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Amplifier Classes Explained

Discussion in 'Receivers and amplifiers' started by Oriphus, Apr 3, 2004.

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  1. Oriphus

    Oriphus Senior member

    May 31, 2003
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    Just like to clarify a few things for people who are unsure of the different amplifier classes. There are currently 5 classifications:

    [bold]Class A:[/bold] Traditional and Expensive. Its a fully linear amplifier with active circuit elements biased into their linear operating region. This means that the region must have enough voltage range to encompass the entire amplitude of an incoming signal in order to reproduce it without clipping or compressing at either extreme. An amplifier's output power supply voltage must equal, roughly, 200% of the maximum output signal swing expected. This method of operation is pure, but inefficient.

    [bold]Class B:[/bold] These amplifiers are somewhat more efficient by utilizing two drive elements operating in a push-pull configuration. On the positive excursion of the signal, the upper element supplies power to the load while the lower is turned off. During negative going signal excursions, the opposite operation occurs. This increases operating efficiency, but suffers from the nonlinear turn-on, turn-off region created where the driver elements switch from their ON state to their OFF state. This switching error creates a condition commonly called cross-over distortion.

    [bold]Class A/B:[/bold] amplifiers remedy cross-over distortion to a great degree by combining the best features of both classes. The push-pull drivers are carefully biased just above their fully OFF state so that the transition between drivers is smoother. Therefore, each driver is never completely turned OFF. This alleviates most of the cross-over distortion at the expense of efficiency. An A/B amplifier is still more efficient (60 Ð 65%) than a Class A amplifier.

    [bold]Class C:[/bold] amplifiers, biased at or below cutoff, are commonly used for certain types of RF transmission, but not commonly used in audio applications.

    [bold]Class D:[/bold]Class D amplifiers are not digital in the true sense. They are not driven directly by coherent binary data. They do act digitally in that the output drivers operate either in the fully ON-region or fully OFF-region. Think of Class D amps as being similar to a switch-mode power supply, but with audio signals modulating the switching action.

    A switch-mode power supply uses pulse-width modulation (PWM) to control the on/off duty cycle of the power switching transistor(s) providing power to a load. The efficiency is high because there is little voltage drop across the switch transistor during conduction. This means very low power dissipation in the switch while virtually all the power is transferred to the load. During the OFF period, there is essentially zero current flow. The quality and speed of MOSFET (metal oxide semiconductor field effect transistor) devices has led to compact, efficient, high frequency power supplies. Switch-mode power supplies are more efficient at high frequencies. At higher operating frequencies, components may become smaller and the power supply becomes very compact for the power delivered. In addition, the output filter components may be much smaller. Today, switching frequencies over 1 MHz are not uncommon. But, as you probably know, switch-mode supplies generate considerable noise.

    Audio signals can be used to modulate a PWM system to create a high power audio amplifier at nominal voltages using small components. Class D audio utilizes a fixed, high frequency carrier having pulses that vary in width based on signal amplitude. Class D amplifiers reach efficiencies as high as 90%. This is of great importance to portable applications relying on battery power. Class D portable, battery-powered audio gear may have battery life extended by 2.5 times or more.

    Thanks to Extron Electronics for saving me the time of having to write a lot of the Class D stuff myself ;-)
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