Class A,B,AB,C and D amplifier (Udemy Course)

Summary

This video script delves into various amplifier classes, focusing on their applications, characteristics, and limitations. It covers Class A, known for high gain and linearity but with high power loss; Class B, which improves efficiency but suffers from crossover distortion; and Class AB, a compromise between A and B with reduced distortion but lower efficiency. The script also touches on Class C, a tuned amplifier with high efficiency but significant distortion, and Class D, a popular switching amplifier with high power efficiency and low heat dissipation, ideal for digital audio and motor control applications.

Takeaways

• 🔊 Class A amplifiers are high gain with high linearity, using a single active device for the entire input signal.
• 🔧 Class A amplifiers are easy to construct but have limitations due to continuous conduction, leading to high power loss and heat generation.
• 🌡️ Class C amplifiers have a 360-degree conduction angle, meaning they are active throughout the entire input signal, but suffer from poor efficiency (25-30%) and require careful component selection.
• 🔥 Class C amplifiers emit heat and require heatsinks, but their efficiency can be improved with inductively coupled configurations, reaching up to 40-50%.
• 🎵 Class B amplifiers use two active devices, each conducting for half of the cycle, improving efficiency over Class A (over 60%) but introducing crossover distortion.
• 🔄 The Class AB amplifier mitigates crossover distortion by using an intermediate conduction angle, reducing distortion but at the cost of efficiency.
• 📡 Class C amplifiers are tuned amplifiers used in RF applications, with efficiencies up to 80%, but they introduce significant distortion in engine mode.
• 🛠️ Class D amplifiers are switching designs using pulse width modulation, offering high efficiency and low heat dissipation, making them suitable for digital audio players and motor control.
• 🔀 Class D amplifiers convert analog signals into a pulse stream and reconstruct them with a low-pass filter, providing high power efficiency but not suitable for complex waveforms.
• ⚙️ The script suggests simulating various amplifier classes in future courses to better understand their operation and characteristics.

Q & A

What is a Class A amplifier and what are its characteristics?

A Class A amplifier is a high gain amplifier with high linearity. It is easy to construct and uses a single active device, such as a transistor, which remains on all the time. It provides better high frequency and feedback loop stability but has limitations due to continuous conduction, leading to high power loss and the need for heatsinks.

How does the conduction angle in a Class C amplifier differ from that in a Class A amplifier?

In a Class C amplifier, the conduction angle is 360 degrees, meaning the amplifier device remains active for the entire input signal cycle. This is different from a Class A amplifier where the conduction angle is less than 360 degrees.

What are the typical efficiency percentages for Class C amplifiers?

The efficiency of Class C amplifiers theoretically varies between 25 to 30 percent. With inductively coupled configurations, the efficiency can be improved but is typically not more than 40 to 50 percent.

What is the main limitation of Class C amplifiers?

The main limitation of Class C amplifiers is high power loss, which results in heat emission and the requirement for heatsink space. This makes them less efficient and suitable only for low signal or low power level amplification purposes.

How does a Class B amplifier differ from a Class A amplifier in terms of active devices and conduction cycle?

A Class B amplifier differs from a Class A in that it uses two active devices which conduct for half of the actual cycle, or 180 degrees of the cycle. This means each device provides current drive for the load during alternating half cycles, improving efficiency compared to Class A.

What is crossover distortion and why is it a significant issue in Class B amplifiers?

Crossover distortion occurs in Class B amplifiers because the two devices provide each half of the sinusoidal waves that are combined across the output. There is a mismatch or crossover in the region where two halves are joined, leading to distortion in the output signal. This is a major limitation for precision audio applications.

How does a Class AB amplifier attempt to overcome the limitations of a Class B amplifier?

A Class AB amplifier uses an intermediate conduction angle, allowing each device to conduct a small amount of the input on the other half cycle, thus reducing the crossover mismatch during the dead zone. However, this comes at the cost of reduced efficiency compared to Class B.

What is unique about the operation of a Class C amplifier compared to other classes?

Class C amplifiers are tuned amplifiers that work in either tuned or untuned modes. They use less than 180-degree conduction angles and are not used for linear amplification due to their nonlinear output amplitude function. They are typically used in radio frequency applications like oscillators and modulators.

What is the efficiency range of a Class C amplifier and where are they commonly used?

Class C amplifiers can achieve up to 80% efficiency, especially in radio frequency operations. They are commonly used in RF applications, including oscillators with constant output amplitude and modulators where a high-frequency signal is controlled by a low-frequency signal.

How does a Class D amplifier differ from Class A, B, and AB in terms of design and operation?

Class D amplifiers are switching designs, unlike the analog designs of Class A, B, and AB. They use pulse width modulation to vary the amount of current that flows to a device, and the analog signal is converted into a pulse stream before being amplified. They are known for high power efficiency and low heat dissipation.

What is pulse width modulation and how is it used in Class D amplifiers?

Pulse width modulation (PWM) is a technique that uses digital control to create a square wave signal switched between on and off. In Class D amplifiers, PWM is used to modify the width of square waves to vary the current flow, and the analog signal is reconstructed using a low-pass filter at the output.

Outlines

00:00

🔊 Class A and Class C Amplifiers Explained

The script discusses two types of amplifiers: Class A and Class C. Class A amplifiers are characterized by high gain and linearity, with a continuous conduction nature, making them suitable for high fidelity audio applications. They are easy to construct but suffer from high power loss and require heat sinks due to poor efficiency, typically between 25% to 30%. On the other hand, Class C amplifiers have a 360-degree conduction angle, meaning they are active throughout the entire input signal cycle. They are more efficient but introduce distortion and noise, requiring careful component selection to minimize these issues. Class C amplifiers are not ideal for high-frequency or feedback loop stability but are easier to construct with fewer components.

05:01

🔧 Class B and AB Amplifiers: Theory and Limitations

The script moves on to explain Class B and AB amplifiers. Class B amplifiers use two active devices that conduct for half of the cycle each, leading to improved efficiency over Class A but introducing crossover distortion due to the mismatch when the devices switch on and off. This distortion makes them unsuitable for precision audio applications. The AB class amplifier is an improvement over Class B, using an intermediate conduction angle to reduce crossover distortion. However, this comes at the cost of reduced efficiency compared to Class B. The script also touches on the practical aspects of designing these amplifiers, such as the need for careful biasing and component selection to minimize distortion and ensure stability.

10:02

📡 Class C and D Amplifiers: Efficiency and Modulation

The final paragraph delves into Class C and D amplifiers. Class C amplifiers are tuned amplifiers that can achieve high efficiency, up to 80%, but are prone to distortion and are typically used in radio frequency applications. They operate with less than 180-degree conduction angle and are not suitable for linear amplification due to their nonlinear output amplitude. Class D amplifiers, on the other hand, are switching amplifiers that use pulse width modulation to achieve high efficiency and are popular in digital audio players and motor control applications. They convert the analog signal into a pulse stream and then reconstruct it using a low-pass filter, making them highly power-efficient with minimal heat dissipation. However, it's important to note that Class D amplifiers are not digital converters and still require careful design to ensure optimal performance.

Keywords

💡Class A Amplifier

A Class A amplifier is an amplifier with high linearity and gain, where the active device (usually a transistor) conducts throughout the entire input signal cycle, leading to continuous operation. In the video, it is explained as a high-frequency amplifier with stability advantages but is noted for being inefficient due to continuous power consumption, resulting in high heat generation and low efficiency (25-30%).

💡Conduction Angle

The conduction angle is the duration, expressed in degrees, during which an amplifier's active device is conducting current. For example, a Class A amplifier has a conduction angle of 360 degrees, meaning it conducts continuously over the entire signal cycle, while a Class B amplifier has a 180-degree conduction angle, conducting only half the time. The concept is crucial in understanding the operational differences among amplifier classes.

💡Class B Amplifier

A Class B amplifier operates using two active devices, each conducting for 180 degrees of the input signal. This configuration improves efficiency (up to 60%) compared to Class A, as it reduces heat dissipation by only operating during half of the signal cycle. However, it introduces 'crossover distortion' due to the gap when switching between the two devices, a key limitation mentioned in the video.

💡Crossover Distortion

Crossover distortion is a form of signal distortion that occurs in Class B amplifiers due to the transition point when one transistor stops conducting and the other starts. This results in a small gap in the signal output, affecting audio fidelity. The video mentions this as a significant drawback for precision audio applications and highlights how Class AB amplifiers are designed to mitigate this problem.

💡Class AB Amplifier

Class AB amplifiers combine features of both Class A and Class B amplifiers. They conduct for slightly more than 180 degrees of the signal, reducing crossover distortion by ensuring that each device remains slightly active even when the other device begins conducting. The video discusses how this configuration balances efficiency and linearity, with efficiency higher than Class A but lower than Class B.

💡Class C Amplifier

A Class C amplifier operates with a conduction angle of less than 180 degrees, making it highly efficient (up to 80% in some cases), but with significant distortion. It is not suitable for linear amplification, which is why it is often used in radio frequency applications where the output is constant, such as oscillators or modulators. The video explains its use in situations where high efficiency is more critical than signal fidelity.

💡Class D Amplifier

A Class D amplifier is a switching amplifier that uses pulse width modulation (PWM) to convert an analog signal into a series of pulses. The primary advantage is its high efficiency, as the transistors are either fully on or off, minimizing power loss and heat dissipation. The video describes Class D as the most power-efficient class, suitable for digital audio and motor control applications.

💡Pulse Width Modulation (PWM)

Pulse Width Modulation (PWM) is a technique used in Class D amplifiers to create an analog-like output by switching the amplifier on and off rapidly. The width of the pulses is modulated to represent the amplitude of the analog signal, allowing for efficient power usage. In the video, PWM is highlighted as a key feature of Class D amplifiers, which rely on this method to achieve their high efficiency.

💡Linearity

Linearity in amplifiers refers to the ability of the amplifier to produce an output signal that is a direct, proportional representation of its input signal. Higher linearity results in less distortion. The video discusses how Class A amplifiers are highly linear, whereas Class B and Class AB amplifiers trade off some linearity for improved efficiency, and Class D amplifiers prioritize switching efficiency over linearity.

💡Efficiency

Efficiency in the context of amplifiers refers to the ratio of the output power delivered to the load to the total power consumed from the supply. Class A amplifiers are the least efficient (around 25-30%), while Class D amplifiers are the most efficient due to their switching design. The video highlights how different amplifier classes manage the trade-off between efficiency, linearity, and distortion.

Highlights

Class A amplifiers are high gain with high linearity, commonly used in power and audio amplifiers.

Class A amplifiers feature a single active element, such as a transistor, that remains on all the time.

Class C amplifiers have a 360-degree conduction angle, meaning they are active throughout the entire input signal.

Class A amplifiers offer better high-frequency stability and feedback loop stability compared to other classes.

Class A amplifiers are easy to construct with minimal components, making them cost-effective.

Class C amplifiers suffer from high power loss and require heatsinks, which can be a limitation.

The efficiency of Class C amplifiers is poor, ranging between 25 to 30 percent.

Class B amplifiers use two active devices that conduct for half of the cycle each, improving efficiency over Class A.

Class B amplifiers are prone to crossover distortion due to the way the two devices combine their outputs.

Class AB amplifiers mitigate crossover distortion by using an intermediate conduction angle between Class A and B.

Class C amplifiers are tuned amplifiers that can operate in either tuned or untuned modes.

Class C amplifiers achieve high efficiency, up to 80%, but are not suitable for linear amplification due to distortion.

Class D amplifiers are switching designs that use pulse width modulation for efficiency and are popular in digital audio players.

Class D amplifiers are the most power-efficient, requiring smaller heatsinks and low on-resistance switching components.

Class D amplifiers convert analog signals into a pulse stream and reconstruct them with a low-pass filter.

Class D amplifiers are not digital converters but are widely used in applications requiring high efficiency and low heat dissipation.

Transcripts

[Music]

the classes that we are going to talk

about in this course are not only used

in power amplifiers but also used in

audio amplifiers in quotes the first

class that we are going to examine is

Class A amplifier this amplifier circuit

must look familiar to you because this

configuration is nothing else but the

common emitter circuit amplifier so we

already talked about this circuit and we

have also seen how to design one so

let's just point out some facts about

this configuration

okay so Class A amplifier is a high gain

amplifier with high linearity in case of

Class C amplifier the conduction angle

is 360 degree as I explained in the

previous course a 360 degree conduction

angle means that the amplifier device

remains active for the entire time and

use complete input signal as we can see

in this image there is only one active

element which is nothing else but a

transistor so the bias of the transistor

remains on all the time right dooty is

never turn off feature Class A amplifier

provides better high frequency and

feedback loop stability other than

disadvantages Class A amplifier is easy

to construct with a single device

component and minimum parts count

despite the advantages and high

linearity certainly it has some it has

many limitations due to continuous

conduction nature the Class C amplifier

introduced high power loss also due to

high linearity Class C amplifier

provides distortion and noise ease the

power supply and the BIOS construction

need careful component selection to

avoid unwanted noise and to minimize the

distortion so we mentioned that Class C

amplifier has high power loss right this

means that it emits heat and requires

heatsink space which sometimes is not

available the efficiency is very poor in

Class C amplifiers theoretically the

efficiency varies between 25 to 30

percent if used with the usual

configuration the efficiency can be

improved using inductively coupled

configuration

but the efficiency in such case is not

more than forty or fifty eight percent

use it is only suitable for low signal

or low power level amplification

purposes now that we highlighted some

important facts about this class let's

move to Class B the cleanse B amplifier

is a bit different from the Class A as

you can see here it is created using two

active devices which conduct half of the

actual cycle in other words they conduct

180 degree of the cycle so two devices

provide combined current drive for the

load if the circuit is used to provide

audio signal amplification then the load

is nothing else but a speaker right you

might wonder how are these transistors

biased

well the bias is provided by the input

signal which means that the input signal

has to be higher than 0.6 volt and also

the input signal has to go a negative in

order to bias the PNP transistor

don't worry if you don't understand how

this circuit works because in the next

course we try we will try to simulate

some of the classes so that you get a

better understanding of how they work

now let's just concentrate on theory a

little bit and don't bother yourself if

you don't understand exactly how the

circuit works

so this Class B amplifier consists of

two active devices which get biased one

by one during the positive and negative

half cycle of the sinusoidal wave and

use the signal gets pushed or pulled to

the amplified level from both positive

and negative side and by combining the

result we get complete cycle across the

output because these two active devices

are turned on one after another and

because they conduct only half cycle the

efficiency gets improved compared to the

efficiency of a Class C amplifier which

is twenty five to thirty percent the

Class B amplifier can have an efficiency

more than sixty percent also the heat

dissipation is minimized in this class

providing a low heatsink space but this

class

also have limitations a very profound

limitation of this class is the

crossover Distortion

as two devices provides each half of the

sinusoidal waves which are combined and

joined across the output there is a

mismatch on in other words crossover in

the region where two halves are combined

this is because when one device complete

the half cycle the other one needs to

provide the same power almost at the

same time when other one finished the

job it is difficult to fix this error in

Class B amplifier as during the active

device the other device remains

completely inactive the error provides a

distortion in the output signal and due

to this limitation it is a major fail

for precision audio amplifier

applications

an alternate approach to overcome the

crossover distortion is to use the a B

amplifier class a B amplifier uses

intermediate conduction angle of both

classes a and B use we can see the

property of both Class A and Class B

amplifier in this a B Class of amplifier

topology same as Class B it has the same

configuration with two active devices

which conducts during half of the cycles

individually but each device by us

differently so they do not get

completely off during the unusable

moment or the crossover moment each

device does not leave the conduction

immediately after completing the half of

the sinusoidal waveform instead they

conduct a small amount of input on

another half cycle using this biasing

technique the crossover mismatch during

the dead zone is dramatically reduced

but in this configuration efficiency is

reduced as the linearity of the devices

is compromised the efficiency remains

more than the efficiency of typical

Class A amplifier but it is less than

the Class B amplifier system also the

diodes need to be carefully chosen with

the exact same rating and need to be

placed as close as possible to the

output device in some sync

reconstruction designers tend to add

small value resistors to provide stable

quiescent current across the device to

minimize the distortion across the

output apart from the Class A B and a B

amplifier there is another amplifier

called as Class C it's a traditional

amplifier which works differently than

the other amplifiers classes Class C

amplifier is tuned amplifier which works

in two different operation modes tuned

or untuned the efficiency of Class C

amplifier is much more than the a B and

a B so a maximum 80% efficiency can be

achieved in radio frequency related

operations the interesting thing here is

that Class C amplifier uses less than

180 degree conduction angle and during

the engine mode the class II amplifier

unfortunately gives huge distortion

across the output but in typical uses

Class C amplifier gives 60 to 70 percent

efficiency the output amplitude is a

nonlinear function of the input so Class

C amplifiers are not used for linear

amplification they are generally used in

radio frequency applications including

circuits such as oscillators that have a

constant output amplitude and modulators

where a high-frequency signal is

controlled by a low frequency signal so

it works fine with simple sinusoidal

waveform but may have unacceptable

distortion with complex waveform as the

Class C amplifier the transistor

conducts for less than 180 degree of the

input signal on the other hand Class D

amplifiers are way more popular and

wider spread as Class C amplifiers it's

one of my favorite in terms of

functionality and we will also try to

simulate the circuit in our simulator in

the next course

while Class A B and a B are classified

as analog designs Class D amplifiers are

classified as switching designs as you

can see the circuit is very different

from the others this is just a semi

block diagram of Class D amplifier but

in the last course we will see a full

functional circuit so Class D amplifier

is a switching amplifier which uses

pulse width modulation if you are

wondering what post width modulation is

I advise you to look that up on the

internet because it's very important

technique used in F Ronix basically

positive modulation or PVM is a

technique for getting analog results

with digital means so digital control is

used to create a square wave which is a

signal switched between on and off right

as the name implies the width of the

square waves are modified in order to

vary the amount of current that flows to

a device

all right so the conduction angle is not

a factor in such case as the direct

input signal is changed with a variable

pulse width in this class the amplifier

system the linear gain is not accepted

as they work just like a typical switch

which have only two operations on or off

before processing the input signal the

analog signal is converted into a poor

stream by various modulation techniques

and then it is applied to the amplifier

system as the pulses duration is related

with the analog signal it is again

reconstructed using low-pass filter

across the output remember that Class D

amplifier is the highest power efficient

amplifier class so it has smaller heat

dissipation so small heatsink is needed

the silk which requires whereas

switching components like MOSFETs which

has low on resistance it is widely used

apology in digital audio players or

controlling the motors as well but we

should keep in mind that it is not a

digital converter