How it works : Amplifier

AMPLIFIER

An amplifier is a device for increasing the strength of a weak signal fed into it. Electronic amplifiers, which are the best known and most important type,are used in a huge variety of devices such as RADIO and TELEVISION receivers, RECORD PLAYERS, TAPE RECORDERS and HI-Fl SYSTEMS, RADAR, analogue COMPUTERS, SERVO MECHANISMS and electronic equipment generally. Other devices which amplify in a different way include mechanical amplifiers such as the PANTOGRAPH, which enlarges drawings, hydraulic amplifiers such as the POWER BRAKES of a car, acoustic amplifiers such as the horn of an old fashioned gramophone [phonograph], the fluid amplifiers used in FLUIDICS and magnetic amplifiers, used as theatre light dimmers and in computers.
All electronic amplifiers work in much the same way, though they differ widely in design and in the gain (degree of amplification) they produce. Gain can be measured as a proportional increase in voltage (the usual method for amplifiers), in current or in wattage- total electrical power. The heart of an amplifier, and the device that actually does the amplifying, is either a thermionic VALVE [vacuum tube] or a TRANSISTOR. Nearly all electronic amplifiers have several of these plus a set of RESISTORS, CAPACITORS, POTENTIOMETERS and related devices to control the flow of electricity through the basic amplifying components.
Valves [tubes] and transistors use different principles to perform the same function. Basically, they act as variable switches where the flow of a small current through one part of the device controls the flow of a larger current through another part. When the small current flows, the large current, which is drawn from a separate power source, flows too. When the small current stops flowing, the large one is shut off, and when the small current flows at, say, half power, so does the large one.
The proportion of the smaller current to the larger one is constant (at least in a linear amplifier, the most usual type). So if the small current is modulated (varied) by adding a signal from a record, tape or other source, the signal will be reproduced more or less faithfully at a much higher power in the form of a modulation of the large current. This large current can then he fed to a loudspeaker to convert it into an intelligible sound (or elsewhere, depending on what the amplifier is used for).
In the case of a transistor, the small current is fed in between the terminals known as the emitter and the base, and the large current flows between the emitter and the collector. So a transistor has only three terminals, not four, to carry the ingoings and outgoings of two currents. The collector,emitter and base are represented in the thermionic valve by the cathode, anode and grid. The names and principles are different, but the function is the same.

Amplifiers in practice
A simple amplifier as described would not normally produce enough gain for practical purposes. It might reach a 30 times increase in voltage. But an ordinary hi-fi amplifier operating in normal conditions would probably give a 100,000 times voltage increase. Some amplifiers used for other purposes have much higher gains than this. Gains of this degree are produced by using an amplifier consisting of several stages. The output of the first stage is passed to the second stage and amplified further, and so on through as many stages as are needed to yield the necessary gain. A hi-fi amplifier usually consists of two stages, the first a pre-amplifier with a fixed gain setting which boosts the incoming signal from the record, tape or radio to a manageable level at which it can be handled by the second stage, or main amplifier, which provides sufficient power to drive the speakers. This stage includes a volume control to adjust the final gain..
Amplifiers are normally designed with an inbuilt gain much higher than is actually needed or used. This is then moderated by the use of negative FEEDBACK, where a portion of the output signal is fed back to the input with a reversed polarity (current direction) to reduce the gain. In this way, the volume can be controlled by varying the amount of negative feedback. More importantly, distortion will be reduced and any changes in the supply voltage or the electronic components will have less effect on the gain.
If positive feedback were used, part of the output signal being fed back to the input with the same polarity to boost the gain, the result would probably be to produce unwanted oscillations, which are sometimes heard in public address systems as a loud howling noise. This is caused by the output boosting the input, the input increasing the output accordingly, the increased output further boosting the input, and so on up to uncontrollable levels, causing the amplifier to stop working. In public address systems this is caused by sound from the loudspeakers (the output end) reaching the microphone (the input end).
The quality of a linear amplifier is assessed by its ability to magnify the input signal 'faithfully', that is, without altering its essential shape. But amplifiers, like other physical systems, are not perfect. To give a faithful reproduction of, say, a musical instrument, an amplifier must respond to all the frequencies (pitches of sound) that the instrument produces, giving an equal response to all of them. In practice, this means that a hi-fi amplifier must respond equally to the whole range of audible frequencies, from about 30Hz to 18 kHz (30-18,000 cycles per second). This range of frequency response is known as the bandwidth.
No actual amplifier can live up to this ideal, but high quality hi-fi amplifiers come closest. The frequency response can be partially altered by adjusting the treble and bass controls. The video amplifiers used to form the pictures of TV and radar receivers have an enormous frequency bandwidth from 0 Hz (that is, direct current) to 6 MHz (6,000,000 cycles per second).
Amplifiers also suffer from harmonic distortion-output at frequencies twice, three or more times that of the signal-and from amplifier noise, a random jumble of different frequencies independent of the input signal. This is termed 'white noise' because it includes all frequencies just as white LIGHT includes light waves of all frequencies between red and violet. Amplifier noise can never be totally eliminated. It can always be heard in a sound amplifier as a slight hiss. But a good hi-fi amplifier can have a signal to noise ratio better than 5,000,000 to 1. For a 10 watt amplifier this would mean a noise power of less than 3 microwatts.
AMPLITUDE (see sine wave)

Reproduced from HOW IT WORKS p101