What is a power amplifier?
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A power amplifier is an electronic amplifier designed to increase the power amplitude of a given input signal. The power of the input signal is increased to a level sufficient to drive the load of output devices such as speakers, headphones, RF transmitters, etc. Unlike voltage/current amplifiers, power amplifiers are designed to drive the load directly and serve as the final module in the amplification chain.
The input signal to the power amplifier must be above a certain threshold. So instead of passing the raw audio/RF signal directly to the power amplifier, it is first pre-amplified with a current/voltage amplifier and sent to the power amplifier as an input after necessary modifications. Below you can observe the block diagram of the audio amplifier and the usage of the power amplifier.
In this case, the microphone is used as the input source. The signal strength from the microphone is not strong enough for the power amplifier. So first preamp it and then slightly increase its voltage and current. The signal then passes through tone and volume control circuits, which make aesthetic adjustments to the audio waveform. Ultimately, the signal passes through a power amplifier whose output is fed to the speakers.
Types of Power Amplifiers
Depending on the type of output device connected, power amplifiers are divided into the following three types:
audio power amplifier
RF Power Amplifier
DC power amplifier
audio power amplifier
This type of power amplifier is used to increase the power level of weaker audio signals. Amplifiers used in speaker drive circuits for TVs, mobile phones, etc. fall into this category.
Audio power amplifiers have outputs ranging from a few milliwatts (like headphone amplifiers) to several kilowatts (like power amplifiers in Hi-Fi/home theater systems).
RF Power Amplifier
Wireless transmission requires modulated waves to be sent over long distances through the air. The antenna is used to transmit the signal, and the transmission range depends on the power of the signal fed to the antenna.
For wireless transmissions such as FM radio, antennas require input signals with powers of thousands of kilowatts. Here, an RF power amplifier is used to increase the power amplitude of the modulated wave to a level sufficient to achieve the desired transmission distance.
DC power amplifier
A DC power amplifier is used to amplify the power of a PWM (pulse width modulation) signal. They are used in electronic control systems that require high power signals to drive motors or actuators. They take input from a microcontroller system, increase its power, and feed the amplified signal to a DC motor or actuator.
Power amplifier class
There are several ways to design a power amplifier circuit. The operation and output characteristics of each circuit configuration are different from each other.
To differentiate the characteristics and performance of different power amplifier circuits, power amplifier classes are used, in which letter symbols are assigned to identify the method of operation.
They are roughly divided into two categories. Power amplifiers designed to amplify analog signals are class A, B, AB or C. Power amplifiers designed to amplify pulse-width modulated (PWM) digital signals are located under D, E, F, etc.
The most commonly used power amplifiers are those used in audio amplifier circuits, and they are class A, B, AB or C. So let's look at them in detail.
Class A Power Amplifier
An analog waveform consists of positive highs and negative lows. In this type of amplifier, the entire input waveform is used in the amplification process.
A single transistor is used to amplify the positive and negative halves of the waveform. This makes their design simple and makes Class A amplifiers the most commonly used type of power amplifier. Although such power amplifiers have been replaced by better designs, they are still popular among hobbyists.
In this type of amplifier, the active elements (electronic components used for amplification, in this case transistors) are always in use, even when there is no input signal. This generates a lot of heat and reduces the efficiency of the Class A amplifier to 25% in the normal configuration and 50% in the transformer coupled configuration.
The conduction angle of a Class A amplifier (the portion of the waveform used for amplification in 360 degrees) is 360°. Therefore, the signal distortion level is very small, which can provide better high frequency performance.
Class B power amplifier
Class B power amplifiers are designed to reduce the efficiency and heating issues present in class A amplifiers. Such amplifiers use two complementary transistors instead of a single transistor to amplify the entire waveform.
One transistor amplifies the positive half of the waveform and the other transistor amplifies the negative half of the waveform. Therefore, each active device turns on half (180°) of the waveform, two of which when on will amplify the entire signal.
Due to the two-transistor design, the efficiency of class B amplifiers is greatly improved compared to class A amplifiers. They can achieve around 75% of the theoretical efficiency. Such power amplifiers are used in battery powered devices such as FM radios and transistor radios.
Since the two halves of the waveform overlap, there is very little distortion in the intersection area. To reduce this signal distortion, class AB amplifiers are designed.
Class AB power amplifier
A Class AB amplifier is a combination of Class A and Class B amplifiers. These amplifiers are designed to reduce the problems of inefficient class A amplifiers and signal distortion in the crossover area of class B amplifiers.
It can maintain high frequency response like a class A amplifier and maintain good efficiency like a class B amplifier. A combination of diodes and resistors is used to provide a small bias voltage, thereby reducing waveform distortion near the crossover region. Therefore, the efficiency drops slightly (60%).
Class C power amplifier
Class C power amplifiers are designed to achieve higher efficiency at the cost of reduced linearity/conduction angle (below 90°). In other words, the quality of amplification is sacrificed for efficiency.
A smaller conduction angle means more distortion, so this type of amplifier is not suitable for audio amplification. They are used for high frequency oscillators and amplification of radio frequency signals.
Class C amplifiers typically contain a tuned load that filters and amplifies the input signal at certain frequencies, while suppressing waveforms at other frequencies.
In this type of power amplifier, the active elements are turned on only when the input voltage is above a certain threshold, reducing power consumption and increasing efficiency.
Other power amplifier classes
Power amplifiers D, E, F, G, etc. are used to amplify PWM modulated digital signals. They fall into the category of switching power amplifiers that turn the output on or off continuously without any other level in between.
Due to this simplicity, power amplifiers belonging to the above categories can achieve theoretical efficiencies as high as (90-100)%.
The following are the applications of power amplifiers in different fields:
Consumer Electronics: Audio power amplifiers are used in virtually all consumer electronics devices, from microwave ovens, headphone drivers, televisions, mobile phones and home theater systems to theater and concert enhancement systems.
Industrial: Switching power amplifiers are used to control most industrial actuator systems such as servos and DC motors.
Wireless Communications: High power amplifiers are important for transmitting cellular or FM broadcast signals to users. Thanks to the power amplifier, higher power levels are possible, thereby increasing the data transfer rate and availability. They are also used in satellite communication equipment.
A quick introduction to the concept of a power amplifier. You know what a power amplifier is and its requirements, the different types and classes of power amplifiers, and several applications.