Amplifiers

Video Gallery

The Video Gallery of Amplifiers is appended below:

 

1. How does an Amplifier Work? (Class-A)

VIDEO TRANSCRIPT: How does an Amplifier Work? (Class-A)

An amplifier is an electronic device used for increasing the amplitude of electrical signals they are available in different classes a, b, ab, c, d, and so on In this video we will explore the class a amplifier which uses only one transistor You would know that the transistor can amplify the base current by its gain to the collector current Hence, we will use the transistor to amplify the voltage applied at the base to a higher voltage at the collector. In this circuit the output will always be Vcc. As it is directly connected to the source. Thus we add a resistor between them. The voltage drop across a resistor is directly proportional to the current flowing through it, and the current through the resistor is equivalent to the collector's current and the collector current depends on the base current which depends on the voltage of the input signal thus any change in input voltage will change the output voltage and the low voltage at the input is amplified at the output. When the input voltage is high the current is high hence the voltage drop is high and the output is low, similarly when the input turns low the output goes high The output has a phase shift of 180 degrees. But the waveform looks distorted. Because the gain of the transistor is very high and it is getting saturated. To limit the gain we need to provide the feedback. Hence we add a resistor between the emitter and the ground As you know the voltage drop across a resistor is directly proportional to the current flowing through it. Thus when the base current increases the collector current also increases The increasing current increases the voltage drop across the resistors. Here as the supply is fixed the voltage at the other end falls this drops the output voltage and here as the ground is fixed the voltage at the other end rises and this increases the output voltage This provides negative feedback for the output limiting the gain of the amplifier. The gain of the amplifier is now more dependent on these two resistors rather than the gain of the transistor. Allowing us to have better control over the total gain of the amplifier. The gain of this amplifier is given by the ratio Rc to Re. At last we add a capacitor to remove the DC bias and get the amplified input signal You would know that a transistor requires 0.7 volts or above at the base to turn on Thus this circuit cannot amplify voltages below 0.7 volts To make this actually work we need to add a dc bias to the input signal. Such that the input signal is always above 0.7 volts. And to do that we add a voltage divider here. This will provide the required bias to the voltage of the signal Now a complete waveform of the signal can be obtained at the output, but now the transistor is always on also this dc bias voltage can interfere with the source of the input signal Hence we add a capacitor here. The capacitor will block dc from any of the directions and allow only the ac signal to pass. The input signal is first biased such that it's above 0.7 volts, then the signal is amplified by the transistor with the gain provided by the relation between rc and re and at last the dc bias is removed to obtain the amplified signal This is how a class a amplifier works. This is the simplest type of class a amplifier circuit. We can add more components to it for improvements. In this circuit the biasing resistor is connected after the Rc. When the input voltage increases the base current increases which increases the collector current and the voltage at the output falls This also drops the bias voltage thus the base current drops collector current drops and the voltage at output rises. This resistor now provides the negative feedback and limits the gain of the amplifier. This circuit has a capacitor across this resistor This increases the ac gain of the amplifier. The capacitor will block the DC thus the amplification of dc will depend on the resistors But for ac the capacitor will act as a short circuit. Its gain will depend on the beta of the transistor which is in hundreds thus you can amplify a very weak signal to strong signal This circuit uses the Darlington transistor in place of the single transistor Darlington transistor is two transistors within a single package One small pilot transistor and another larger switching transistor This increases the current handling capacity and gain of the circuit. Also this provides high impedance at the input and low impedance at the output This circuit has a transformer in place of Rc to increase the efficiency of the circuit. However the transformer is an inductive device due to its windings in core So using inductive components in amplifier switching circuits is best avoided as any back emfs generated may damage the transistor. A class a amplifier can reproduce full waveform with very low distortions But as the transistor is biased it's always on. This and some other factors reduce its efficiency to less than 30 percent which is very low Now you know how a class A amplifier works. In the upcoming videos we will explore the remaining classes of amplifiers. Thank you for watching

 

2. Amplifier basics, Types & Characteristics | Basics of Electronics

VIDEO TRANSCRIPT: Amplifier basics, Types & Characteristics | Basics of Electronics

amplifier is an electronic device or circuit that is used to increase the voltage current or power of an input signal the process of increasing these parameters in a signal is called as amplification amplifiers are broadly categorized into three types based on their function these are voltage amplifier current amplifier and power amplifier let's take a look at the most important applications of an amplifier amplifiers are most popular in audio systems where a weak signal from source is fit to amplifier this amplifier will boost the signal power high enough to drive the speakers to reach large audience it is also used widely in radio transceivers in these devices incoming signal received from antenna will be too weak to use directly by the user therefore an amplifier is used to amplify the weak signal and then passed on to the user similarly when the user need to send a signal back this signal will be amplified by the amplifier in the radio transceiver before modulating them and transmitting back through antenna now let's take a look at four important characteristics of an amplifier linearity the property where amplifier does not alter the shape of a signal and produces only the amplified version of original signal this majorly determines the quality of output signal gain the ratio between the magnitude of key parameters of input and output signal for example voltage gain is given by the ratio between voltage of signal output to the voltage of input signal noise a measure of an amplifier to amplify input signals without introducing undesired signals in it bandwidth the range of input signal frequencies within which an amplifier can effectively amplify it can also be described as a range of input signal frequency within which an amplifier will operate and that's all about amplifier grab our free ebook electronics mini dictionary which has simplified definitions of more than 500 commonly used terms in electronics you can download that from the link given in the description below subscribe to our channel and hit the notification icon to get notified about our videos visit our website http://www.gadgetronix.com for more resources on electronics if you want us to make video about a specific concept in our channel mention them in the comment section below see more of the concept videos here thanks for watching

 

3. How does an Amplifier Work? (Class-B & AB)

VIDEO TRANSCRIPT: How does an Amplifier Work? (Class-B & AB)

In one of our previous videos, we did the design and working of a class A amplifier. We went through and explored all the components, figured out how each of them affected the output, and at last, we explored some of its variations. In this video, we will move further and look at other class amplifiers, specifically class B and class AB amplifiers. This is the simplest class B amplifier. Unlike class A, it uses two transistors as an emitter follower, in a push-pull configuration. This is an NPN transistor and this is a PNP transistor. The positive cycle of the input signal will be followed by the NPN transistor. When the voltage at the base of this transistor changes, the voltage at its emitters also changes following the base. And, the negative cycle of the input signal will be followed by this PNP transistor. When the voltage at the base of this transistor changes, the voltage at its emitters also changes following the base. During the positive cycle, the voltage of the base of the PNP transistor is higher than its emitter, hence this transistor is in its cut-off region. Similarly, in the negative cycle, the voltage of the base of the NPN transistor is lower than its emitter, hence this transistor is now in its cut-off region. Now, you would know that a transistor requires 0.7 volts between base and emitter to turn on. Thus, during the positive cycle, until the voltage at the base is above the cut-off voltage, the transistor is in its off stage and the output voltage is zero. And when the voltage at the base increases above the cutoff voltage, the voltage at the emitter starts to increase, following the base voltage, until it reduces to below the cutoff voltage. Similarly, in the negative cycle, this transistor is off until the voltage at the base reduces below the cutoff voltage and then it follows the base voltage. You can see that the output wave is not similar to the input wave. It has these regions of distortions. They are called crossover distortions as they are caused during the switching of the transistors. Now, if we bias the input voltage such that, when there is 0 volt at the input, we have 0.7 volts at the base of the NPN transistor, and minus 0.7 volts at the base of the PNP transistor. Thus, if the input signal increases, the NPN transistor will turn on and start conducting, And we will have the same signal at the output. Also, the PNP transistor will be in the cutoff region. And, during the negative cycle, the PNP transistor will be on, with the output following the input. But, you may know that the cut-off voltage varies for each transistor, it depends on its manufacturing and the temperature (and other factors). If we use a bias of 0.7 volts, it may be sufficient for one transistor but not for other transistors, resulting in distortions in the output. Hence, we increase the bias from 0.7 to 0.8 volts. Thus we can now be sure that the transistor will be on. As the bias voltage is 0.8 volts, at 0 volts of the input signal, both the transistors will be turned on. Now, both of the transistors will try to control the output for different voltages. This will cause distortions in the output. Hence, we add two small resistors to separate both the transistors from fighting for the output. This is how a class AB amplifier works. Class A amplifier is an amplifier where the transistor is always on. Class B amplifier is an amplifier where the transistor operates for half of the cycle only. Class AB amplifier is an amplifier where the transistor operates for more than half cycle but less than complete cycle. A class A amplifier produces little to no distortions because the transistor is always on, but it has very high losses, Its efficiency is less than 30 percent. The class B amplifier negates this drawback of class A and has an efficiency of 79 percent, but it creates distortions in the output. Thus, a class AB is a compromise between both of them, which has fewer distortions than class B amplifier and better efficiency than class A. You may wonder, how do we bias the input signal. We use the PN junction diodes. When a diode is active, it has a voltage drop across its terminals. Let's say this diode has a forward voltage drop of 0.7 volts. And this point of the circuit is at 0 volts, as the signal is at 0 volts. Thus the voltage at this terminal of the diode will be 0.7 volts. And for the other one, the voltage at its terminal will be -0.7 volts. Now any changes at this point will cause the voltage at another end of the diode to change with the bias of 0.7 volts. This is how we bias both the transistors. You can also connect the input signal at other terminals of the diode. We also add capacitors to filter any DC from any direction, and to avoid short-circuiting the diodes. This is how Amplifier of class B and AB work. Thank you for watching.

 

4. Amplifiers: Class B & AB

 

5. The Basic Concept of An Amplifier

VIDEO TRANSCRIPT: The Basic Concept of An Amplifier

so let us start by introducing the concept of amplifier an amplifier is a circuit where the output signal is an amplification of the input signal so what does that means if we have an amplifier and this amplifier will take an input signal this input signal is a voltage that is called VI it could be a current as well but we decided to make it a voltage and then this amplifier will take this input signal VI and we'll send it to the output which is the voltage across this resistor that is called ro and the output signal is the output voltage real so the amplifier takes the input signal at the input and amplifies it which means makes it bagel and send it to the output so if you see for example the voltage at the input could be shown here by this small magnitude voltage and the amplifier going to amplify this input signal which means going to make it bigger then you may observe that the output signal which is the voltage across the resistor is amplified which means became bigger at the output basically amplifiers take the input signal and make it bigger or amplify it and then send it to the output ideal amplifiers are linear what does that mean basically it says that V out is proportional to vi and if you take the proportionality and replace it with the proportionality constant then we can state that V out will equal to a times VI we're a is called the gain of the amplifier that is basically the ratio of the output over the input the gain tells us how many times the output is bigger than the input so how many times the output is bigger than the input is the gain actual amplifiers are nonlinear so let me show you that through a graph if we would like to plot V out versus VI and we would like to plot the relationship between VI and V out you can see that it could be governed by this curve different amplifiers will have different curve we call it the voltage transfer color so if we look at the voltage transfer curve it is clear that the relationship between the input and the output is nonlinear we can express this curve using a power series so we can say that V out will equal to a naught plus a 1 times VI plus a 2 times VI squared plus a3 times VI cubed and so on so it is clear that from this equation that V out is not a function of VI only but it is also a function of VI squared VI cubed VI to the power 4 V R to the power of 6 and so on so the relationship between the output and the input is nonlinear now we engineers like to use simple models but they work one way to achieve that is we are going to simplify this realistic model so the real word is more tedious and more complex as a first cut approximation we are going to simplify this model and we do that by using assumptions so if VI is less than one which is small then VI squared is even smaller and VI cubed is even smaller and VI to the four is even smaller so now we can ignore this high order terms the VI squared and the VI cube and the VI to the power 4 and so on can be ignored so let us state that actual amplifiers are approximated to be linear by ignoring the higher order term and this approximation is valid for small VI then we can say that V out will equal to a 0 plus a 1 times VI now a naught is the DC offset which may be blocked using DC coupling capacitors now the output voltage will equal to a 1 times VI now it is clear that the output voltage is directly proportional to the input voltage so basically what we said here is an amplifier is a device or a circuit that will take the input signal and amplify it makes it bigger and send it to the output then we said that ideal amplifiers are linear however the real world amplifiers are nonlinear and we can approximate or assume the amplifier is linear by making sure that VI or the N book signal is very small and that's all what we say it so far

 

6. What are the Types of Amplifiers | Classification of Amplifiers | Electronic Devices & Circuits

VIDEO TRANSCRIPT: What are the Types of Amplifiers | Classification of Amplifiers | Electronic Devices & Circuits

what are the types of amplifier most of the electronic system requires at least one stage of amplification hence amplifiers can be seen in almost all the electronic devices amplifiers are the devices that increase the amplitude of the input signal the output of the power supply is modulated by the amplifier amplifiers increase only the amplitude and the other parameters such as frequency and shape remain constant there are three categories of amplifiers depending on the property of their output voltage amplifier current amplifier and power amplifier all these amplifiers these are most common amplifiers used in electronic devices these amplifiers increase the amplitude of the output voltage of the signal current amplifiers these amplifiers increase the amplitude of the input current compared to the input current waveform power amplifiers the purpose of the power amplifiers is to increase the power that is the product of output voltage and current is greater than the product of input voltage and current either the voltage or current at the output may be less than the input the overall voltage or current product will be greater than the input when an AC signal is applied to the amplifier only a part of it is amplified depending on the portion of wave amplified these are classified into four classes they are Class A Class B Class A B Class C amplifiers can be further classified based on the signal the amplifier they are as follows number one audio frequency amplifiers AF amplifiers two intermediate frequency amplifiers high F amplifiers three radio frequency amplifiers RF amplifiers for ultrasonic amplifiers five wide band amplifiers six direct coupled amplifiers called DC amplifiers video amplifiers buffer amplifiers operational amplifiers transistor amplifiers exception

 

7. Audio amplifier basics

 

8. How does an audio amplifier work? (AKIO TV)

VIDEO TRANSCRIPT: How does an audio amplifier work? (AKIO TV)

an audio amplifier is obviously a device that can amplify an audio signal so what it can do is it can take a low-power weak signal from something like a headphone jack and then it can turn that into a more powerful signal that can be used to drive a speaker so why don't we start by taking a look at the main concept of what the amplifier does what it is and more importantly what it isn't so a transformer we've talked about transformers before on this channel a transformer is not an amplifier so what a transformer is is a device that you you put electricity into it then it will change the voltage in the current it will transform those and then electricity comes out of it and a different voltage in current so it transforms the voltage in the current so what it it could be a transformer that doubles the voltage but the twist with the transformer is that the power that goes into it is exactly the same as the power that comes out of it so if you have a transformer that doubles the voltage that does mean it cuts the current in half or maybe doubles the current but that means it cuts the voltage in half so there is always a sacrifice and that's perfectly logical because if the transform would magically produce more power than that you put into it it would create free energy and that of course doesn't exist the amplifier does do that but it doesn't doesn't quite create free energy because it has an external power source of it has some kind of battery or some kind of power outlet so it uses power from a power source but what it does is takes the input signal and the output signal will actually have greater power than the input so it can raise the voltage without changing the current or raise the current without changing the voltage or just raise both that's what an amplifier can do but of course like I said it does need a power supply because it can't create free energy so that's basically what an amplifier does now let's take a closer look at how it does start most amplifiers use transistors to do the amplification a transistor is a small electronic device that has three pins on it one of those pins is the input which we call the collector one of them is the output which we call the emitter and there is a third which we call the base now wallah transistor basically is is some sort of valve for electricity and this valve can be operated using that third pin which is the base now it's kind of difficult to explain but have got a very nice simulation on my screen why here which I'm going to use to explain properly how it works so right here in front of me I have got an electric circuit simulator and I'll put a link to it in the video description so that you can try this out yourself because it's actually a really handy web application this time I am going to use it to demonstrate what the transistor does so as you can see this is our transistor this is the input so the collector this is the output which is the emitter and this is the base now here we've got a 50 volt power supply which it goes into this light bulb and then the light bulb is connected to the input of the transistor the output of the transistor goes to ground which if you don't know it basically means it's it's the equivalent of a wire that goes back to the negative side of the power supply but this is quicker to draw then we've also got a variable power supply which is now off it on zero volts as you can see which is connected to the base of the transistor okay so as you can probably see because of the yellow dots that are not moving there is no current running through this circuit which means a transistor is now blocking all of the current is like a valve that is closed but when I apply a small voltage to the base of the transistor you can see that this is actually if we apply some higher current speed we can see that the current starts running into the base and then also to the ground but more importantly this current through our main circuit now starts running as you can see here so by applying this very tiny voltage to the base of the transistor we open up the valve and the electricity can now flow through the light bulb if we increase that voltage we can turn on the light bulb so now the current is sufficient to actually power the and this is the main principle of the transistor so using a small voltage we can control a much more powerful circuit which is 50 volts and the amplifier makes use of this so now let's take a look at a very simple amplifier circuit okay so I've quickly designed an absolutely terrible amplifier so this is this is awful I apologize to all the electrical engineers out there has many flaws but because it's so incredibly simple it is also incredibly easy to understand what it does and that's what this is all about of course so up here we've got our 50 volt power supply which is the power supply of the amplifier then this is a resistor an 8 ohm resistor which represents our speaker because the typical impedance of a speaker is 8 ohms that is then connected to the collector of a transistor and the emitter then goes to ground this right here is our input signal so this is a 40 Hertz power supply in this case which basically means a 40 Hertz audio signal so the voltage goes up in down 40 times per second and that weak audio signal goes into the base of the transistor ok then so now let's see what happens when we run this simulation just observe this animation for a moment as you can see when the voltage on the input signal goes up the transistor conducts electricity better so it opens up and therefore more current runs through the speaker so the Volt the alternating voltage of the input signal now controls the alternating voltage on the speaker and you can see that down here so this graph is a graph of the input signal so it's 12 volts as you can see it's not that much it's quite quite weak and this is the output signal so this is the speaker and that's 50 volts as you can see so it's a lot more so as you can see these waveforms look exactly the same they're an exact copy of each other but this one's bigger because this is 50 volts and this is 12 volts so we've now successfully amplified this weak input signal and that's what an amplifier does and of course this is terrible as I just explained this is an absolutely awful amplifier which has tons of flaws but it does explain the basic principle of what most amplifiers do anyway now you know a little bit more about how amplifiers work I hope you've enjoyed this video and thank you for watching

 

9. DIY Class D Audio Amplifier

VIDEO TRANSCRIPT: DIY Class D Audio Amplifier

in a previous video I showed you how [to] create a simple class AB audio amplifier that consists of the NE5554 op-amp and the Push-pull Bipolar junction Transistor output stage Now even though the circuit did work decently is there was one problem... its power loss We can identify it's cause by taking a look at the simplified functional block diagram Firstly our originally applied audio signal gets it's voltage amplified by an op-amp Since such Op amps can only supply very little power the next part of the circuit is a BJT output stage That modulates the supply voltage of 12 volts [to] become the audio signal that is not only voltage amplified, but can also supply a sufficient current for the speaker. The only problem is that the transistors work in the active region. Which means there exists a noticeable voltage drop across the collector-emitter path and thus power loss is created Which leads to an overall efficiency of around 50 to 60% To increase the efficiency though we can utilize another audio amp kinds, a class D amp to be precise. With an efficiency of up to 95%! So in this video. I will show you how such an audio amp works, and how we can create our own diy version Which consists of common components. Let's get started! [Intro music playing] First off let's have look at the simplified functional block diagram of the class D amp. On the left side we got our audio signal which is connected to the non-inverting inputs of a comparator. The inverting input however, gets connected to a triangle wave with a frequency up above 200 Kilohertz Now whenever the voltage of our audio signal is higher than the triangle voltage the output of the comparator gets pulled high, and Vice-versa. This way we basically modulated our audio signal with maximal frequencies of around 20 Kilohertz Into a high frequency square wave which then connects to a MOSFET driver the driver obviously, turns on and off two mosfets. The high side one according to the high voltage levels and the low side one according to the low voltage levels. This way, we get a powerful high frequency square wave at the MOSFET outputs. Which, since the mosfets were switched on and off in their ohmic region with a low drain to source voltage drop, created very little power losses. Now we can recreate the original audio signal by adding an LC Low-pass filter which like the name implies filters out all the high frequencies and leaves us with our original now amplified audio signal. As suitable components, we can use a 555 timer (IC), LM393, 74HC04 to create an inverted signal of the High frequency wear wave Which is mandatory for the IR2113 MOSFET driver and two IRLZ44N MOSFETS. Before we can solder a proper circuit though, I utilize the free, Easy EDA circuit design software to create an appropriate Schematic By utilizing the online component library, I imported all the required ICs and passive components And afterwards connected them all to one another according to the previously discussed functional block diagram. The only question left to answer was what kind of values should be used for the LC filter? According [to] a commercial class D amp that I had lying around, Inductances of 22uH should be suitable. The only inductors I had lying around though were 33uH ones. So I connected two of them in parallel in order to create a 16.5uH value and wrote down the two formulas to calculate inductance and capacitance* of a LC low-pass filter if We rearrange the inductor formula and insert the load impedance of 4 Ohms According to the speaker properties and the inductance of 16.5uH We can calculate a cut-off frequency of around 40 Kilohertz At which the original voltage amplitudes will be lowered by three decibel or 30% of the original amplitudes. By inserting this frequency into the capacitance* formula, we get a capacitance of around 1.03 uF Which I created by connecting 50.2 uF capacitors in Parallel and now that this schematic was complete, I gathered all the required components and started soldering them to a piece of perf board. As always, I try to utilise silvered copper wire for the most part but still had to use a bit of hook up wire at the end. And of course you can find reference pictures of my perf board layout as well as the schematic for this project in the video description aftert three hours of soldering the circuits was complete it was time to insert all the ICs connect A 15 volt power supply to the inputs and the speaker to the outputs As the first test, I hope that my function generator set to a sine wave with a frequency of only 1 Hertz to the audio inputs since the 555 timer, Created writing a voltage between around 1/3 and 2/3 of the supply voltage, We also have to use a potentiometer to add a DC offset to the Sine wave that is completely submerged Inside the triangle wave if we now take a look at the output of the comparator We can see how the slowly changing 1 Hertz sine wave creating modulated high frequency square wave and If we increase the frequency to a value that is closer to proper music The output definitely looks crazy and confusing, but the theory pretty much stays the same but by using lower frequencies We can [actually] create a lot of visible vibrations of the speaker cone which showcases that this class D amp is pretty damn powerful for its simplicity and As you would have expected it by increasing the frequency of the sine wave we produce higher tones Which means that the amp works without a problem. so I replaced the function generator with my smartphone and tested the music playback capabilities of the amp. As you can hear the music playback also worked like a charm, but the volume was a bit low The reason for that was that the music signal only features a peak to peak voltage of around half a volt while the fashion generator easily created Peak to Peak voltages of 4 volts So as an afterthought I added an LM386 op-amp in between the input audio signal and the comparator Which didn't even require complementary components Afterwards the Audio signal consisted of a peak to peak voltage of four volts as well Which was suitable to create a much louder playback of music through the class D audio amp. Now of course this circuit is definitely not perfect However the audio quality is pretty decent and more importantly, maybe you learnt something new! If so don't forget to like share and subscribe Stay creative, and I will see you next time!

 

10. Transistor Amplifiers – Class A, AB, B, & C Circuits

VIDEO TRANSCRIPT: Transistor Amplifiers - Class A, AB, B, & C Circuits

in this video we're going to talk about the different types of transistor amplifiers such as the class a amplifier the class a b amplifier the class b amplifier and the class c amplifier there's also the class d amplifier but we're not going to touch that one in this video so let's draw the circuit for the class a amplifier we're going to start with an npn transistor this is the base of the transistor that's the collector and this is the emitter we're going to connect the emitter to a resistor which we'll call re the emitter resistor rb is the base resistor and then rc the resistor associated with the collector now rc and rb they will be connected to vcc the collector supply voltage which would be the positive terminal of the battery and then typically there's another resistor which we'll call r1 rb and r1 they form a voltage divider now the capacitors c1 and c2 are coupling capacitors they're used to block dc but they will pass an ac signal and then we have the ground and then typically you'll find connected across the emitter resistor is a bypass capacitor it allows the ac signal to bypass the emitter resistor thus increasing the voltage gain of this circuit so here we have an input signal and then at the output we're going to have an inverted but larger output signal and so the function of an amplifier is to increase the power level of an ac input by transferring power from the dc power supply to the input signal the voltage gain of the amplifier is the ratio of the output voltage to the input voltage the efficiency of the amplifier is the ratio of the ac load power divided by the power delivered by the dc power source times 100 so this is the class a amplifier also known as the common emitter amplifier it has a maximum efficiency that is a maximum theoretical efficiency of 25 percent the actual efficiency will usually be less than that but that's the maximum theoretical efficiency and so as you can see this amplifier is not very good in terms of or being used as a power amplifier it's more appropriately used as a small signal amplifier because its efficiency is so low now this particular transistor conducts for the entire 360 degrees of the input cycle so q1 which is this transistor that's a terrible circle is always on in this case one advantage of the class a amplifier is that it has very little to no distortion which is good even though the efficiency is very low now in order to design a circuit with an efficiency that is close to 25 as possible you want to set vce that is the voltage between the collector and the emitter pins of the transistor to one half of the collector supply voltage so if the collector's supply voltage is 9 volts you want to design a circuit in such a way that vce is as close to 4.5 as possible now keep in mind vbe the voltage between the base and the emitter is going to be between 0.6 and 0.7 volts and ic is equal to beta times ib where beta is equal to hfe beta is the ratio of the collector current to the base current so those are some things that you want to know when dealing with the class a amplifier or the common emitter amplifier now there's something that you could do to increase the efficiency of the class a amplifier and in order to do that you need to replace rc with another element that is a transformer and so what we have here is a transformer coupled class a power amplifier so the output is going to be across the secondary part of the transformer what you could do is replace it with a load resistor if you want to and so with this particular transformer coupled class a amplifier the theoretical efficiency is no longer 25 but the maximum theoretical efficiency is 50 percent now to actually achieve this is not easy in actuality your actual efficiency might be less than 40 you might be getting efficiencies of 35 39 or something like that but the maximum theoretical efficiency is 50 now here's a question for you why is it that replacing rc with a transformer increases the efficiency of the circuit why is that it has to do with the fact that the transformer like an inductor can absorb and release stored energy using its magnetic fields let's set the collector supply voltage to 12 volts now keep in mind the input current is fluctuating so the base current is fluctuating and thus the collector current is fluctuating as well as the collector current increases the magnetic field that is generated by the transformer that field is going to increase in strength and so during this process energy is being stored by the transformer and so there's going to be an induced emf generated by this transformer at this point the transform is absorbed in energy and just to keep things simple let's say that the potential across the transformer is eight volts so these two voltages they go against each other so at point c the voltage will be four volts with respect to ground now the current won't increase forever eventually the current is going to come back down when the current decreases the magnetic field collapses and that stored energy is released back to the circuit the polarity across the transformer reverses and so this is now positive and the other side is negative and let's use the same value just to keep it simple 8 volts so now the potential at point c will be the sum of 8 and 12 volts so it's going to be 20 volts so notice that the potential is higher than the collector supply voltage and as a result this increases the efficiency of the transformer coupled class a amplifier circuit now the next type of amplifier that we're going to talk about is the class b amplifier the class b amplifier it uses two complementary transistors an npn transistor and a pnp transistor let's call this q1 and q2 so q1 is an npn transistor you can easily distinguish it from a pnp transistor due to the fact that the arrow points away from the center of the transistor in the pnp transistor the arrow points towards the center now we need three resistors one of which will be between the bases of the two transistors so we're going to call this r1 r2 and r3 now the input signal is going to be coupled through two capacitors and it's going to be connected to the ground as well the output is going to be taken across the load resistor and here is the collector supply voltage vcc so this is the circuit for a class b amplifier the maximum theoretical efficiency for this amplifier is 78.5 percent now in order for this circuit to work properly the two transistors have to be biased at cut off during the positive half cycle of the input sine wave q1 conducts so q1 is in the on state q2 is off now during the negative half cycle let's put this in blue q1 is going to be off and q2 will be on so for each half cycle only one of the transistors should be conducted if it's designed appropriately the other transistor should be off so each transistor conducts for one half or 180 degrees of the sine wave and so this improves the efficiency of the class b transformer i mean the class b amplifier now there's one problem with this particular circuit besides the efficiency not being 100 and that is crossover distortion the output doesn't look like a nice pure sine wave instead it looks something like this my drawn is not perfect but that's what it is now the crossover distortion is created due to the fact that the emitter base voltage of each of the two transistors is approximately 0.7 volts so until the input sine wave reaches positive or negative 0.7 volts the two transistors will be off and so the output voltage will be zero at this point so that's the one disadvantage of the class b amplifier is the presence of crossover distortion now this circuit can be improved by creating what is known as a class a b amplifier and to create the class a b amplifier what you need to do is replace r2 with two diodes and these are silicon diodes with a voltage drop of approximately 0.7 so to combine the total voltage drop between let's call this point a and point b will be 1.4 which is equal to the voltage drops of the two transistors which is also 1.4 now granted the voltage across the silicon diode and the base emitter junction of a transistor both can be between 0.6 to 0.7 volts so this could be 1.2 and that could be 1.2 but they're relatively close to each other the combined voltage drop will be somewhere between 1.2 and 1.4 but that's how you could design the class a b amplifier is by replacing r2 with two silicon diodes and so this will reduce the problem of crossover distortion so that's the advantage of the class a b amplifier now the last amplifier that we're going to talk about today is the class c amplifier this particular amplifier only uses one transistor it's not like the class b or the class a b amplifier that uses two transistors now this amplifier is different from the other ones in that it is a tuned amplifier so it has an inductor and a capacitor in parallel with each other so we'll call this c2 this is c1 and we'll call that l1 and then we have a resistor rb and that is attached to a negative voltage which we'll call negative vbb so that is the class c tuned amplifier the efficiency the maximum theoretical efficiency for this amplifier is pretty high it's 99 however there's a major disadvantage with this amplifier and that is the distortion the distortion can be very severe for this particular amplifier and the second thing is that it conducts for less than 180 degrees of the input cycle so during the positive half cycle that is when it's above 0.7 volts this side is going to be positive this side will be negative conventional current will flow in this direction charge in c1 activating the npn transistor and when that's activated current will flow from the dc power supply charging c2 and l1 and then oscillations will begin energy will be transferred back and forth between c2 and l1 and so that energy transfer happens during the positive half cycle during the negative half cycle q1 is off if we put a negative sign here and a positive sign here current cannot flow in this direction because the base emitter junction will be reverse bias and so q1 is off during the second half but it's on during the first half when the voltage exceeds 0.7 so it conducts for less than 180 degrees of the input cycle so that's the class c amplifier it has a lot of distortion but the maximum theoretical efficiency is very high now i've seen different variations of the class c amplifier here's another one and depending on which variation you're using sometimes the efficiency could be eighty percent ninety percent but it's still relatively higher than the class a and the class b amplifier so rb is going to be connected between the base of the transistor and the ground in this case we're still going to have a capacitor and instead of an inductor we're going to use a transformer this time and so the output will be across the transformer on the other side so we'll put a little resistor here so this is another variation of the class c tuned amplifier so that's another way in which you could design the class c amplifier the resonant frequency of the tuned circuit is going to be 1 over 2 pi times the square root of lc well specifically c1 and then you got to find out what the l value is for that side of the transformer so that's basically it for this video now you know the difference between the class a class a b class b and class c amplifier thanks for watching

 

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