What is a Power Amplifier
An electronic amplifier which is designed to produce a higher magnitude signal from a given input signal is known as a power amplifier. Power amplifiers are commonly used in applications such as headphones, headphones, radio frequency transmitters, etc. In order to do the required amplification, the input of the power amplifier has to be increased to a certain level. Power amplifiers are usually used in the end of the design though the other amplifiers such as voltage/ current amplifiers designed to use directly to drive loads.
As we mentioned above, a power amplifier has a limitation in the input signal. Therefore, we pre amplify the initial signal using current or voltage amplifiers without directly connecting it to the power amplifier. This method modifies the input signal in a way that the power amplifier can function well. The below diagram shows how a power amplifier is embedded in an audio amplifier design.
In the above block diagram, the input source used is a microphone. Since the magnitude of the input signal from the microphone is not adequate, the signal has to be pre amplified. Therefore, the magnitude of the current and voltage of the signal is increased slightly. Before the signal reaches the power amplifier it is subjected to aesthetic adjustment using a tone and volume controls circuit. Finally, the output from the power amplifier is connected to a speaker.
Power Amplifier Gain
The connection between the input signal and the output signals in a power amplifier is called the amplifier gain. This is a basic measure of in which magnitude an amplifier amplifies the given input signal. It is also called as the ratio between the power of the input signal and power of the output signal. Amplifier gain has no units, but there is a common symbol using for this (A). Usually, the gain is represented in decibels, dB
Power Amplifier Gain (dB) = 10 Log 10 (Output Power / Input Power)
Where,
Input power = Vin * Iin and Output Power = Vout * Iout
Ideal Amplifier Model
The ideal amplifier passes the input signal which is fed through the output in an undistorted manner enlarging the signal without any delay. Usually, an ideal signal amplifier will have three main properties, the general amplifier model also known as the ideal amplifier model can be used to represent the following three properties regardless of the complexity of the circuit design.
- Input Resistance (Rin)
- Output Resistance (Rout)
- Amplification Gain (A)
Amplifier Efficiency
Amplifier efficiency is the ratio between power delivered to the load and power taken from the supply.
Amplifier Efficiency (η) = Pout / Pin
Where,
Input power = Vin * Iin and Output Power = Vout * Iout
Types of Power Amplifier
- Digital power amplifier
Digital power amplifiers are mainly used in amplifying the power of pulse width modulated signals and they can be seen in power electronic components such as motor drives. They amplify the input signal which comes through the microcontroller system and feed the amplified signal to the DC motor.
Usually, the analog signal in the digital power amplifiers is encoded in a pulse train and not handled directly and it is restored at the output using a bandpass filter. The above principle is known as class S due to the discretization in the signal amplitude. Digital power amplifiers are widely used in audio frequency applications and now they can be seen also in microwave frequencies.
- MOSFET power amplifier
MOSFET transistors are also known as Metal Oxide Semiconductor Field Effect transistors and they are commonly used in applications with small signal linear amplification. Since in those applications the magnitude of the input impedance is very large, they can be easily biased. In order to develop a linear amplification, the MOSFET needs to be operated in its saturation region and it has to be operated biased around a centralized constant Q point.
These power amplifiers consist of active elements with passive transmission line circuits to critical functioning in microwave related systems and designs. Microwave power amplifiers are usually seen in applications such as antennas, limiter diodes, MMIC based power amplifiers, etc.
- Transistor power amplifier
In this power amplifier, a power transistor is used to amplify the signal. Tape recorders, public address systems, radio and television receivers and other electronic devices use transistor power amplifiers in their circuit design.
- High power amplifier
To amplify a small signal to a significant level we use a high-power amplifier. This is a nonlinear device which is used in applications mostly in applications with high power output.
- Multichannel power amplifier
These kinds of amplifiers produce copies of the same input signal with the same values distributed over multiple output channels.
Power Amplifier Classes
The Power amplifier circuit can be designed in many ways; therefore, the characteristics of every circuit will be different from each other. In order to differentiate the behavior of the power amplifier circuits power amplifier classes are introduced. They are named according to English alphabetical letters which is assigned to easily identify the method of operation. There are mainly two classes for the categories. They are, A, B, AB or C categories which denote the power amplifiers that are designed to amplify the analog signals and categories such as D, E, F which denotes the power amplifiers that are designed to amplify Pulse Width Modulated digital signals. The commonly observable power amplifier classes are Class A, Class B, Class AB, and Class C. They are mostly seen in the applications that are used in audio amplifier circuits.
Classification of Power Amplifiers Based on Frequencies
When considering the frequencies, power amplifiers can be divided into two categories as follows:
- Audio Power Amplifiers
This type of amplifier are developed to increase the magnitude of the power of weaker audio input signal by a larger amount. The audio power amplifier can be seen in applications such as televisions, mobile phones where there is a speaker driving circuit is in use and the output of an audio power amplifier ranges varies from a few milliwatts of power to thousands of watts power output.
- Radio Frequency Power Amplifiers
This type of amplifier are used in applications such as FM broadcasting and they need antennas with input signals with several thousands of kilowatts of power. Above wireless transmission methods need modulated signals to be transmitted in wide distances through the air. These signals are sent through the antennas and the range of the signal transmission depends on the power of the input signals which is going to fed to the antenna. Therefore, we use the radio frequency power amplifiers to get a bigger magnitude of power of the modulated signal which is adequate for good signal transmission.
Classification Based on Mode of Operation
When considering the mode of operation of a power amplifier, we can categories it into three categories. They are as follows.
- Power amplifier Class A– Class A power amplifier has a collector current flowing every time in its collector when considering the full cycle of the input signal. The efficiency of this class is around 40% but has a good signal reproduction and good linearity.
- Power amplifier Class B – In class B power amplifier, the current at the collector flows only in the positive half cycle of the input signal. The maximum efficiency achieved by this class is around 70%.
- Power amplifier Class C – In class C power amplifier, the current at the collector flows less than the half cycle of the input signal. This is the class having the highest efficiency but the signal reproduction is no good when compared to other categories.
- Power amplifier Class AB – To get the advantages of both class A and class B we add those classes and develop another power amplifier class called Class AB. The efficiency of this class is around 40% to 70% and this has a good signal reproduction than class B.
