There are many forms of electronic circuits classed as amplifiers, from Operational Amplifiers and Small Signal Amplifiers up to Large Signal and Power Amplifiers. The classification of an amplifier depends upon the size of the signal, large or small, its physical configuration and how it processes the input signal, that is the relationship between input signal and current flowing in the load.
Amplifiers can be thought of as a simple box or block containing the amplifying device, such as a , or , which has two input terminals and two output terminals (ground being common) with the output signal being much greater than that of the input signal as it has been “Amplified”.
Generally, an ideal signal amplifier has three main properties, or ( ), or ( ) and of course amplification known commonly as or ( ). No matter how complicated an amplifier circuit is, a general amplifier model can still be used to show the relationship of these three properties.
Not all amplifiers are the same and are therefore classified according to their circuit configurations and methods of operation. In “Electronics”, small signal amplifiers are commonly used devices as they have the ability to amplify a relatively small input signal, for example from a such as a photo-device, into a much larger output signal to drive a relay, lamp or loudspeaker for example.
A common type of amplifier distortion is called harmonic distortion. It can arise if any component in the amplifer clips the peaks of the waveform. A common specification for high fidelity amplifiers is the total harmonic distortion. This distortion may be less than 1%, or even less than 0.5% from 20-20,000 Hz for high quality amplifiers.
In the diagram, the input is a single frequency (pure sine wave), but the output waveform is clipped by the amplifier. The result is that harmonic frequencies not present in the original signal are produced at the output (harmonic distortion). This harmonic distortion contains only odd harmonics if the clipping is symmetrical. For example, a geometrical square wave has only odd harmonics, and as a signal is clipped, it approaches a square wave rather than a sine wave.
An amplifier can be said to be linear if the output voltage is strictly proportional to the input signal. Any nonlinearity, such as that arising from the semiconductor devices themselves, will give rise to harmonic distortion. Such defects in the performance of the devices can be minimized by using negative feedback in the circuit so long as the output is not overdriven to the point of clipping.
Plots of frequency spectra such as those illustrated here can be important diagnostic and research tools. Converting a signal from a plot as a function of time to a plot as a function of frequency is called Fourier analysis, and a common display is the Fast Fourier Transform or FFT of the signal.
Sound reproduction concepts