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Electronic amplifier

An electronic amplifier takes a low-power signal and power from a power supply, and produces a high-powered signal and waste heat. Amplifiers always introduce some noise and distortion, because of imperfections in their component parts.

Many types of electronic amplifier have been developed.

The earliest electronic amplifiers used vacuum tubes. An electric current was run from the anode, through a vacuum, through a grid, to a cathode. By varying the electric charge on the grid, the electrons would be repelled or permitted to pass through the grid. Field effect transistors work almost exactly like tubes, except they use a small piece of semiconductor rather than a hot filament and vacuum.

Another amplification scheme uses bipolar junction transistors. By varying the current in the base, a larger varying current is permitted to flow between collector and emitter.

Electronic amplification has also been performed with diodes, transformers, hall effect devices, and various types of gas-filled tubes. The principle is always to achieve a proportional increase in some combination of voltage and current, in order to increase the power of a signal.

One of the problems of amplifiers is to try to lower the resistance of the amplifying circuit element as much as possible. This increases efficiency, so that the amplifier wastes less power, and allows the transistor, tube, or other amplifying element to be more reliable by running cooler.

One basic method of increasing efficiency is to try to either turn the current all the way on, or all the way off. If the device is all the way on, then its resistance is as low as possible. Since it has minimal resistance, the minimal amount of power is converted to heat in the device. This is usually called "saturating" the device. When the device is all the way off, only the smallest amount of "leakage" current flows. Therefore, almost no current is converted to heat. Again, the device is efficient.

This effect is exploited by class C, D, E, and to a lesser extent B, and AB amplifiers. Only class A amplifiers do not use saturation. Clearly the topic fascinates engineers.

Class A amplifiers deliberately operate the amplifying device in its "linear range." They are designed for this purpose, and just accept the inefficiency, lessened reliability and waste heat. Class A amplifiers can waste more than 60% of the available power as heat. However, they introduce an absolute minimum of distortion.

Most hi-fi audio and instrumentation amplifiers are class A, with careful design to remain linear. Most often, transistors with very wide linear ranges are used. The resistor networks that set the bias voltages to power these transistors are very precise (always less than 1%!), to keep the transistors in their linear regions. Finally, very accurate impedance matches are used to move the signal from one part of the amplifer to another. Some people even favor tubes over transistors, claiming that since tubes have more electrons moving than transistors, the error is less, because the tiny bump of noise caused by a single electron is less important in a tube than a transistor (see shot noise). There is a myth that tubes are cleaner and distort less than MOSFETs. While to some ears they may sound better, it is typically because the distortion in tubes is at harmonic multiples of the primary signal frequency, much in the same way a bell sounds better than beating on a cardboard box. The amount of distortion from the bell is higher than from the box, but the distortion from the bell is at multiples of 1, 2, 3 etc. So if your bell resonates at 1000 hertz, it also has harmonics (distortion) at 250 Hz, 500 Hz, 1500 Hz, 2000 Hz, 2500 Hz, etc. Although the bell has a much higher distortion, most people enjoy the sound more than beating a cardboard box because the box's distortion frequencies are more or less non-multiple harmonics or even "random".

The most exotic instrumentation amplifiers are helium-cooled ruby MASERs, used in radio telescopes.

Linear amplifiers are also useful for single sideband and AM transmitters. Although they can be too expensive for commercial operators, amateur and military users favor them.

Class B amplifiers chop off the top of the waves, a process called "clipping." This improves efficincy a lot, but introduces distortion. Class B amplifiers tend to create unwanted harmonics at odd multiples of the fundamental frequency, an effect called "harmonic distortion." The more the wave is clipped, the closer it will approach to a square wave, and the greater the harmonic distortion will become. A perfect square wave has all the odd harmonics up to infinity!

A common way make a clipping amplifier is to bias the input signal so that the output of the amplifying device saturates- the clipping bias can usually be adjusted by adjusting the value of a bias resistor. Another way to make an amplifier clip is to have the linear range of the amplifying device pass the maximum voltage provided by the amplifer's power supply. This is sometimes used in very high power amplifiers, where a bias resistor would waste a lot of power.

There is a lot of historical hair-splitting about compromises on the percentage of clipping. A 30% clipping is popular because it seems to be a good compromise. Some purists insist that a class "B" amplifer has to have 50% clipping, and call a 30% clip an "AB" amplifier. A good portable AM receiver will have an amplifier with about a 30% clip. This also seems to be a good compromise for small single-sideband and AM transmitters.

Class C amplifiers approximate a wave by turning an amplifying element on or off. They're used for some radio transmitters, notably frequency-modulated (FM) and data transmitters. They're also used for high-efficiency audio amplifiers, like battery-powered bullhorns. They have terrible harmonic distortion. In transmitters, class C amplifiers create unwanted out-of-band harmonics. For example, when an amateur radio transmitter at low, shortwave frequencies (1.8 to 30MHz) creates television interference (near 100 MHz). In audio PAs, class C amplifiers create very distorted audio, so bad that it requires skilled listeners.

Class D, "digital" amplifiers use several, usually a binary number, of high power "switches" (i.e. transistors usually). The input signal must be "sampled" more than twice as often as the highest frequency of interest. The samples are turned into numbers. The numbers are used to switch combinations of the switches off and on, and the current from these is fed through a summing electronic mixer, usually a network of resistors. Since any individual transistor is either on or off, the amplifer is efficient.

A badly-designed digital amplifier can be a multiplying electronic mixer. It can add difference-frequencies between the sample rate and the desired signals. these show up as unwanted low frequency noise. In order to avoid these, an electronic filter must be placed on the input to throw away all signals with frequencies more than half that of the sampling frequency. A clever man named Nyquist proved that one must have at least one sample for the high part of the wave, and one for the low part of the wave. This sample rate, two times the maximum interesting frequency, is called the "Nyquist" frequency.

Another problem with class D amplifiers is that the sets of switches can't perfectly match what the output should be. One can get closer with more switches and resistors. This error is called "quantization error" and appears as harmonic distortion. In practice, most people can't hear quantization error in speech when the range of numbers is wider than about one in 4000, and the sampling rate is greater than 8,000 times per second. This is the rate and width that long distance telephone lines use.

Class E amplifiers use pulse width modulation to amplify. The plan here is that the output signal is turned off and then on periodically. The time off and on is made to be proportional to the desired output amplitude. Since this amplifer just has one switch, it can be cheaper than any other class.

However, it also has more types of error than any other class. Class E amplifiers take samples, and thus have a Nyquist frequency and require a filtered input. It's also common to use digital circuits to control the duty cycle by counting some very fast periodic clock signal. When this is done, class E amplifiers also have quantization error. Last but not least, class E amplifiers have sharp edges, so they can have severe harmonic distortion[?] though it occurs at frequencies above the frequencies of interest.

Class E amplifers are used to control motors. In fact, it's hard to find any other type of motor controller for small DC motors. They have also been used as transmitters for commercial AM and shortwave radio, although more modern systems use class D amplifiers, which introduce lower distortion.



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