Q. I have learned about Class A, B and C amplifiers, but have heard that there is also a kind called Class D. What is a Class D amplifier, and how does it differ from the first three classes?

A. In a Class D amplifier, the power transistors are operated as switches. They are driven by pules so that they turn on and offvery rapidly. The duty cycles of their control pulses are made to follow variations in the signal voltage.

In other words, the on” and “off” times are varied, so that the output from the power transistors is a series of pulses of varying widths. The pulses are smoothed into a continuous waveform in the output by LC filter circuits.

The advantage of Class D is that the power transistors are either on or off. When they are on, the current through them is high, but the voltage across them is low.

Since power is the product of current times voltage, the power dissipated in the output transistors is low. The same is true when the transistors are off. Although the voltage across them is high, the current through them is low, so the power they must handle is also low.

The transistors spend very little time in the range where both the current through them and the voltage across them are substantial, and the power is high. Only during the relatively short rise and fall times of the pulses are they in this range.

(One instructor likens this to someone running quickly back and forth over hot coals. He spends most of his time at one end or the other of the coals, where the ground is cool, and little of the time actually over the hot coals.)

Consequently, these amplifiers can produce a powerful output signal without having to dissipate much power. Class D amplifiers are very efficient. Unlike Class C amplifiers, they do not require a tuned load. They are often used in the power output stages of audio amplifiers and radio transmitters.



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