Class A Amplifiers Class B Amplifiers Class AB Amplifiers


The term amplifier is very generic. In general, the purpose of an amplifier is to take an input signal and make it stronger (or in more technically correct terms, increase its amplitude). Amplifiers find application in all kinds of electronic devices designed to perform any number of functions. There are many different types of amplifiers, each with a specific purpose in mind.

Small signal amplifiers are generally referred to as "Voltage" amplifiers as they convert a small input voltage into a much larger output voltage. Sometimes an amplifier is required to drive a motor or feed a loudspeaker and for these types of applications where high switching currents are needed Power Amplifiers are required.

The main function of Power amplifiers (also known as large signal amplifiers) is to deliver power, which as we know from above, is the product of the voltage and current applied to the load. The power amplifier works on the basic principle of converting the DC power drawn from the power supply into an AC voltage signal delivered to the load.


Class A Power Amplifiers

The transistor is biased in class A which means that collector current flows all the time..

The collector current can increase or decrease.

The input signal increases and decreases the forward bias causing the collector current to change.

These changes in current in the primary of the transformer induce signal currents in the secondary.

The transformer matches the output impedance of the transistor to the loudspeaker impedance.

The disadvantage of this circuit is that the collector current is high even if there is no signal input.

Class B push-pull is more efficient


Class B Power Amplifiers

As the output transistors for each half of the waveform, both positive and negative, requires a base-emitter voltage greater than the 0.7v required for the bipolar transistor to start conducting, the lower part of the output waveform which is below this 0.7v window will not be reproduced accurately resulting in a distorted area of the output waveform as one transistor turns "OFF" waiting for the other to turn back "ON".

This type of distortion is called Crossover Distortion and is looked at later on in this section.


Class AB Amplifiers

The Class AB Amplifier is a compromise between the Class A and the Class B configurations above. While Class AB operation still uses two complementary transistors in its output stage a very small biasing voltage is applied to the Base of the transistor to bias it close to the Cut-off region when no input signal is present.

An input signal will cause the transistor to operate as normal in its Active region thereby eliminating any crossover distortion. A small Collector current will flow when there is no input signal but it is much less than that for the Class A amplifier configuration. This means then that the transistor will be "ON" for more than half a cycle of the waveform. This type of amplifier configuration improves both the efficiency and linearity of the amplifier circuit compared to Class A

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