Power amplifier classes


In electronics, power amplifier classes are letter symbols applied to different power amplifier types. The class gives a broad indication of an amplifier's efficiency, linearity and other characteristics.
Broadly, as you go through the alphabet, the amplifiers become more efficient but less linear, and the reduced linearity is dealt with through other means.
The first classes, A, AB, B, and C, are related to the time period that the active amplifier device is passing current, expressed as a fraction of the period of a signal waveform applied to the input. This metric is known as conduction angle. A class-A amplifier is conducting through the entire period of the signal ; class-B only for one-half the input period, class-C for much less than half the input period.
Class-D and E amplifiers operate their output device in a switching manner; the fraction of the time that the device is conducting may be adjusted so a pulse-width modulation output can be obtained from the stage.
Additional letter classes are defined for special-purpose amplifiers, with additional active elements, power supply improvements, or output tuning; sometimes a new letter symbol is also used by a manufacturer to promote its proprietary design.
By December 2010, classes AB and D dominated nearly all of the audio amplifier market with the former being favored in portable music players, home audio and cell phone owing to lower cost of class-AB chips.
In the illustrations below, a bipolar junction transistor is shown as the amplifying device. However, the same attributes are found with MOSFETs or vacuum tubes.

Class A

In a class-A amplifier, 100% of the input signal is used. The active element remains continuously conducting. Their output stage transistors are biased for class-A operation, leading to a continual drain current. This means class-A amplifiers have poor efficiency and heat is generated in the transistor, typically requiring thermal management.
Subclasses A1 and A2 are sometimes used to refer to vacuum-tube class-A amplifiers that drive the grid slightly negative or positive respectively on signal peaks for slightly more power than normal class-A. This, however, incurs higher signal distortion.

Advantages of class-A amplifiers

  • Simplicity. Class-A amplifiers are typically single-ended, requiring just a single device. The usual push–pull output configuration for class-AB and -B amplifiers requires two connected devices in the circuit, one to handle each half of the waveform.
  • The amplifying element is biased so the device is always conducting, the quiescent collector current is close to the most linear portion of its transconductance curve.
  • Because the device operates continuously there is no "turn on" time, no problems with charge storage, and generally better high-frequency performance and feedback loop stability.
  • The point where the device comes closest to being 'off' is not at 'zero signal', so the problems of crossover distortion associated with class-AB and -B designs is avoided.
  • Good for amplifying the weak signals received by radio receivers due to low distortion.

    Disadvantage of class-A amplifiers

  • Class-A amplifiers are inefficient. A maximum theoretical efficiency of 25% is obtainable using usual configurations, but 50% is the maximum for a transformer or inductively coupled configuration. In a power amplifier, this not only wastes power and limits operation with batteries, but increases operating costs and requires higher-rated output devices. Inefficiency comes from the standing current, which must be roughly half the maximum output current, and a large part of the power supply voltage is present across the output device at low signal levels. If high output power is needed from a class-A circuit, the power supply and accompanying heat becomes significant. For every watt delivered to the load, the amplifier itself, at best, uses an extra watt. For high-power amplifiers this means very large and expensive power supplies and heat sinks.
  • Because the output devices are in full operation at all times, they will not have as long a life unless the amplifier is specifically designed to take this into account, adding to the cost of maintaining or designing the amplifier.
Class-A power amplifier designs have largely been superseded by more efficient designs, though their simplicity makes them popular with some hobbyists. There is a market for expensive high fidelity class-A amps considered a "cult item" among audiophiles mainly for their absence of crossover distortion and reduced odd-harmonic and high-order harmonic distortion. Class-A power amplifiers are also used in some "boutique" guitar amplifiers due to their unique tonal quality and for reproducing vintage tones.

Single-ended and triode class-A amplifiers

Some hobbyists who prefer class-A amplifiers also prefer the use of thermionic valve designs instead of transistors, for several reasons:
  • Single-ended output stages have an asymmetrical transfer characteristics curve, meaning that even-order harmonics in the created distortion tend to not cancel out. For tubes, or FETs, most distortion is second-order harmonics, from the square law transfer characteristic, which to some produces a "warmer" and more pleasant sound.
  • For those who prefer low distortion figures, the use of tubes with class A together with symmetrical circuits results in the cancellation of most of the even distortion harmonics, hence the removal of most of the distortion.
  • Historically, valve amplifiers were often used as a class-A power amplifier simply because valves are large and expensive; many class-A designs use only a single device.
Transistors are much less expensive than tubes so more elaborate designs that use more parts are still less expensive to manufacture than tube designs. 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-A amplifiers may be used in output stages of op-amps. They are sometimes used as medium-power, low-efficiency, and high-cost audio power amplifiers. The power consumption is unrelated to the output power. At idle, the power consumption is essentially the same as at high output volume. The result is low efficiency and high heat dissipation.

Class B

In a class-B amplifier, the active device conducts for 180 degrees of the cycle. Because only half the waveform is amplified, significant harmonic distortion is directly present in the output signal. Therefore, class-B amplifiers are generally operated with tuned loading - where harmonics are shorted to ground by a series of resonators. Another method of reducing distortion, especially at audio frequencies, is to use two transistor devices in a push-pull configuration. Each conducts for one half of the signal cycle, and the device currents are combined so that the load current is continuous.
At radio frequency, if the coupling to the load is via a tuned circuit, a single device operating in class B can be used because the stored energy in the tuned circuit supplies the "missing" half of the waveform. Devices operating in class-B are used in linear amplifiers, so called because the radio frequency output power is proportional to the square of the input excitation voltage. This is more easily understood if stated as "output voltage is proportional to input voltage, thus output power is proportional to input power." This characteristic prevents distortion of amplitude-modulated or frequency-modulated signals passing through the amplifier. Such amplifiers have an efficiency around 60%.
When Class-B amplifiers amplify the signal with two active devices, each operates over one half of the cycle. Efficiency is much improved over class-A amplifiers. Class-B amplifiers are also favoured in battery-operated devices, such as transistor radios. Class-B has a maximum theoretical efficiency of π/4.
A practical circuit using class-B elements is the push–pull stage, such as the very simplified complementary pair arrangement shown at right. Complementary devices are each used for amplifying the opposite halves of the input signal, which is then recombined at the output. This arrangement gives good efficiency, but usually suffers from the drawback that there is a small mismatch in the cross-over region at the "joins" between the two halves of the signal, as one output device has to take over supplying power exactly as the other finishes. This is called crossover distortion. An improvement is to bias the devices so they are not completely off when they are not in use. This approach is called class AB operation.

Class AB

In a class-AB amplifier, the conduction angle is intermediate between class-A and -B ; each one of the two active elements conducts more than half of the time.
Class-AB is widely considered a good compromise for amplifiers, since many types of input signal are nominally quiet enough to stay in the "class-A" region, where they are amplified with good fidelity, and by definition if passing out of this region, will be large enough that the distortion products typical of class-B will be relatively small. The crossover distortion can be reduced further by using negative feedback.
In class-AB operation, each device operates the same way as in class-B over half the waveform, but also conducts a small amount on the other half. As a result, the region where both devices simultaneously are nearly off is reduced. The result is that when the waveforms from the two devices are combined, the crossover is greatly minimised or eliminated altogether. The exact choice of quiescent current makes a large difference to the level of distortion. Often, bias voltage applied to set this quiescent current must be adjusted with the temperature of the output transistors. Another approach is to include small value resistors in series with the emitters.
Class-AB sacrifices some efficiency over class-B in favor of linearity, thus is less efficient. It is typically much more efficient than class-A.