 Hello everyone. In this session, we are going to learn about feedback amplifier, its input impedance and output impedance. Learning outcome of this session, at the end of this session, students will be able to analyze the effect of negative feedback on amplifier's input impedance and output impedance. So, this is the content of this session. Now, let us discuss what is the effect of negative feedback on input impedance of overall negative feedback amplifier. So, high input impedance is desirable in a voltage amplifier. Since, it reduces a loading effect on the preceding stage or it will not load the previous stage of multistage amplifier. So, high input impedance or resistance is a required feature of a voltage amplifier. So, this can be achieved with the use of negative feedback. So, in a voltage series feedback amplifier, the feedback signal voltage V f. So, that is the out of phase with respect to the input signal V n or external applied input voltage source signal V s. Listen once again. In a voltage series feedback amplifier, the feedback signal V f. So, that is the out of phase with respect to the input signal V n of the amplifier. So, due to this effect, the net effective input for amplifier V n reduces and due to that the effective input current of a voltage series feedback amplifier reduces. So, hence the effective input resistance of a voltage series feedback amplifier decreases. As the input current decreases, the input impedance or the input resistance increases. Now, this figure shows the equivalent circuit for voltage series feedback amplifier. In this Z in F stands for the input impedance of amplifier with negative feedback and Z in stands for the input impedance of amplifier without feedback. So, now this block diagram shows the input is a voltage signal V s and effective input voltage of this amplifier is V n. V f stands for output voltage of feedback circuit. So, that is a feedback voltage signal and the output at the output side of this amplifier. So, output voltage V out is applied to load resistor R L. So, output resistance of amplifier is Z out or impedance is Z out and the output voltage is applied to output load resistance R L. So, considering this block diagram, the feedback factor beta, so that is the ratio of output voltage to the input voltage. Output voltage of feedback circuit is V f. The input voltage for feedback circuit is V out that is the output of amplifier. So, considering this block diagram, now let us justify the effect of negative feedback on input impedance or input resistance of overall voltage series feedback amplifier. So, from figure one series feedback connection, the input impedance can be determined as follows. So, the effective input current of amplifier N equal to V in upon Z in. So, that is equal to V s minus V f. So, that is divided by Z in. Z in stands for the input impedance of amplifier without feedback. Effective input voltage of amplifier is an algebraic sum of V s and V f that is V s minus V f. So, that is equal to V s minus beta into V out. For V f we can write beta into V out. So, this is divided by Z in. So, where V f that is equal to beta into V out. So, this is simplified to V s minus beta into A into V in, where A stands for voltage gain of amplifier without feedback A or A V. So, V s minus beta into A V into V in. So, this is divided by Z in. So, V out that is equal to A into V in or I in multiplied by Z in. So, that is equal to V s minus beta into A into V in or V s equal to for V s we can write I in into Z in plus beta A V or A into V in. So, that is equal to I in into Z in plus beta A into I in into Z in. So, I in is effective input current of amplifier and Z in is input impedance of amplifier without feedback. So, V in equal to I in into Z in. So, effective input voltage of amplifier that is equal to effective input current multiplied by input impedance of amplifier without feedback. So, V s upon I in. So, that is equal to Z in plus beta A into Z in. So, overall input impedance of amplifier with feedback with negative feedback Z in F. So, that is equal to V s upon I in input voltage upon input current that is equal to Z in plus beta into A. So, multiplied by Z in. So, that is equal to Z in into 1 plus beta A beta A or A V. So, overall input impedance of voltage series feedback amplifier with negative feedback it is increased by factor 1 plus beta A V. So, Z in F that is equal to Z in multiplied by 1 plus beta A V. So, input impedance is increased by factor 1 plus beta A V. The input impedance with series feedback is seen to have the value of input impedance without feedback multiplied by the factor 1 plus beta A V. So, input impedance with voltage series feedback is increased by the factor 1 plus beta A V or A. So, this is applicable to both voltage series and current series feedback configuration. For voltage series amplifier and current series amplifier the input impedance is increased by factor 1 plus beta A V. Now, voltage and feedback amplifier for voltage and feedback amplifier the effect of a negative feedback on input impedance. Now, this figure shows the block diagram for voltage and feedback amplifier. So, in this the feedback signal I f is applied in parallel to the input current I s and the output voltage is applied to load resistor R L. So, from output side voltage is taken and at the input side of amplifier the proportional current is applied in parallel to the input signal source current I s. So, considering this block diagram we can find out the effect of a negative feedback on input impedance. So, using a voltage and feedback. So, from figure 2, voltage and feedback connection. So, I s equal to I n plus I f. So, input signal source current I s equal to the effective input current I n plus feedback current I f and input impedance can be calculated as Z in f. So, input impedance with feedback that is equal to voltage upon current V in upon I s. So, that is equal to V in upon for I s we can write I n plus I f. So, this is equal to V in upon I n plus beta into V out. For I f we can write beta into V out where beta stands for feedback factor. So, for feedback circuit the input is the output voltage of amplifier and output is feedback current I f. So, for I f we can write beta into V out. So, this is simplified too. So, V in upon I n divided by I n upon I n plus beta into V out upon I n. So, after simplification. So, this is equal to Z in upon 1 plus beta into A. So, overall input impedance of amplifier with negative feedback Z in F. So, it is decreased from Z in without input impedance without feedback by factor 1 plus beta A. So, input impedance of amplifier with voltage and feedback is reduced by factor 1 plus beta into A V A V R A. So, that is a voltage gain of amplifier without feedback. So, the effect of negative feedback that is a voltage and feedback the overall input impedance of amplifier is reduced by factor 1 plus beta A V. Now, let us say the effect of negative feedback on output impedance of feedback amplifier. So, the low output impedance is a desirable feature for amplifier to drive a lower impedance load. The required feature of amplifier to drive a low resistance output load the output impedance should be low. So, voltage for voltage series feedback amplifier. Now, let us find out the output impedance that can be obtained as follows. The short circuit input terminals of amplifier short circuit input signal source. So, that input terminals of amplifier are shorted. So, that minus V f minus V f that is feedback voltage is only input voltage at the input side of amplifier and voltage source V out voltage source whose voltage equal to V out is applied at the output of amplifier. So, that causes the output current I out. So, the I out is a drawn from applied source and it flows inside the output side of amplifier. So, consider that V out equal to I out into Z out where Z out stands for output impedance or resistance of amplifier without feedback plus A into V in. So, we can write I out into Z out minus A v A into V f. So, that is equal to I out into Z out minus A into beta into V out. So, where V in is equal to minus V f and V f is nothing but beta into V out or V out plus A into beta V out. So, that is equal to I out into Z out or after simplification V out into bracket 1 plus beta A or A v. So, that is equal to I out into Z out or we can write ratio of output voltage to output current that is equal to Z out upon 1 plus beta A or A v. So, this is nothing but the output impedance of amplifier with feedback. So, that is equal to Z out upon 1 plus beta A or beta A v. So, output impedance of amplifier with the voltage series feedback is reduced by factor 1 plus beta A v. So, this is a required feature for the amplifier. So, output impedance is decreased by factor 1 plus beta A v. Similarly, the effect of negative feedback on output impedance of current series feedback amplifier. In current series feedback amplifier now this figure shows the block diagram for current series feedback amplifier. So, in this output quantity taken sampled is a current and it is paid back to the input side of amplifier. So, the output quantity taken is a current that is series connection is used and the signal proportional to output quantity that is the voltage signal it is applied in series with the input signal V s or V in. So, this is the current series feedback amplifier and now let us find out the effect of negative feedback on output impedance of this current series feedback amplifier. So, from this block diagram of current series feedback amplifier the output impedance with current series feedback can be obtained by disconnecting a load RL by disconnecting output load RL and apply a signal voltage V to the output with the input signal source voltage V s shorted out and taking the ratio of voltage to current gives the output impedance. So, current I equal to voltage V upon Z out. So, that is minus A into V in. So, that is equal to V upon Z out minus A into V f. So, this is equal to V upon Z out minus A into beta into I out where V in equal to V f that is equal to beta into I out because input for feedback circuit is a current output and output is a voltage V f. So, output impedance Z out multiplied by 1 plus beta A multiplied by I that is equal to V. So, here output current of amplifier I out equal to I. So, output impedance of this amplifier with negative feedback designated as Z out f. So, that is equal to V upon I voltage upon current that is equal to Z out multiplied by 1 plus beta A V where Z out is a output impedance without feedback and Z out f is a output impedance with feedback. So, that is equal to Z out multiplied by 1 plus beta A. So, output impedance of amplifier current series feedback amplifier is increased by factor 1 plus beta A V. Now, student can pause video here and think over this question, try to answer this question. Shunt feedback connection tends to decrease the input impedance of amplifier. Why? The answer is this is because the current drawn from input signal source is increased by the amount equal to the feedback current I f. So, the effect to input impedance of the amplifier decreases. As the effect to input current increases, the effect to input impedance decreases. This is the reference. Thank you.