Another article on the classification of amplifiers. This type of classification is based on the position of the Q point on the load line.
Learning Objectives:
Learn about the difference between classes of power amplifiers
FAQs
Why is power dissipation considered a big issue in power amplifiers?
Is a large-signal operation different from a small-signal operation?
How to eliminate crossover distortion?
Introduction
Large signal amplifiers or power amplifiers provide a large amount of power to the load. This amplifier category has different classes. Every class has a different region of operation on the load line. The types of amplifiers are usually at the output stage of the system to drive or power the device connected as a load. Some important features are
Power efficiency
Maximum power
The power dissipated should be as low as possible
Impedance matching to the output device. Low output resistance so that the power loss is minimum.
This classification can also be viewed as the output stage of an amplifier. It means different classes of amplifiers can be used as an output stage of a multistage amplifier system. Each class has a different shape of the output waveform. The following figure compares the output waveforms obtained from each class.
How does a power amplifier look like?
A power amplifier can either be a simple power transistor (not a common transistor but designed with some extra thermal stability properties) or an integrated circuit along with special thermal properties. Power transistors have a larger surface area for efficient heat sinking.
Power amplifiers process large signals, and a larger amount of power is required. So, all of the power devices must dissipate the internally generated heat. For power, the BJT collector is a critical junction. a heat sink is necessary to attach with it. Also, the case of power transistors has a large contact area. Internally generated heat flows from the case then heat sinks and then dissipates in the surrounding air.
Why is power dissipation considered a big issue in power amplifiers?
Power is defined as the rate at which energy is consumed or dissipated in a device. Power generated must be dissipated fast enough to avoid temperature buildup. The heat generated is equal to heat dissipated or consumed.
Large signal operation is also applied to digital switching circuits. As you know switching devices operate either in cut-off or saturation modes. But its operation is limited to either of the two states only. So, heat dissipation is not an issue in a digital switching system.
Is large-signal operation different from small-signal operation:
For signals that have a power greater than 1W are considered large signals. In large signal operations, approximation and models are not applicable. In a small signal, it is assumed that the incoming signal is small enough and can not be able to change device parameters that are beta and re`.
A small-signal swings over a small part of the AC load line. While a large signal approaches the limits of the AC load line.
Class A Amplifiers - A brief overview:
It necessarily operates in a linear region for 360 degrees of the input cycle and cut off at 0 degrees. Following is the simple BJT class A amplifier circuit and its load line.
The maximum possible signal swing is achieved when the Q point is centred at the AC load line. With the help of a load line, we can conclude the following points.
Collector current is maximum (peak value of current or peak of the sinusoid) at IC(sat) and minimum at its cut off value.
Similarly, VCE collector to emitter voltage is maximum (peak voltage or peak of the sinusoid) is at Vce(sat) and minimum at its cut off value.
Class B Amplifiers - A brief overview:
Class B amplifiers are biased such that they operate in linear region for 180 degrees of the input signal half cycle of the sinusoid) and then cut off for the next 180 degrees. Q point is at cut off or it is biased at cut off.
Since half of the input cycle is clipped, the output of the class B amplifier is highly distorted. As I discussed earlier, they are linear amplifiers, the shape of the output waveform is the same as that of the input wave. To get the same output waveform similar to the input waveform (similar in shape) there are two complementary transistors. This class of amplifiers use two complementary transistors. One of the transistors amplifies the positive part of the signal and the other transistor amplifies the negative part of the signal.
A simple class B amplifier with the help of two complementary BJTs (one is NPN and the other is PNP) and the load line is given below.
During the positive half cycle, Q1 is ON Vce varies from its Q point value (that is 0V) to its maximum value (that is Vcc). During the negative half-cycle, Q1 remains turned off. Similarly, during the positive half cycle, Q2 remains turned off. During the negative half-cycle, it produces the negative half of the Vce.
During the positive half cycle, Q1 is ON Ic varies from its Q point value (that is 0V) to its maximum value (that is IC(sat)). During the negative half-cycle, Q1 remains turned off. Similarly, during the positive half cycle, Q2 remains turned off. During the negative half-cycle, it produces the negative half of the Vce.
Why does cross over distortion occur?
One of the most simple classes B circuits is shown in the figure above.
To turn on the transistor VBE ≥ 0.7V. When the base voltage is 0 (VBE = 0 or VBE < 0.7V) neither of the transistors conducts. So there is no waveform at the output. Because of this output waveform is not the same in shape as that of an input waveform. This type of distortion is known as cross over distortion.
Class AB Amplifiers - A brief overview:
This class of amplifiers are biased such that they operate in a linear region and conduct slightly more than 180 degrees. The amplifier is then cut off for the rest of the cycle. The Q point is slightly above the cut off point.
How to eliminate crossover distortion?
This class is specifically designed to eliminate crossover distortion. Slight changes made in the class B circuit will eliminate the distortion.
Class C Amplifiers - A brief overview:
The class C amplifiers are biased such that they operate less than 180 degrees of the input signal. The output is highly distorted. They are also known as tuned amplifiers and are used at radio frequencies.
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| Class C Amplifier |
The figure above shows a simple CE amplifier. The idea of biasing is noticeable. The base is negatively biased with the voltage source VBB. The signal applied must have an amplitude (peak value) greater than VBB+VBE. The transistor is ON for a very short interval of time and produces a pulse (pulse of Ic) at the output. The output is highly distorted. To get the output voltage similar to the input voltage in shape and tank circuit is used.
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