Amplifier classes

Introduction

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

Class A

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

Class AB

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

Class B

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

Class C

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

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 analogue amplifier, the signal is applied to the input terminal of the device (base, gate or grid), and this causes a current to flow in proportion to the input between the output terminal and ground (collector, drain or anode). This 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. This 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.

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Class A

Class A amplifiers amplify over the whole of the input cycle. They are the usual means of implementing small-signal amplifiers. They are not very efficient—a theoretical maximum of 50% is obtainable, but for small signals, this waste of power is still extremely small, and can easily be 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 designs over transistors, for a number of reasons. One is that the characteristic curve of a valve means that distortion tends to be in the form of even harmonics, which, they claim, sound more "musical" than odd harmonics. 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; the 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.

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. 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, and are sometimes used for RF linear amplifiers as well.

Class C

Class C amplifiers conduct less than 50% of the input signal. As such the distortion at the output is gross, but very high efficiencies can be reached - up to 90% or so. 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 one particular frequency, and so the unwanted harmonics are suppressed, and the wanted full signal (sine wave) will be developed across the tuned load. Provided the transmitter is not required to operate over a very wide band of frequencies, this arrangement works extremely well. Any residual harmonics can be removed using a filter.

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Class D

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 pulse frequency. These amplifiers use pulse width modulation, pulse density modulation (sometimes referred to as pulse frequency modulation) or some combination of the two. 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 (ie. 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 analogue 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 are widely used to control motors, and almost exclusively for small DC motors. They are also used as audio amplifiers. While class D amplifiers with excellent audio quality do exist, 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". 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.

Letter D

The letter D used to designate this type of amplifier is simply the next letter after C . It does not stand for digital, although class D and class E amplifiers are sometimes classified as 'digital' because the transistors operate in a switched rather than a linear mode. This is incorrect, as digital implies converting a signal into discrete symbols, and a class D amplifier merely converts the waveform into a continuously pulse-width modulated analog signal. The only similarity between digital signals and the output of a class D amplifier is that they often consist of signals with two discrete states. This is not even true for many digital signals, however.

Class E

A class E amplifier is a tuned power amplifier particularly suitable for radio and microwave frequencies. It consists of a switching transistor driving a current through an inductor and then a resonant filter. The filter ensures that a high current never coincides with a high voltage in the transistor, greatly reducing power dissipation and making this the most efficient class of amplifier. The first class E amplifier ever built [1] had an efficiency of 96%.

Class E amplifiers have been used to transmit commercial AM and shortwave radio, although class D amplifiers introduce lower distortion. They are not used for audio signals.

The class E amplifier was invented in 1972 by Nathan O. Sokal and Alan D. Sokal, and details were first published in 1975 [1].

A practical circuit

For the purposes of illustration, this practical amplifier circuit is described. It could be the basis for a moderate audio power amplifier. It features a typical design 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, 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 amplified signal from Q1 is directly fed to the second stage, Q3, which provides further amplification of the signal, and the d.c. 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 d.c 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. Calculating the values of the resistors is left as an exercise for the reader

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 nonlinear 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.

Class G

A class G amplifier is a more efficient version of a class AB amplifier, which uses "rail switching" to decrease power consumption. 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.