with a bandwidth well below the switching frequency. These amplifiers use pulse width modulation, pulse density modulation (sometimes referred to as pulse frequency modulation) or more advanced form of modulation such as Sigma delta modulation (see for example Analog Devices AD1990 Class-D audio power amplifier). The input signal is converted to a sequence of pulses whose averaged value is directly proportional to the amplitude of the signal at that time. The frequency of the pulses is typically ten or more times the highest frequency of interest in the input signal. The output of such an amplifier contains unwanted spectral components (i.e.. the pulse frequency and its harmonics) that must be removed by a passive filter. The resulting filtered signal is then an amplified replica of the input.

The main advantage of a class D amplifier is power efficiency. Because the output pulses have a fixed amplitude, the switching elements (usually MOSFETs, but valves and bipolar transistors were once used) are switched either on or off, rather than operated in linear mode. This means that very little power is dissipated by the transistors except during the very short interval between the on and off states. The wasted power is low because the instantaneous power dissipated in the transistor is the product of voltage and current, and one or the other is almost always close to zero. The lower losses permit the use of a smaller heat sink while the power supply requirements are lessened too.

Class D amplifiers can be controlled by either analog or digital circuits. A digital controller introduces additional distortion called quantisation error caused by its conversion of the input signal to a digital value.

Class D amplifiers were widely used to control motors, and almost exclusively for small DC motors. But they are now also used as audio amplifiers. The relative difficulty of achieving good audio quality means that the vast majority appear in applications where quality is not a factor, such as miniature audio systems and "DVD-receivers". But now, high quality Class D audio amplifiers are starting to appear in the market. Some of them even outperform classical designs. Recently, several manufacturers are offering built modules ready to be integrated in audio systems. An early and prolific area of application is high-powered, high-fidelity subwoofer amplifiers in automobiles. Because subwoofers are generally limited to a bandwidth of no higher than 150 Hz, the switch speed for the amplifier does not have to be as high as for a full range amplifier. They have become so inexpensive that a true 1 kW of power output can be had for less than 250USD (retail). Efficiencies are in the 80% to 95% range.

D does not stand for "digital"

The letter D used to designate this type of amplifier is simply the next letter after C , and does not stand for digital. Class D and Class E amplifiers are sometimes mistakenly described as "digital" because the output waveform superficially resembles a pulse-train of digital symbols, but a Class D amplifier merely converts an input waveform into a continuously pulse-width modulated analog signal. (A digital waveform would be pulse-code modulated.)

Specialty classes

Class E/F

The class E/F amplifier is a highly efficient switching power amplifier, typically used at radio frequencies. Its efficiency depends on the fact that perfect switching operation does not dissipate power. An ideal switch has zero impedance when closed and infinite impedance when open, implying that there is zero voltage across the switch when it carries current (on state) and zero current through it when there is a voltage across (off state). Consequently the product of voltage and current (power loss) is zero at any time.

This class of amplifier is unique to radio frequency ranges, where the amplifier analysis is usually done in the frequency domain and not in the voltage/current domain. This class is further divided to subclasses depending on which harmonics of the signal are taken into account during zero-voltage switching (ZVS) and zero-current switching (ZCS), with names such as E/F2,odd; class F^-1; and so on.

The figure below shows a schematic of a class-E/F amplifier that uses this principle to achieve high efficiency.

File:Class-e.jpg

The switch is periodically opened and closed at the frequency of operation. Usually, but not always, the switching duty ratio is 50%. The RF choke has comparatively large inductance so that in effect it functions as a constant current source. Other passive device values are chosen such that the following conditions are satisfied simultaneously. (1) The voltage across the switch at the instant of closing is zero. (2) The time derivative of voltage across the switch at is zero when the switch turns on. Moreover, Ls and Cs forms a resonating filter at the frequency of operation.

In practical implementations a transistor is substituted for a switch, but is operated either in saturation (on) or in cut-off (off). The theoretical efficiency of a class-E amplifier is 100% with ideal components. However, practical circuits do exhibit a number of weaknesses that makes them less than 100% efficient. These effects include finite switching speed, finite on-resistance and non-zero saturation voltage of the transistor as well as lossy passive components at high frequencies. Typical efficiency is about 60% at an operating frequency of 1-2 GHz.

This amplifier class is specially designed for the amplification of square waves, such as those used to transmit data in purely digital form. “Square” waves or pulses have special needs due to their frequency characteristics, since they require the faithful reproduction of the very high frequencies present in their leading and trailing edges, without adding artifacts such as ringing or overshoot during the amplification process. Consideration must be made as well for the lower frequency components introduced by the switching levels, such as the impedance of the output load, which is often in the form of a transmission line.

The class E amplifier was invented in 1972 by Nathan O. Sokal and Alan D. Sokal, and details were first published in 1975 . Some earlier reports on this operating class have been published in Russian.

Class F amplifiers are used in radio frequency communications such as cell phones or data links. They have provisions designed into them that accommodate for the modulation (or decoding) of the output by both amplitude modulation (AM) and frequency modulation (FM). This gives the designer multiple multiplex paths, whereby different information channels can be encoded into the signal, not just the AM or the FM, but also secondary paths such as side bands, pilot signals, or data streams. This also allows for redundancy to be designed into the signal, to ensure the prevention of error, or loss of data. This type of amplifier often uses Pulse Frequency Modulation (PFM) techniques to provide this complex modulation.

Class G

Class G amplifiers are a more efficient version of class AB amplifiers, which use "rail switching" to decrease power consumption and increase efficiency. The amplifier has several power rails at different voltages, and switches between rails as the signal output approaches each. Thus the amp increases efficiency by reducing the "wasted" power at the output transistors.

Class H

Class H amplifiers are similar to Class G, except that the power supply voltage "tracks", or is modulated by, the signal. The power supply is always kept slightly higher than the actual power required. Often it has two power supplies, like the class G, and only the higher is modulated. The modulated power supply is generated by a circuit similar to a class D amp.

Other classes

Several audio amplifier manufacturers have started "inventing" new classes as a way to differentiate themselves. These class names usually do not reflect any revolutionary amplification technique, and are used mostly for marketing purposes. This can easily determined by the fact that the class name is trademarked or copyrighted. For example, "Class T" is a trademark of TriPath company, which manufactures audio amplifier IC's. This new class "T" is just a common class D amplifier, operating at a frequency of 650kHz, with a proprietary modulator.

See also

• Electronics (the section about electronic stages)
• Low noise amplifier
• Operational amplifier
• Instrumentation amplifier
• Charge transfer amplifier

External links

• Rane audio's guide to amplifier classes
• Conversion: distortion factor to distortion attenuation and THD
• An alternate topology called the grounded bridge amplifier (PDF)
• Contains an explanation of different amplifier classes (PDF)
• Amplifier classes
• Reinventing the power amplifier (PDF)
• Anatomy of the power amplifier, including information about classes