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ELECTONIC DEVICES & CIRCUITS (BATU), , , , When the EMF is induced in the secondary coil due to the, varying magnetic field of the primary coil, then such, phenomenon is called as the Mutual Inductance., , The Fig, 3 40 below gives an idea about this., , , , , , , , CIAAXD, , , , , , , , Fig. 3.40, , The current i, in the Fig. 3.40 indicate the source current while, ing indicates the induced current The flux represents the, magnetic flux created around the coil. This spreads to the, secondary coil also,, , With the application of voltage, the current i, flows and flux, gets created When the current is varies the flux gets vaned,, producing tine in the secondary coil, due to the Mutual, inductance property., , Coupling, , Under the concept of mutual inductance coupling will be as, shown in the Fig. 3.41 betow., , , , , , , , 2 Load, , at, P, , , , , , , , , , , , , , , , , , Fig. 3.41, , When the coils are spaced apart, the flux linkages of primary, coil Ly will not link the secondary coil L;, At this condition, the, coils are said to have Loose Coupling The resistance, reflected from the secondary coil at this condition is small, and the resonance curve will be sharp and the circuit Q is, high as shown in the Fig. 3.42 below,, , t Loose coupling, Gain) ‘, , Tigh coupling, , , , | Frequency —», , (3.24), , , , ELECTRONICS AMPLIFIERS, , * On the contrary, when the primary and secondary coils are, brought close together, they have Tight Coupling. Under, such conditions, the reflected resistance will be large and the, arcuit Q is lower. Two positions of gain maxima, one above, and the other below the resonant frequency are obtained., , Bandwidth of Double Tuned Circuit, , * The above Fig. 3.42 clearly states that the bandwidth, increases with the degree of coupting, The determining factor, in a double tuned circuit is not Q but the coupling., , * We understood that, for a given frequency, the tighter the, coupling the greater the bandwidth will be,, , The equation for bandwidth is given as, BWa = ki, BWa = bandwidth for double tuned circuit,, K = coefficient of coupling,, , Where, , f, = resonant frequency,, 3.9 FEEDBACK AMPLIFIERS, , * An amplifier circuit simply increases the signal strength. But, while amplifying, It just increases the strength of its input, signal whether it contains information or some noise along, with information., , * This nolse or some disturbance is introduced in the amplifiers, because of their strong tendency to introduce hum due to, sudden temperature changes or stray electric and magnetic, fields. Therefore, every high gain amplifier tends to give noise, along with signal in its output, which is very undesirable,, , « The noise level in the amplifier circuits can be considerably, reduced by using Negative Feedback done by injecting a, fraction of output in phase opposition to the input signal., , Principle of Feedback Amplifier, , * A feedback amplifier generally consists of two parts. They are, the Amplifier and the Feedback Circuit. The feedback circuit, usually consists of resistors. The concept of feedback, amplifier can be understood from the following Fig. 3.43, , +, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , + Amplifier “oY,, awe vy - with gain A. U =o Output, +, Fig. 3.43, , * From the above Fig 3.43, the gain of the amplifier is, represented as A the gain of the amplifier is the ratio of, output voltage Vo to the input voltage V. the feedback, network extracts a voltage Vi = B V. from the output V. of, the amplifier.

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ELECTONIC DEVICES & CIRCUITS (BATU), , This voltage is added for positive feedback and subtracted, for negative feedback, from the signa! voltage V, Now,, Vio= Wet Wa Vy + BV, Vi= Vir VeVi Bo, The quantity B = ¢ is called as feedback ratio or feedback, fraction., Let us consider the case of negative feedback, The output Vo, must be equal to the input voltage (¥, - BV.) multiplied by, the gain A of the amplifier,, Hence,, .~ BVala = Vo, Or, AV, - ABV, = Vo, Or, AV, = Voll + AB), Therelore,, , A, VMs = Ty AB), , Let A; be the overall gain (gain with the feedback) of the, amplifier. This is defined as the ratio of output voltage V. to, the applied signal voltage Vy i., , he Output voltage, ' = Input signal voltage, Vo, “Ve, So, from the abave two equations, we can understand that,, the equation of gain of the feedback amplifier, with negative, , feedback is given by, , , , A, (1 + AB), The equation of gain of the feedback amplifier, with positive, feedback is given by, , , , Af =, , Af = —le, ~ (2 - AB), , These are the standard equations to calculate the gain of, feedback amplifiers., , 3.9.1 Types of Feedbacks, , The process of injecting a fraction of output energy of some, device back to the input is known as Feedback, It has been, found that feedback is very useful in reducing noise and, making the amplifier operation stable., , Depending upon whether the feedback signal Aids or, , Opposes the input signal, there are two types of feedbacks, used, , Positive Feedback, , The feedback in which the feedback energy ie, either voltage, of Current is in phase with the input signal and thus aids it is, called as Positive Feedback., , , , G25), , ELECTRONICS AMPLIFIERS, , Both the input signa! and feedback signal introduces a phase, shift of 180° thus making a 360” resultant phase shift around, the loop, to be finally in phase with the input signal., , Though the positive feedback Increases the Gain of the, amplifier, it has the disadvantages such as, , > Increasing distortion, , > Instability, , It is because of these disadvantages the positive feedback is, not recommended for the amplifiers. If the positive feedback, is sufficiently large, it leads to oscillations, by which oscillator, circuits are formed, This concept will be drscussed in, oscillators tutorial, , Negative Feedback, , 1, , The feedback inn which the feedback energy ie, either voltage, of current is out of phase with the input and thus opposes it,, is called as Negative Feedback., , In negative feedback, the amplifier introduces a phase shift of, 180° into the circuit while the feedback network is so, designed that it produces no phase shift or zero phase shift, Thus the resultant feedback voltage V; Is 180° out of phase, with the input signal Vy,, , Though the Gain of negative feedback amplifier is Reduced,, there are many advantages af negative feedback such as, , > Stability of gain is Improved, , > Reduction in distortion, , * Reduction in noise, , > Increase in input impedance, , > Decrease in output impedance, , 7 Increase in the range of uniform application, , Tt ts because of these advantages negative feedback is, frequently employed in amplifiers., , Negative feedback in an amplifier is the method of feeding a, portion of the amplified output to the input but in opposite, phase. The phase opposition occurs as the amplifier provides, 180° phase shift whereas the feedback network doesn't., ‘While the output energy is being applied to the input, for the, voltage energy to be taken as feedback, the output is taken, in shunt connection and for the current energy to be taken as, feedback, the output is taken in series connection., , There are two main types of negative feedback circuits. They, are:, , 1, Negative Voltage Feedback, , 2, Negative Current Feedback, , Negative Voltage Feedback, , In this method, the voltage feedback to the input of amplifier is, Proportional to the output voltage.

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ELECTONIC DEVICES & CIRCUITS (BATU), , This is further classified into twa types :, , 2., , i, Voltage-series feedback, iL Valtage-shunt feedback, Negative Current Feedback, , In this method, the voltage feedback to the input of amplifier is, proportional to the output current. This is further classified into, two types =, , i. Current-series feedback, iL Current-shunt feedback, , Let us have a brief idea on all of them., 3.9.2 Voltage-Series Feedback, , (3.26), , ELECTRONICS AMPLIFIERS, , As the feedback circuit is connected in shunt with the output, and the input as well, both the output impedance and the, input impedance are decreased,, , 3.9.4 Current-Series Feedback, , , , , , In the voltage series feedback circuit, a fraction of the output, yoltage is applied in series with the input voltage through the, feedback circuit. This |s also known as Shunt-Driven SeriesFed feedback, ie, a parallel-series circuit., , The following Fig. 3.44 shows the block diagram of voltage, series feedback, by which it is evident that the feedback, circuit is placed in shunt with the output but in series with the, , , , , , , , , , , , , , , , , , , , , , , , , , , , input, g Amplifier, Ve Vi }wity gain A U Mo 3 R, Vy=P¥o, V, Feedback, ‘ circuit B, Fig. 3.44, , As the feedback circuit is connected in shunt with the output,, the output impedance is decreased and due to the series, connection with the input, the input impedance is increased., , 3.9.3 Voltage-Shunt Feedback, , In the current series feedback circuit, a fraction of the output, voltage is applied in series with the input voltage through the, feedback circuit. This is also known as Series-Driven SeriesFed feedback i.e, a series-series circuit., , The following Fig, 3.46 shows the block diagram of current, series feedback, by which it is evident that the feedback, circuit is placed in series with the output and also with the, input., , , , Basic, Amplifier, with gain A, , , , o, Vs vs Vo 3 R, , , , , , , , , , , , , , V, | Feedback, li circuit, , , , , , , , , , , , , , Fig. 3.46, , As the feedback circuit is connected in series with the output, and the input as well, both the output impedance and the, input impedance are increased., , 3.9.5 Current-Shunt Feedback, , , , , , In the voltage shunt feedback circuit, a fraction of the output, voltage is applied in parallel with the input voltage through, the feedback network, This is also known as Shunt-Driven, Shunt-Fed feedback ie, a parallel-parallel proto type., , The below Fig. 3.45 shows the block diagram of voltage, shunt feedback, by which it is evident that the feedback, circuit is placed in shunt with the output and also with the, , , , , , , , , , , , , , , , , , input., > Basic, V, Vv, Amplifier Vo SR,, _ d —| with gain A rR 33 ., Vr BV,, Feedback, “ circuit B, , , , , , , , , , Fig. 3.45, , , , In the current shunt feedback circuit, a fraction of the output, voltage is applied in series with the input voltage through the, feedback circuit. This is also known as Series-Driven ShuntFed feedback i.e, a series-parallel circuit., , The below Fig. 3.47 shows the block diagram of current shunt, feedback, by which it is evident that the feedback circuit Is, placed in series with the output but in parallel with the Input,, , mT, , , , Basic, Amplifier, with gain A, , , , Vo 3 Re, , , , , , , , , , , , , , , , ve Feedback, E circuit, , , , , , , , , , Fig. 3.47, , As the feedback circuit is connected in series with the output,, the output impedance is increased and due to the parallel, connection with the input, the input impedance is decreased., , Let us now tabulate the amplifier characteristics that get, affected by different types of negative feedbacks.

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ELECTONIC DEVICES & CIRCUITS (BATU) B27 ELECTRONICS AMPLIFIERS, Table 3.6 Operation, Characteristics Types of Feedback * The input signal voltage applied between base and emitter,, Vottage- | Voltage: | Current” | Current develops an output voltage Vo across Re, which is in the, Serles Shunt Series Shunt Z :, Vollaga Gam | Decreases | Decreases | Decreases Decreases emitter section: Therefore,, Bandwidth Increases | Increases | Increases Increases Vo = ERr, Inputresistance | Increases | Decreases | Increases _| Decreases « The whole of this output current is applied to the input, Output Dacteasas | Decreases | Increases Increases through feedback. Hence,, resistance, Harmonic Dacteases | Decreases | Decreases | Decreases VE= Ma, distortion « As the output voltage developed across R. is proportional to, Noise Decteasos | Decreases | Decreases | Decreases the emitter current, this emitter follower circuit is a current, , , , , , , , , , , , , , , , Emitter follower and Darlington amplifier are the most common, examples for feedback amplifiers. These are the mostty used ones, with a number of applications., , 3.9.6 Emitter Follower, , , , Emitter follower circuit has a prominent place in feedback, amplifiers. Emitter follower is a case of negative current feedback, circuit. This is mostly used as a last stage amplifier in signal, generator circuits,, , The important features of Emitter Follower are —, , * = Ithas high input impedance, + — Ithas low output impedance, * Itis ideal circuit for impedance matching, , All these ideal features allow many applications for the emitter, follower circuit. This is a current amplifier citcuit that has no, voltage gain,, Construction, , ., , The constructional details of an emitter follower circuit are, nearly similar to a normal amplifier. The main difference is, that the load Ri is absent at the collector terminal, but, present at the emitter terminal of the circuit, Thus the output, is taken from the emitter terminal instead of collector, terminal,, The biasing is provided either by base resistor method or by, potential divider method. The following Fig, 3.48 shows the, circuit diagram of an Emitter Follower., Mec, , , , eR,, , , , ), , Fes Output, , on), , Sk, , Input, signal, , , , , , , , , , , , Fig. 3.48, , , , feedback circuit. Hence,, , p= 7 It is also noted that the input signal voltage to the transistor, (= ¥)) is equal to the difference of V, and Vo ie,, , Vi = Ve-Mo, , Hence the feedback is negative., , Characteristics, , The major characteristics of the emitter follower are as follows , No voltage gain, In fact, the voltage gain Is nearly 1,, Relatively high current gain and power gain., High input impedance and low output impedance., , Input and output ac voltages are in phase., , Valtage Gain of Emitter Follower, , As the Emitter Follower circuit is a prominent one, let us try, to get the equation for the voltage gain of an emitter, follower circuit. Our Emitter Follower circuit looks as follows ;, , , , , , +NVeo, SR,, Input $, signal Ry ‘Oulput, i *, + +, Fig. 3.49, , If an AC equivalent circuit of the above circuit is drawn, it, would look like the below one, as the emitter by pass, capacitor is absent.

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ELECTONIC DEVICES & CIRCUITS (BATU), , Cin, |, Input, signal Ry WR, , 4 :, , Fig. 3.50, The AC resistance re of the emitter circuit is given by, , te = Pet Re, Where, , . . 25mv, , re, , In order to find the voltage gain of the amplifier, the above, Fig. 3.50 can be replaced by the following Fig. 3.51, , Oun |, , , , , , Fig. 3.51, , Note that input voltage is applied across the ac resistance of, the emitter circuit ie, {r'y + Re). Assuming the emitter diode, to be ideal, the output voltage Vou will be, , Vou = ieRe, Input voltage V;, will be, Vin = let's + Re}, Therefore, the Voltage Gain of emitter follower is, _ Vout iRe, , , , Av = Vin = ial + RD, _ ke, (re + Re), - i, ce Av = 4 Re), , (3.28), , ELECTRONICS AMPLIFIERS, , In most practical applications,, , Re > re, So, Ay = 1. In practice, the voltage gain of an emitter follower, is between 0.8 and 0.999,, , 3.9.7 Darlington Amplifier, , , , , , The emitter follower circuit which was just discussed lacks to, meet the requirements of the circuit current gain (A) and the, input impedance (2). In order to achieve some increase in, the overall values of circuit current gain and input, impedance, two transistors are connected as shown in the, following circuit diagram, which is known as Darlington, configuration., , , , , , , , , , , , , , Meee, iY, en I,, cz, a,, By, Q,, J Re ler* lez €) :, i |f_,, ¥ IN Vi, ve |, ea, ¥, Fig. 3.52, , As shown in the above Fig. 3.52 the emitter of the first, transistor is connected to the base of the second transistor., The collector terminals of bath the transistors are connected, together., , Biasing Analysis, , Because of this type of connection, the emitter current of the, first transistor will also be the base current of the second, transistor. Therefore, the current gain of the pair is equal to, the product of individual current gains i.e,, , B = Bib:, 4 high current gain is generally achieved with a minimum, number of components., As two transistors are used here, two Vat drops are to be, considered, The biasing analysis is otherwise similar for one, transistor,, , Voltage across R2,, , Ne, w= (te) x