 Hello everyone. In this session, we are going to learn about negative feedback amplifier characteristics. At the end of this session, students will be able to understand the effect of negative feedback on amplifier characteristics and different advantages of negative feedback amplifier. So, these are the contents of this session. So, let us go for what are the advantages of negative feedback in short. So, with the application of negative feedback in the amplifier, there are many advantages for amplifier. Like the first gain stability, the overall voltage gain of amplifier with negative feedback becomes stable. So, gain stability is the most important advantage of amplifier and second is reduced noise non-linear distortion. So, with the application of negative feedback, non-linear distortion in the amplifier is reduced. Noise level, noise signal in the amplifier is reduced. So, bandwidth of the amplifier increases. Whereas, effect of negative feedback, the input and output impedance of the negative feedback amplifier are modified, either increased or decreased as per a requirement or type of feedback used at the input and output side of amplifier. Now, let us discuss what are the characteristics of negative feedback amplifier. How the amplifier is able to stabilize the gain, overall gain of amplifier with the use of a negative feedback, the stabilization of gain with negative feedback. The amplifier gain may be affected by the change in surrounding temperature or variation in power supply voltage or due to variation in different transistor parameters, due to aging of a transistor or a old transistor is replaced by a new transistor. So, due to variation of a transistor parameters, the operating point or this operating point or a Q point of the amplifier may change. So, due to that the gain of amplifier may change. So, with the application of a negative feedback, the gain of this negative feedback amplifier becomes stable. So, voltage gain with negative feedback is given by ABF, where ABF stands for voltage gain with feedback. So, that is equal to AB upon 1 plus beta into AB. So, where AB stands for voltage gain of basic amplifier or internal gain of amplifier, basic internal gain of amplifier and ABF stands for overall voltage gain of negative feedback amplifier. So, if the product of feedback factor and voltage gain of basic amplifier is much greater than 1, if this product beta into AB is much greater than 1 unity, then the overall voltage gain of amplifier with negative feedback becomes independent of internal gain of amplifier. If product beta into AB is much greater than 1, so ABF can be approximated to 1 upon beta. So, ABF depends upon only beta, that is beta stands for a feedback factor, gain of feedback circuit. So, the gain ABF is independent of internal gain of amplifier and depends on only feedback factor beta. So, feedback factor beta which depends on passive elements used in that feedback circuit like registers. So, once feedback circuit is designed using certain registers, the resistance value will not change. So, the gain of feedback circuit, that is feedback factor, will not change. So, as gain of feedback circuit becomes stable, constant, the overall gain of amplifier with negative feedback becomes stable, a constant. So, mostly in feedback circuit, registers are used. So, for registers, the resistance value are constant once the circuit is designed. So, in this way, the overall gain of negative feedback amplifier is stabilized, becomes stable. The second characteristic or effect of negative feedback on amplifier, so it reduces frequency distortion. So, with the use of negative feedback, frequency distortion caused in the amplifier is reduced. So, as far as negative feedback amplifier is concerned, if product of feedback factor and voltage gain of basic amplifier is much greater than 1, the gain with feedback, the overall voltage gain with feedback, negative feedback is given as ABF. So, that is approximately equal to 1 upon beta. So, beta stands for gain of feedback circuit. So, overall voltage gain depends upon the gain of feedback circuit, feedback factor. So, it shows that if feedback network is purely resistive, if feedback network used in the amplifier is purely resistive, so ABF, that is voltage gain with feedback, becomes independent of a frequency. Independent of frequency, even though the basic amplifier is a frequency dependent. So, the gain is totally depends upon feedback factor. So, that becomes constant. So, once the feedback circuit is designed and its feedback factor is finalized. So, practically this frequency distortion is caused due to change in gain of amplifier, changing voltage gain of basic amplifier. So, frequency distortion is considerably reduced. So, frequency distortion is considerably reduced with the use of negative feedback. So, basically you know the frequency distortion is caused due to change in a voltage gain of amplifier. So, with the use of negative feedback, overall voltage gain of this amplifier becomes independent of the gain of voltage gain of basic amplifier. It depends upon only feedback factor. So, the frequency distortion in negative feedback amplifier is reduced. Now, next characteristics or effect of negative feedback on a feedback amplifier. So, reduction in non-linear harmonic distortion with a negative feedback. So, harmonic distortion occurs in the amplifier when transistor goes beyond, the transistor operation goes beyond its range of linear operation. When transistor goes beyond its range of linear operation, this harmonic distortion occurs. If negative feedback is used, if negative feedback is used, the strength of input signal, effective input, strength of effective input of amplifier is increased with the use of negative feedback. So, by the same factor by which gain is reduced. The factor by which the overall gain of this amplifier is reduced by the same factor, the effective input signal strength of this amplifier is increased. So, magnitude of input signal remains unchanged and thus there is a reduction in non-linear harmonic distortion. If feedback, negative feedback is used and strength of input signal is increased by the same factor by which gain is reduced, this thus magnitude of input signal remains unchanged and thus there is a reduction in non-linear harmonic distortion. The next, let amplifier basic gain A B produces a distortion D in output signal. If negative feedback is applied, then output signal, amplified output signal as well as a distortion is fed back to the input side of amplifier. So, let distortion in output is D F in case of overall voltage gain with feedback A B F. A part of beta into D F of this distortion is fed back to the input side of amplifier. So, it gets amplified by factor A B and becomes beta into D F into A B. So, net distortion in output, amplified output signal D F. So, that is equal to a distortion without feedback minus feedback factor beta multiplied by D F that is distortion with feedback multiplied by basic gain of, voltage gain of basic amplifier. So, the distortion with feedback is equal to D upon 1 plus beta A. So, distortion in a negative feedback amplifier is reduced by factor 1 plus beta A, A or A B. So, thus harmonic distortion is reduced by the same factor as the gain. So, gain is reduced by factor 1 plus beta A. So, distortion is also reduced by the same factor 1 plus beta A or 1 plus beta A B. Now, next characteristic of a feedback amplifier that is effect of a negative feedback on amplifier. So, reduction in noise with negative feedback. So, noise in amplifier is reduced in the same way as a distortion is reduced with the application of a negative feedback. So, it is noise signal is noise is an unwanted electrical signal with random frequency and a random amplitude. So, that is generated in the amplifier. So, the internally due to active device that is amplifying device like transistors or it is introduced at any stage of amplifier in the multistage amplifier by external source. So, this noise can be reduced with the use of a negative feedback. If its source lies, source of noise signal is lying within the feedback loop. If negative feedback is used with the feedback factor beta, similar to distortion noise n can be reduced by factor 1 plus beta A B. So, noise with feedback n f designated as n f is equal to noise without feedback n divided by 1 plus beta A B. So, the noise is reduced by factor 1 plus beta A B. So, the performance of amplifier is improved. Now, last the effect of negative feedback on amplifier characteristic that is increased bandwidth that is improved frequency response. So, as we know the gain of a feedback is given as gain of feedback amplifier is given as A B f equal to A B upon 1 plus beta A B. So, but if product of a feedback factor and voltage gain is much greater than 1, then A B f is approximately equal to 1 upon beta. So, gain depends on feedback factor. So, the main property of feedback amplifier is that it maintains gain bandwidth product constant. So, for amplifier always product of gain and bandwidth remains constant, maintain constant. So, with the use of negative feedback gain reduces by factor 1 plus beta A B. So, at the same time, so bandwidth is increased by the same factor. So, with the application of negative feedback in the amplifier, the factor by which the gain is reduced by the same factor, the bandwidth of amplifier increases. Now, consider this figure. It is a graph of voltage gain versus input signal frequency. So, this graph there are two graphs. One graph of gain versus frequency without feedback. So, whose cut-off frequencies are f 1 and f 2. So, this is the difference between f 2 and f 1 is a bandwidth of amplifier without feedback and bandwidth of amplifier with feedback. So, there is another plot or graph whose lower cut-off frequency is f 1 dash and upper cut-off frequency is f 2 dash. So, bandwidth of amplifier with negative feedback. So, that is the difference between f 2 dash and f 1 dash measured in hertz. So, bandwidth is increased. So, the voltage gain of amplifier is reduced. At the same time, the bandwidth is increased. So, with the application of negative feedback, the lower cut-off frequency of amplifier, so that is f 1, the reduced by factor 1 plus beta A v. At the same time, the upper cut-off frequency of amplifier f 2. So, that is increased by factor 1 plus beta A v. So, that is designated as f 2 dash. So, f 2 changes to f 2 dash and f 1 changes to f 1 dash. So, bandwidth is increased and the gain is decreased. Now, student can pause video here and think over this question and try to answer this question. Which type of negative feedback amplifiers are commonly used? So, generally, voltage series and voltage shunt feedback amplifiers are used because voltage series feedback amplifier and voltage shunt feedback amplifier satisfy all desirable features or properties of amplifier. So, most commonly, voltage series and voltage shunt feedback amplifiers are used. So, this is the reference. Thank you.