ECT281 ELECTRONIC CIRCUITS|| KTU MINOR || RC COUPLED AMPLIFIER || FEEDBACK AMPLIFIER

So, today we are discussing about classification of amplifiers, this topic, amplifiers, classification of amplifiers, RC coupled amplifiers, its design and working, voltage gain and frequency response, multistage amplifiers. Also, we are discussing about feedback amplifiers and effect of negative feedback, if time permits we will discuss the effect of negative feedback on amplifiers also. So, first what is an amplifier? It is used to strengthen a signal, it may be a current signal or a voltage signal anyway. If it is used for amplifying a current signal, we can say it is a current amplifier, for a voltage signal we say it is an voltage amplifier. So, amplifiers are working, amplifiers are mainly classified into different classes. Based on first, based on the type of signal, we can classify based on the type of signal amplifiers as small signal amplifier and large signal amplifier. What is small signal amplifier? Our normal voltage amplifiers, the voltage amplifiers that we normally use, the common emitter amplifiers, they are all examples of small signal amplifiers. Small signal amplifiers are like voltage amplifiers, small signal amplifiers. Next classification is large signal amplifiers, large signal amplifiers means it can amplify high power signals, power amplifiers, power amplifiers are examples of large signal amplifiers. For example, power amplifiers are examples of large signal amplifiers, small signal amplifiers are examples of voltage amplifiers. Now, based on configuration, we can configure a transistor into three different configurations. On the basis of this configuration, we can classify our amplifiers into three different categories, that is, common emitter, common base, and common collector amplifier. Okay, next is the classification, classification based on the Q-point, we said large signal amplifiers, example of large signal amplifiers is power amplifiers, we classify power amplifiers based on the Q-point, we can classify class A amplifiers, class B amplifiers, and class A-B amplifiers. We can classify class A, class B, class A-B, class A-B, class C, etc. Next is the frequency of operation, based on the frequency of operation, we can classify class A, class B, class A-B, class C, etc. So, this is the classification of amplifiers, we can classify amplifiers based on the type of signal, type of configuration, also we can classify power amplifiers based on the Q-point, how Q-point is based on that different power amplifiers, this is based on frequency of operation. Okay. So, this is the main classification of amplifiers. If you are asking the classification of amplifiers, it is better to draw a box and show it. Otherwise, you can write class A, class B, class A-B, class C, and compare class A, class B, class A-B, and class C. Okay. So, this is the classification of amplifiers. Vcc is the power supply, Cb is the capacitor used to couple the input signal to the base of the transistor. This is a blocking capacitor. If the DC component is not present in the input signal, we are using this capacitor to block the DC component. Next, we are using a coupling capacitor called Cc. If the DC component is present in the output of the coupling capacitor, we are using Cc to block the DC component. CE is a bypass capacitor. It is parallel to the resistor called Re. It will provide a low reactance path to the amplified AC. We are using CE to provide a low reactance path to the amplified AC signal. R1 and R2 are voltage divider biasing techniques. R1 and R2 provide voltage divider bias. Re is a resistor for thermal stability. RC is a collector resistance to decide the value of IC and hence Q. RC is a resistor to decide the value of IC and hence Q. This was the one stage RC coupled amplifier. Two stage RC coupled amplifier means we are cascading a single stage RC coupled amplifier to another single stage RC coupled amplifier. One stage RC coupled amplifier is a common emitter amplifier. This is a common emitter amplifier. The output of this common emitter amplifier is the input of the next common emitter amplifier. That is what we call two stage RC coupled amplifier. The overall gain of two stage RC coupled amplifier is increased when we cascade. Look at the input signal. This is the output signal after the first stage. This amplified signal is fed to the second stage. This is the output signal after the second stage. Overall gain is increased. If the overall gain is 10 for the first stage and 10 for the second stage, the overall gain is 10 volts when the first stage is amplified. The overall gain can be increased by using cascading. Another advantage of cascading is that bandwidth shrinkage occurs. Gain bandwidth product is the frequency response of a common emitter amplifier. This is gain. Gain is av. When we draw the frequency response of a common emitter amplifier, gain versus frequency is called frequency response. In the last class, we talked about frequency response in the case of RC, low pass and high pass. Gain versus frequency is the frequency response. In the case of a common amplifier, the frequency response is like this. It is like a single stage. The frequency response is like this. The higher the frequency, the lower the gain in the low frequency region. The higher the frequency, the lower the gain in the high frequency region. The gain in the middle frequency region is maximum. Why does the gain decrease in the low frequency region? The reason for the decrease in the low frequency region is due to coupling and bypass capacitor. The gain in the low frequency region is due to the effect of the coupling capacitor and bypass capacitor in the amplifier circuit. The reason for the decrease in the gain in the low frequency region is due to the effect of the coupling capacitor and bypass capacitor in the amplifier circuit. The equation of frequency is f is equal to 1 by j omega c or j into 2 pi 2 pi 2 into 3.14 into 2 pi fc. In the case of a capacitor, the capacity reactance fc is equal to 1 by j omega c or 1 by j into 2 pi fc. In the case of a capacitor, the reactance is more when the frequency is less. The reactance is inversely proportional in the case of a capacitor. The reactance is more when the frequency is less. The reactance is more when the resistance of the circuit is more. The gain decreases due to the effect of the coupling capacitor and bypass capacitor in the low frequency region. The reason for the decrease in the gain in the high frequency region is due to the effect of the junction capacitance in the high frequency region. The gain decreases due to the effect of the junction capacitance in the middle frequency region. The gain in the middle frequency region is constant due to the absence of the effect of the coupling capacitance and junction capacitance. That is why the gain in the middle frequency region is maximum. This is the case of a single stage RC coupled amplifier. In the case of a multistage cascade, the output of one stage is fed to the input of the next stage. When this is done, the gain increases. The gain increases due to the increase in the frequency response. When the gain increases, the bandwidth automatically decreases. The bandwidth is the maximum frequency of the amplifier. From the maximum amplitude, if we draw 3 dB downwards, we get two points. This is the point. If we draw like this, we can write this as fL and this as fH. The low frequency point is denoted as the lower cut-off frequency and the high frequency point is denoted as fH. In this region between fH and fL, the gain of the amplifier is maximum. This region is called bandwidth. Bandwidth is the band of frequencies that the amplifier gives maximum gain. The equation of bandwidth is fH bandwidth is equal to fH minus fL. Bandwidth is the range of frequencies at which the amplifier will provide maximum gain. In this case, the bandwidth was this much. But when we look here, this is the maximum value, maximum gain. From there, we drew 3 dB downwards. We drew two points. The bandwidth here is this much. At the same time, when it was a single stage, the bandwidth was this much. When we did a two-stage cascade, the bandwidth was this much. From this, we can say that when we do a cascade, the gain increases and the bandwidth reduces. The gain increases but the bandwidth reduces. This is the concept of gain-bandwidth product. When the gain increases, the bandwidth naturally reduces. The concept is that the product of gain-bandwidth is a constant value. When the gain increases, the bandwidth automatically reduces. This is my concept. This is it. The advantage of doing a cascade is that we can increase the gain of the amplifier. But the bandwidth reduces. The bandwidth is the range of frequencies at which the amplifier will provide maximum gain. In the case of a single stage, a common emitter amplifier provides a 180 degree phase shift. If we give an input signal here, it will be amplified with an out-of-phase 180 degree phase shift. Similarly, if we use a two-stage common emitter amplifier, the input signal here will get a 180 degree phase shift. That will be the input signal for the next stage. When the phase shift is 180 degrees, the total phase shift is 360 degrees. We can say that there is no phase shift. It will be amplified but there is no phase shift. These are all about RC coupled amplifiers. This is a portion. Classification of amplifiers. RC coupled amplifiers. Design and working. Design is not necessary. Design and working. Voltage gain. Frequency response. This is what we drew. I will provide you with the proper material. Frequency response is between gain and frequency. This is the frequency response. If we take 3 dB below, we get two frequency points. Lower cut-off frequency and upper cut-off frequency. The difference between these two is called bandwidth. Bandwidth is the set of frequencies at which the amplifier will provide maximum gain. That is called bandwidth. What happens to the gain when we cascade this amplifier? The gain increases but the bandwidth decreases. In this case, the maximum amplitude gain is 3 dB below. This is 3 dB. This is 3 dB below. In this case, we get FH and FL. These are the two points. This is the bandwidth. When it is a single stage, the bandwidth increases. But the gain is low. Why do we feedback the amplifier? There are many uses for feedback. It can reduce distortion, improve bandwidth, improve the input resistance of an amplifier, improve the output resistance. An amplifier works for an application. We use amplifiers for many applications. We use feedback to meet the application requirements. By using feedback, we can increase the bandwidth of an amplifier. We know which feedback is used for amplifiers. Amplifiers use negative feedback. We use positive feedback for oscillators. There are two types of feedback. Positive feedback and negative feedback. We use negative feedback for amplifiers. By providing feedback, we can improve bandwidth, reduce distortion, improve the desensitivity of an amplifier, improve bandwidth, improve input and output resistance. There are many advantages. That is why we apply feedback to amplifiers. This is a block diagram. This is a block diagram of the feedback given by an amplifier. Here the output quantity is sampled by means of a suitable sampling network and is fed to the feedback network. A portion of the output is fed to the input. This is the feedback concept. Output can be voltage or current. Input can be voltage or current. A portion of the output voltage or current is sampled and mixed with the input voltage or current. This is the block diagram of a feedback amplifier. You can see different blocks in this. For example, signal source. What is a signal source? A signal source is an input. It may be a voltage source with a series resistor or a current source with a parallel resistor. You have learned about current source and voltage source in basic electrical. How to draw a current source? For a current source, draw an IS and a parallel RS resistor. For a voltage source, draw a VS and a parallel RS resistor. A signal source is either a voltage source or a current source. Next is a feedback network. A feedback network is usually in the form of a passive 2 port network and may be formed of resistors, inductors and capacitors. A feedback network is a passive 2 port network. It is a circuit using a resistor and a capacitor. A signal source is a feedback network. Next is a sampling network. What does a sampling network do? It takes a portion of the output and inputs it to the feedback network. The output may be voltage or current. Either it is voltage or current. Based on this, we can sample output voltage or output current. If we are sampling output voltage, we can draw a line parallel to the feedback network after the amplifier. If we are sampling current, we have to draw a line in series instead of parallel. For example, if we are sampling RL, we have to draw a line in series. If we are sampling voltage, we have to draw a line in series. If we are sampling current, we have to draw a line in series. This is how current is sampled. If we are sampling voltage, we have to draw a line in series. We have to draw these two diagrams properly. This is how we base our configurations. Next is a Comparator or Mixer. Mixer is another name for Comparator. First, we mix voltage here. If the input is voltage, we can mix the feedback voltage here. Or we can mix feedback current here. This Comparator works based on this. Either we mix voltage here, or we mix current here. We have to learn how to mix voltage, or how to mix current. If we mix voltage, we are mixing series here. If we mix current, we are mixing shunt here. If we are mixing current, we are mixing shunt here. If we are mixing current with our circuit, we are mixing shunt. Earlier I said voltage source, signal source. I said signal source may be either voltage source or current source. If it is a voltage source, it will be like this. This is a voltage source. If it is a current source, it will be like this. This is a current source. If we mix series to this source, we are adding voltage. Or we are feedbacking voltage to the input. Or we are feedbacking current to the input. If we are feedbacking current to the input, we are doing shunt mixing. Based on this, we can categorize the feedback amplifier in many ways. We can categorize the feedback amplifier in many ways. Based on input and output. Please pay attention. Input and output. Input can be voltage. Output can be voltage. Input and output can be voltage. Input can be voltage. Output can be current. Input can be current. Output can be current. Input can be voltage. Input can be current. Output can be voltage. Based on sampling and mixing, we can categorize the feedback amplifiers into four different categories. First is the input voltage. Please pay attention. Input is voltage. Output is also voltage. Output sampling is also voltage. If input and output are voltage, it is called voltage series amplifier. Next, input can be voltage. Output can be current. Input can be voltage. Output sampling can be current. Input can be voltage. Output can be current. It is called voltage shunt amplifier. If input is voltage and output is current, it is called current series amplifier. Don't get confused. Input can be voltage or current. Output can be voltage or current. Based on this, we can categorize the feedback amplifiers into four categories. Voltage series feedback, voltage shunt feedback, current series feedback, current shunt feedback. Voltage series feedback is voltage sampling series mixing feedback. Voltage sampling shunt mixing feedback. Current sampling series mixing feedback. Current sampling shunt mixing feedback. Voltage series feedback, voltage shunt feedback, current series feedback, current shunt feedback. Voltage sampling series mixing feedback. Voltage sampling shunt mixing feedback. Current sampling series mixing feedback. Current sampling shunt mixing feedback. Let's look at them one by one. First is voltage sampling series mixing. That's all. First, draw an amplifier. Then draw a feedback network. These two are in this diagram. Amplifier A, feedback network B. Draw these two. Draw the amplifier and feedback network. Draw A and B. Draw Beta. Voltage sampling series mixing. Voltage sampling is happening in the output. Mixing is happening in the input. Voltage sampling is happening in the output. We learned how to do voltage sampling. How to do voltage sampling? Connect shunt to the output. Where should this part be drawn? Here. Output is drawn as shunt. Series mixing. Series mixing is done with voltage. This is the diagram of series mixing. Let's look at series mixing. Draw A and B for each configuration. Voltage sampling series mixing. Voltage sampling is happening in the output. Series mixing is happening in the input. This is one category. The name of this amplifier is voltage amplifier. Because what we get in the output is voltage. Voltage sampling is happening in the output. Series mixing is happening in the input. Gain is Vout by Vin. Feedback factor is opposite to gain. Input is the output. A portion of the output is sampled and a small portion is fed to the output. Feedback voltage is beta times Vout. Or beta is equal to Vf by Vout. This is it. Voltage sampling series mixing. Amplifier is drawn. Feedback network is drawn. Voltage sampling is happening in the output. Series mixing is happening in the input. Voltage sampling shunt feedback. What is happening in the output? Voltage sampling. Voltage sampling is happening in the output. We can draw the same diagram as before. Voltage sampling is happening in the output. There is no change here. But what is shunt feedback? It is shunt mixing. What is shunt mixing? Shunt mixing is current source for input. Current source and current source are mixed in parallel. We can see the change in this figure. What is the specialty of this? What is happening in the input? Current source is input. What is happening in the output? Voltage sampling. Gain is Vout by In. Beta is the current mixed with feedback. Current is shunt feedback. Current is mixed. If by input is Vout. If by Vout. In this case, gain is voltage by current. What is voltage by current? Voltage by current is resistance. What is voltage by current? Voltage by current is resistance. So, this is called transresistance amplifier. This amplifier is called transresistance amplifier. Next configuration is current series. Current sampling, series mixing. What is happening in the output? Current sampling. Voltage sampling is like this. There is a difference in the output. This is called current sampling. In order to do current sampling, we have to draw this diagram on the output side. This is called current sampling. What is current sampling? Current sampling is series mixing. When series mixing is done, voltage is mixed. Voltage is mixed. Current sampling is output current. Series mixing. Input is voltage. Current by voltage is conductance. Voltage by current is resistance. Current by voltage is conductance. This is called transconductance amplifier. Next feedback is current shunt feedback. Current sampling, shunt mixing. Current sampling is output current sampling. This is called current sampling. Input is shunt mixing. This is called current source mixing. Current source is drawn here. Output is current. Input is current. Amplification factor is output by input. Output is current. Input is current. This is called current amplifier. These are the four different configurations of feedback amplifiers. How can we define feedback amplifiers? Based on mixing and sampling. Based on output sampling and input mixing. We can categorize into four different categories. Voltage series feedback. Voltage shunt feedback. Current series feedback. Current shunt feedback. Each has its own advantages. We use different feedback amplifiers for different applications. These are the four different types of feedback amplifiers. I hope this topic is clear to all. Do you have any doubts? Sir. Go ahead. Voltage, input and current output are called current series feedback. Voltage, input and? Current output. No, no. Current, input and voltage output. Okay, okay. You have a chance of getting it wrong. Thank you. Voltage series. You don't have to look at this. Look at this. Voltage shunt. Input is current. Output is voltage. Voltage shunt feedback. Voltage sampling, shunt mixing. Voltage sampling. Sampling is output. Sampling is input. Sampling is output. Voltage sampling. Output is voltage. Shunt mixing. Input is current. Is it clear? Yes, sir. Okay. Is it clear? Yes.