Power Amplifier

A power amplifier is an electronic device that increases the power of an input signal

to drive output devices, such as speakers or headphones.

It is an amplifier with high output power.

Power amplifier is also called large signal amplifier, as in order to get large power at

the output.

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 Cont”d…

Aspect Op-Amp Power Amplifiers Transistor Power Amplifiers

Complexity Easier to design, pre-configured ICs Requires more detailed design and biasing

Power Handling Limited power, external transistors needed Handles high power directly with transistors

Efficiency Moderate (Class AB), less efficient alone Higher efficiency (Class D or AB possible)

Distortion Very low distortion, high linearity Dependent on design, potentially low with Class A

Cost Lower for small to medium power Higher cost for high-power designs

Use Cases Low to medium power (audio, instrumentation) High power (audio, RF, industrial applications)

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Bandwidth
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 The basic characteristics /features of a large signal amplifiers are, includes

 Circuit power efficiency

 Power dissipation/Heat Dissipation Mechanism

 Distortion

 Low output resistance

 High Power Output

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 Thermal Stability

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 Cont”d…
 Amplifier Efficiency: It is defined as the ratio of AC output power to the DC input power.

 Efficiency indicates how well an amplifier converts the DC input power to AC output power.
 Power dissipation: The amount of power dissipated by the transistor is the difference between the DC input
power and output power.

Includes heat sinks, fans, or other mechanisms to handle the heat generated during operation.
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 Distortion:
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The distortion can be reduced by using negative feedback in the amplifier.

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 Cont”d…

 Output resistance: Lower output resistance ensures maximum power transfer to the load, particularly when the

load impedance is low.

 In large-signal amplifiers, the output resistance is designed to match or complement the load impedance for

optimal performance.

 Lower output resistance reduces power loss in the amplifier itself, improving overall efficiency.

 High Power Output: These amplifiers are capable of delivering high power to the load, which is their primary

purpose.
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 Thermal Stability: Due to the high power levels, thermal management is crucial. Proper design prevents
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overheating and ensures reliable operation over time.

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 Applications of Power Amplifier?

 It is used in public address system: like speakers

 Power Amplifiers are used in Wireless communication: like Cellular phones, wireless routers, and satellite

communication systems to amplify the signal strength of wireless communication systems.

 Power Amplifiers are used in radio receivers and TV receivers

 Power Amplifiers are used in Industrial control: to control motors, actuators, and other mechanical

devices in industrial control systems.

 Power Amplifiers are used in Power electronics: to boost and regulate the power sent to electrical loads
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like lighting and motors.
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 Cathode Ray Tubes, etc.

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 Classification of Power Amplifier's

 Power amplifiers can be classified in a number of ways. Let's explore some common

classifications:

 Classification based on frequency signal usage: Audio frequency, radio frequency, and video

frequency power amplifiers.

 Classification based on their mode of operation (Conduction period): Class A, B, AB, and C

power amplifiers, which are distinguished by the period of conduction for the output
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transistors.

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 Cont”d…

 Classification based on their mode of operation and efficiency. Power amplifiers can be
categorized into 4-classes based on their mode of operation and efficiency.

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 Class A Power Amplifier

 Class A Power Amplifier is one of the most basic types of amplifiers used in electronic circuits.

 It is widely known for its simplicity and linearity but is less efficient compared to other amplifier classes.

 Linear Operation/Conduction Angle: The output current flows during the entire cycle (360°) of the input signal,

ensuring minimal distortion and high fidelity.

 Efficiency: Class A amplifiers have poor efficiency, typically around 20-30%, because the transistors are always

conducting and dissipating power even when there is no input signal.

 Heat Dissipation: Significant heat is generated due to continuous power consumption. Proper heat sinks or
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cooling mechanisms are essential.
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 Cont”d…

 Simplicity: The circuit design is straightforward, making it easy to implement and understand.
 Low Distortion: They exhibit very low harmonic distortion, which is critical in high-quality
audio systems.
 Biasing: Class A amplifiers are biased to keep the transistor in the active region throughout the
signal cycle.

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Figure : Waveform representation of Class A power amplifier

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 Class B Power Amplifier

 Class B Power Amplifier is a type of amplifier designed to improve the efficiency of Class A amplifiers by

operating transistors in a different way. However, this comes with trade-offs in terms of linearity and distortion.

 Conduction Angle: Each transistor conducts only during half (180°) of the input signal cycle, either the positive

or the negative half.

 Efficiency: Class B amplifiers are more efficient than Class A amplifiers, with theoretical efficiencies of up to

78.5%. This improvement is because the transistors do not conduct continuously.

 Reduced Heat Dissipation: Since the transistors are not always on, there is less heat generated compared to
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Class A amplifiers
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 Cont”d…

 Higher Power Output: Class B amplifiers are suitable for applications requiring higher power output, such as
driving speakers.
 Crossover Distortion: A major drawback is crossover distortion, which occurs during the transition between the
two transistors' operation.
 This happens because there is a small dead zone when neither transistor is conducting

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Figure : Waveform representation of Class B power amplifier

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 To reduce or eliminate crossover distortion:
 Class AB Operation: By biasing both transistors to be slightly on even at zero crossing (i.e
having a small idle current), the overlap ensures that there is no gap where neither transistor
conducting. This minimizes crossover distortion.
 Negative Feedback: Using negative feedback in the amplifier circuit can help correct any non
linearity introduced by crossover distortion.
 Diode Biasing: In Class AB amplifiers, diodes or a similar circuit can be used to bias th
transistors so that they remain slightly conducting around the zero-crossing point, furthe
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reducing distortion

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 Class AB Power Amplifier

 As the name states, Class 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.

 In a Class AB amplifier the transistor operates between Class A and Class B amplifier.

 Conduction Angle: Each transistor conducts for slightly more than half (greater than 180° but less than 360°) of

the input signal cycle. This overlap reduces crossover distortion.

 Efficiency: Class AB amplifiers are more efficient than Class A amplifiers (around 50-70%) but slightly less
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efficient than Class B amplifiers.
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 Cont”d…

 Push-Pull Configuration: Similar to Class B, Class AB uses a push-pull configuration with two transistors.

However, they are biased to allow slight conduction overlap during the signal transition.

 Reduced Heat Dissipation: While not as cool as Class B, Class AB amplifiers generate less heat than Class A,

making them more practical for higher power applications.

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Figure-5: Waveform representation of Class AB power amplifier

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 Class C Power Amplifier

 A Class C Power Amplifier is designed for high efficiency and is primarily used in applications where distortion

is not a significant concern, such as radio frequency (RF) transmission.

 It is the least linear amplifier class but achieves very high efficiency compared to Class A, B, and AB.

 Conduction Angle: The transistor conducts for less than half of the input signal cycle (less than 180°). This is

achieved by biasing the transistor far below the cutoff point.

 High Efficiency: Class C amplifiers have the highest efficiency among linear amplifiers, typically ranging from

75% to 90%, because the transistor is on for only a small fraction of the input cycle.
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 Cont”d…

 High Distortion: They produce significant distortion in the output waveform due to their short conduction angle.

However, this distortion can be corrected using a tuned circuit or filter in RF applications.

 High Power Output: These amplifiers can deliver high power at specific frequencies, making them suitable for

high-power RF applications.

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Figure 5: Waveform representation of Class C power amplifier

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 Conditions for Power Amplification

Conduction Power Output

Class Efficiency Linearity Distortion Applications
Angle Dissipation Power
High-fidelity audio, low-
A 360 20-30% Excellent High Low Very Low
noise amplifiers
B 180° 50-70% Poor Moderate Moderate High Basic audio amplifiers
Consumer electronics,
AB 180° to 360° 50-70% Good Moderate High Low
audio amplifiers

C < 180° >70% Poor Low High High RF amplifiers, transmitters

Switching High Portable audio,

D
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>90% Low Very Low High
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 Cont”d…

THANK YOU !!!

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