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Old 02-19-2008   #32
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Default Re: Amps for dummies

Originally Posted by kennyt View Post
Class A use full power all the time, so they are always ready to output it. They run hot and eat up electricity.
For Class A output, the transistors are biased so that they are conducting for both halves of AC waveforms, The highest "positive-going" output rides above a midpoint value, while the most "negative-going" output rides below the midpoint value but still out of the non-linear portion of the curve close to zero, rather than running at full power.

Originally Posted by kennyt View Post
Class B switches on and off as needed, saves power and heat
Class B output is biased so that the transistors are off during almost one half of AC waveform. A second transistor is used to carry the opposite portion of the waveform during the time that the first one is off. This allows both transistors a chance to recover (each one has an average 50% duty cycle, and also allows for greater total signal swings, since the peak to peak signal is summed. Class B is non-linear because there is a period near the positive and negative region near zero, where neither transistor conducts.

Originally Posted by kennyt View Post
Class A/B works like class A to a point, then uses class B type switching for the higher outputs.
In Class AB, a small bias is placed on the transistors to keep them conducting a little earlier than they normally would, to keep them out of that non-linear part of their curves close to zero.

Class C is where the transistors are biased to conduct less than 50% of the time, but they still linearly track a portion of the input signal that is higher than some threshold volue. One variation (Class G, I think) uses multiple sets of power supplies that are switched in as the signal swings toward the normal power supply limit, to extend the output signal beyond it. Another variation (Class H, I think) actually had separate transistors and power supplies that were turned on to follow the peaks.

Originally Posted by kennyt View Post
D are digital or switching amps, they've been around a while but people are really just getting them accepted by the high end. No, all solid state amps are not digital amps.

This is obviously only a brief overview, but will give you the idea.
Class D is where the transistors are biased to conduct as switches - when they are on, they are driven to the supply rails - A variation on the pulse amplifier technique actually uses a form of modulation such as pulse width modulation, combined with summing up the pulses (integration) through the load and a capacitor to reconstruct an analog signal.
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