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THIS EXAM INCLUDES SOME PARTS ALL OF THE QUIZZES I OFFER BUT YOU MAY PURCHASE SEPARATELY IF DESIRED. Jump to navigation Jump to search Power amplifier classes are, in electronics, letter symbols applied to different power amplifier types. A, B, AB and C for analog designs—and class D and E for switching designs. The image of the conduction angle derives from amplifying a sinusoidal signal. 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. The active element remains conducting all of the time. Amplifying devices operating in class A conduct over the entire range of the input cycle. A class-A amplifier is distinguished by the output stage devices being biased for class A operation. 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. Best for low signal levels of radio receivers due to low distortion. In a power amplifier, this not only wastes power and limits operation with batteries, but increases operating costs and requires higher-rated output devices.
Class-A power amplifier designs have largely been superseded by more efficient designs, though their simplicity makes them popular with some hobbyists. 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. They are sometimes used as medium-power, low-efficiency, and high-cost audio power amplifiers. In practice, distortion occurs near the crossover point.
In a class-B amplifier, the active device conducts for 180 degrees of the cycle. At radio frequency, if the load of the class-B amplifier is a tuned circuit, a single device in class B can be used. These are used in linear amplifiers, so called because the radio frequency output power is proportional to the square of the input excitation voltage. Class-B amplifiers amplify the signal with two active devices, each operating 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. Complementary devices are each used for amplifying the opposite halves of the input signal, which is then recombined at the output. Class AB is widely considered a good compromise for amplifiers, since much of the time the music signal is quiet enough that the signal stays in the “class-A” region, where it is amplified 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.
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. The result is that when the waveforms from the two devices are combined, the crossover is greatly minimised or eliminated altogether. It is typically much more efficient than class A. A vacuum tube amplifier design will sometimes have an additional suffix number for the class, for example, class B1. A suffix 1 indicates that grid current does not flow during any part of the input waveform, where a suffix 2 indicates grid current flows for part of the input waveform.
This distinction affects the design of the driver stages for the amplifier. Suffix numbers are not used for semiconductor amplifiers. The usual application for class-C amplifiers is in RF transmitters operating at a single fixed carrier frequency, where the distortion is controlled by a tuned load on the amplifier. The input signal is used to switch the active device causing pulses of current to flow through a tuned circuit forming part of the load. The class-C amplifier has two modes of operation: tuned and untuned. The diagram shows a waveform from a simple class-C circuit without the tuned load. This is called untuned operation, and the analysis of the waveforms shows the massive distortion that appears in the signal.
The signal bandwidth of the amplifier is limited by the Q-factor of the tuned circuit but this is not a serious limitation. Any residual harmonics can be removed using a further filter. In practical class-C amplifiers a tuned load is invariably used. In one common arrangement the resistor shown in the circuit above is replaced with a parallel-tuned circuit consisting of an inductor and capacitor in parallel, whose components are chosen to resonate the frequency of the input signal. Power can be coupled to a load by transformer action with a secondary coil wound on the inductor.
The active element conducts only while the collector voltage is passing through its minimum. By this means, power dissipation in the active device is minimised, and efficiency increased. Class-D amplifiers use some form of pulse-width modulation to control the output devices. The conduction angle of each device is no longer related directly to the input signal but instead varies in pulse width. The main advantage of a class-D amplifier is power efficiency. Class-D amplifiers are widely used to control motors—but are now also used as power amplifiers, with extra circuitry that converts analogue to a much higher frequency pulse width modulated signal. High quality class-D audio power amplifiers have now appeared on the market.
These designs have been said to rival traditional AB amplifiers in terms of quality. An early use of class-D amplifiers was high-power subwoofer amplifiers in cars. Because subwoofers are generally limited to a bandwidth of no higher than 150 Hz, switching speed for the amplifier does not have to be as high as for a full range amplifier, allowing simpler designs. The letter D used to designate this amplifier class is simply the next letter after C and, although occasionally used as such, does not stand for digital. Other amplifier classes are mainly variations of the previous classes. Wasted heat on the output devices can be reduced as excess voltage is kept to a minimum. The amplifier that is fed with these rails itself can be of any class.