Class A amplifiers amplify over entire input cycle such that output signal is the exact magnified copy of input. Though class A amplifiers have the conduction angle of 360o, they are not proficient being no more than 50% efficiency at most. This is due to device is always conducting whether or not input signal is applied.
Class A amplifiers find application where efficiency is not primary design criteria, but linearity. Many small signal linear amplifiers are developed as class A amplifiers. Class A amplifiers are usually more linear and less complex than other kinds, but are very incompetent. There exists the subclass designated A2 and that refers to vacuum tube class A stages where grid is permitted to be driven slightly positive on signal peaks, resulting in slightly more power than in normal class A.
Class A amplifier:
Class A amplifiers give lowest distortion but are most expensive, and are least practical to apply for high power applications. Class A amplifiers waste power but generate very clean signal output. Class AB amplifiers dominate market and rival best Class A amplifiers in sound quality. They employ less power than Class A, and can be cheaper, smaller, cooler, and lighter. Class D amplifiers are even smaller than Class AB amplifiers and more competent, as they use high-speed switching rather than linear control.
In some amplifiers, output devices are tubes. Many amplifiers use more than one transistor or tube per function in output stage to increase power. Class A refers to the output stage with bias current greater than maximum output current, so that every output transistors are always conducting current. Biggest benefit of Class A is that it is most linear, it has lowest distortion. Biggest drawback of Class A is that it is inept, it takes a very large Class A amplifier to deliver 50 watts, and that amplifier utilizes lots of electricity and gets very hot. Few high-end amplifiers are Class A, but true Class A only accounts for maybe 10% of small high-end market and none of the middle or lower-end market. Class B amplifiers have output stages that have zero idle bias current. Naturally, a Class B audio amplifier has zero bias current in the very small part of power cycle, to avoid nonlinearities. Class B amplifiers have important benefit over Class A in efficiency as they use almost no electricity with small signals.
Advantages and drawbacks of class A amplifiers:
The merits of the Class A amplifier are as follows:
Drawbacks when the class A amplifier is transformer coupled are:
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