Power Amplifier

Power amplifiers serve as fundamental electronic tools that enhance the strength of electrical signals, holding a pivotal position within numerous electronic systems. They fulfill crucial roles in applications necessitating signal augmentation, such as audio systems, RF (Radio Frequency) transmitters, and various other domains. In this all-encompassing exposition, we will explore the domain of power amplifiers, examining their classifications, operational categories, utility scenarios, and attributes related to performances.

What is a Power Amplifier ?

An electronic circuit exists that enhances the strength of an incoming signal, referred to as a power amplifier. In contrast to small-signal amplifiers, which concentrate on boosting voltage or current while preserving smoothness, power amplifiers are engineered to deliver robust output with minimal aberration. They find extensive utility in situations demanding a substantial augmentation of signal potency, such as managing audio system speakers or transmitting wireless signals over extended distances.

Block Diagram of Power Amplifier

The main features of these types of amplifiers are circuit power η, the maximum amount of power that the circuit can handle, and impedance matching to the output device. Power amplifiers are designed with BJT and normal CE mode is used in power amplifiers.

Amplifier Efficiency: It is defined as the ratio of output AC power to the input DC power.

[Tex]η= \frac{P_{ac}}{P_{dc}}*100% [/Tex]

Distortion: The change in output wave shape from the input wave shape of an amplifier is known as distortion. The distortion can be reduced by using negative feedback in the amplifier.

What are the Types of Power Amplifier?

Power amplifiers can also be classified based on various factors. Let’s explore some common classifications:

Classification Based on Frequencies

1. Audio Power Amplifiers: It is designed for amplifying audio signals. These amplifiers are commonly used in speakers, televisions, and mobile phones, etc, to increase the power of weak audio signal. It ranges from few milliwatts to thousands of watts.
   
2. Radio Frequency Power Amplifiers: It is used in radio frequency applications. The range of wireless transmissions, which rely on antennas to send modulated waves over large distances, is influenced by the signal strength. Antennas need input signals that are thousands of kilowatts in power to broadcast FM. Power amplifiers are used to boost the power so that the modulated waves go to the necessary distance.
   
3. DC Power Amplifiers: DC power amplifiers amplify PWM signals in electronic control systems for high-power motors or actuators. They increase input power from microcontrollers and send amplified signals to DC motors or actuators, ensuring they are driven effectively.

Classification Based on Mode of Operation

1. Class A
2. Class B
3. Class AB
4. Class D

Graphs of Classes of Power Amplifier

We will study about classes of power amplifier in the next topic.

Classes of Power Amplifiers

Power amplifiers can be categorized into several classes based on their mode of operation and efficiency. Classes of power amplifiers are as follows:

1. Class A Power Amplifier
2. Transformer Coupled Class A Power Amplifier
3. Push-Pull Class A Power Amplifier
4. Class B Power Amplifier
5. Class AB Power Amplifier
6. Class C Power Amplifier

Class A Power Amplifier

Class A amplifiers operate in a mode where the output transistors conduct during the entire cycle of the input signal. This results in minimal distortion, but also low efficiency, since the transistors are always conducting, leading to significant heat generation.

Circuit Diagram (Class A Power Amplifier)

• With class A amplifier Q point lies middle of the load line so that signal can swing over the maximum possible range without saturating or cut off the transistor as seen in the figure below.
• Due to this, the output signal is obtained for the full cycle of the AC input. i.e. 360^o.
• Due to changes in Ic, the voltage Vce will also fluctuate sinusoidally.
• The operating point (Q-point) of the power transistor is biased to be roughly in the middle of the load line.
  

Graphical Representation (Class A Power Amplifier)

Important Mathematical Equations of Power Amplifier

DC power drawn from collector battery:

[Tex]P_{in} = V*I [/Tex]

[Tex]P_{in} = V_{cc}*(I_{c})_{Q} [/Tex]

AC power output which is developed across the load resistor:

[Tex](P_{out})_{ac} = I^2R_{c} [/Tex]

[Tex](P_{out})_{ac} = \frac{V^2}{R_{c}} = (\frac{V_{m}}{\sqrt{2}})^2*\frac{1}{R_{c}} [/Tex]

[Tex](P_{out})_{ac} = \frac{V_{m}^2}{2R_{c}} [/Tex]

Transformer Coupled Class A Power Amplifier

Transformer coupled Class A amplifiers use transformers to couple the input and output stages, providing isolation and impedance matching. This configuration can offer excellent linearity and power gain.

Circuit Diagram (Transformer Coupled Class A Amplifiers)

Due to the transformer primary coil’s extremely low resistance, there is less dc power loss. The relationship between the primary and secondary values of voltage, current and impedance are summarized as:

[Tex]\frac{N1}{N2} = \frac{V1}{V2} = \frac{I2}{I1} [/Tex]

[Tex](\frac{N1}{N2})^2 = \frac{Z1}{Z2} = \frac{Z1}{Rl} [/Tex]

N1, N2 = the number of turns in the primary and secondary

V1, V2 = the primary and secondary voltages

I1, I2 = the primary and secondary currents

Z1, Z2 = the primary and secondary impedance ( Z2 = Rl )

• Principle advantage: Lower distortion than Class C, B & AB. And it is simple to construct.
• Principle disadvantage: Lower power efficiency than Class C, B & AB and large power dissipation in the power transistors.
  

Push-Pull Class A Power Amplifier

The push-pull Class A amplifier overcomes the efficiency limitations of pure Class A amplifiers by using a pair of transistors in a push-pull configuration. This design allows one transistor to be on while the other is off, reducing heat generation and improving efficiency.

Circuit Diagram ( Push-Pull Class A Amplifier)

Class B Power Amplifier

The biasing of the transistor in class B operation is in such a way that at zero signal condition, there will be no collector current. The operating point is selected to be at collector cut off voltage. So, when the signal is applied, only the positive half cycle is amplified at the output.

Circuit Diagram (Class B Power Amplifier)

• The collector current only flows for 180 degrees because the transistor only operates for one-half of the input cycle, as depicted in the below picture.
• In order to achieve this, the Q point is changed to be a cut-off, or on the X-axis, as indicated in the picture below. As a result, the transistor remains in the off state in the absence of an ac input signal.
• (Vcc, 0) are the Q-point’s coordinates.
  

Graphical Representation (Class B Power Amplifier)

Operation of Class-B Power Amplifier

• The B-E junction of the transistor is only forward-biased during the positive half cycle of the input. When a sinusoidal input signal is applied to the transistor’s base, base current to begin flowing.

• Utilizing two transistors to generate the output signal’s full cycle on alternate half cycles of the input signal, it will remove the distortion. There is only 180° of conductivity for each transistor. The push-pull class B power amplifier is known as the same.

Class AB Power Amplifier

As the name states, AB amplifiers are a combination of class A and class B amplifier. Class AB amplifier not only solve the reduced efficiency issue of class A, but also solves crossover distortion issue that present in class B amplifier.

Circuit Diagram (Class AB Power Amplifier)

To obtain the output signal for an AC input signal that is angled between 180 and 360 degrees. The Q point is situated just below the midpoint of the load line and slightly above the X-axis.

In contrast to class A and class B operations, the Q point is neither on the X-axis nor in the middle of the load line. Between the two, it lies. Hence, the operation is known as class AB. The transistor conducts for more than 180° (class B) but less than 360° (class A), so its power dissipation is greater in class B operation than in class A operation.

Class C Power Amplifier

When the collector current flows for less than half cycle of the input signal, the power amplifier is known as class C power amplifier. The efficiency of class C amplifier is high while linearity is poor. The conduction angle for class C is less than 180^o. It is generally around 90^o, which means the transistor remains idle for more than half of the input signal. So, the output current will be delivered for less time compared to the application of the input signal.

Circuit Diagram (Class C Power Amplifier)

• This design of power has even greater efficiencies than class B amplifiers but sacrifices the quality of amplification.
• For this, the operating point is modified to be below the X-axis as shown in figure below. As a result, the transistor is biased below the cut-off.
  

Graphical Representation (Class C Power Amplifier)

Operation of Class C Power Amplifier

• The transistor can stay in the active region for less than a half-cycle period because of the biasing below the cut-off. Thus, the collector current flows for a shorter angle than 180°. Conduction angle is therefore less than 180 degrees.

• The output signal is severely distorted because of the smaller conduction angle. Contrary to a class B power amplifier, the % distortion is larger.

• The efficiency is often greater than 95%.

Comparison of Different Classes of Power Amplifiers

The comparison of different classes of power amplifier on the basis of efficiency, linearity, distortion and applications are given below:

Power Amplifier Class	Efficiency	Linearity	Distortion	Applications
Class A	Low	Excellent	Minimal	High-fidelity audio
Push-Pull Class A	Moderate	Good	Moderate	RF transmitters
Transformer Coupled Class A	Moderate	Excellent	Minimal	Audio amplification
Class B	Moderate	Good	Moderate to High	Motor control, power inverters
Class AB	Moderate	Good	Moderate	Audio amplification, car audio
Class C	High	Poor	High	RF signal amplification, broadcasting
Class D	High	Good	Low	Portable audio devices, power-efficient applications
Class E	High	Good	Low	Wireless communication (RFID, NFC)

Comparison between Voltage and Power Amplifiers

Parameter	Voltage Amplifier	Power Amplifier
Objective	Increase voltage of input signal	Increase power of input signal
Mode of Operation	Linear	Non-linear
Efficiency	High	Moderate to low
Distortion	Minimal	Can have higher distortion
Typical Applications	Small-signal amplification	High-power applications
Amplification Range	Low to moderate	High

Solved Examples of Power Amplifier

There are some Solved Examples of Power Amplifier given below :

1. Calculate the efficiency of a Class A power amplifier with an output power of 10 watts and a DC power input of 20 watts.

Solution:

[Tex]Efficiency = \frac{Output Power }{ DC Input Power}*100% [/Tex]

[Tex]Efficiency = \frac{10}{ 20}*100 = 50% [/Tex]

2. Determine the gain of a transformer-coupled Class A amplifier with a turns ratio of 1:5.

Solution:

[Tex]Gain = \frac{Number of Turns in Secondary}{Number of Turns in Primary} [/Tex]

Gain = 5

Advantages and Disadvantages of Power Amplifiers

There are some list of Advantages and Disadvantages of Power Amplifiers given below :

Advantages of Power Amplifiers

• Signal Amplification: It enhances the strength of an incoming signal making it suitable for operating the devices like speakers for delivering of high-quality and clean signal.

• Enhancing the audio quality: Power amplifiers helps in improving the sound quality of the system. This is achieved due to the less distortion and high linearity of the system. For eg. Class A and Class AB Power Amplifier are suitable for improving the sound quality.

• Efficiency: Power amplifiers reduces the power loss and hence improving the efficiency. In case of Class B Power Amplifier, there is no power loss under quiescent circumstances.

• Wide Range of Applications: It finds its use in diverse applications like audio amplification, RF signal transmission, consumer electronics, telecommunications, and industrial sectors, etc.

• Customizability: There are various classes of Power Amplifier available. Each classes has its own specification. This helps the designers to develop the configurations according to the system need.

Disadvantages of Power Amplifiers

• Heat Generation: Power amplifiers generates the heat while working. Hence, cooling systems are used for the prevention of overheating. It leads to the increase in the cost of the device.

• Size and Weight: The high power application Power amplifiers are large and heavy. So, it can’t be used in the portable or small sized devices.

• Non-Linear Distortion: When the transistor change its states from ON to OFF and vice versa, some distortion signals are generated. This leads to the non-linearity of the systems. For example in Class B Power Amplifier, crossover distortion is present in the output waveform. In class C, output waveform is distorted.

• Complex Design: High-power amplifiers often require complex designs and precise component matching, making them challenging and costly to manufacture.

Applications of Power Amplifiers

• Audio Systems: Driving speakers in home theaters and concert venues.

• Wireless Communications: It helps in transmitting RF signals in cell towers and satellite communication. Higher power levels made possible because of power amplifiers increases data transfer rates and usability.

• Radar Systems: In the radar systems it amplifies radar signals for accurate object detection.

• Industrial: It is used in servo motor and DC motor. Switching type power amplifiers are used for controlling most of the industrial actuator systems. It is helpful in controlling motors and actuators in manufacturing processes.

• Medical Equipment: Powering ultrasound and MRI systems.

Conclusion

In this article we have studied about Power Amplifier, classification and different classes. It is the fundamental electronic tools that enhance the strength of electrical signals. It finds its application in various fields like audio systems, RF transmitters, and many more is playing a important role in the technology driven world.

FAQs on Power Amplifier

What is the gain of Power Amplifier?

Gain of he Power Amplifier is the ratio of output power and input power. It is measured in dB. The mathematical formula for gain is [Tex]10 * log_{10}(\frac{Pout}{Pin}) [/Tex]

Why biasing in the power amplifier important?

In order to minimize the distortion, making the system linear and enhancing the performance of the amplifier, biasing is important.

How load impedance and efficiency of Power Amplifier are related to each other?

When the load impedance matches with the output impedance of the power amplifier, efficiency will be the highest.