 Hello, everyone. In this session, we are going to learn about feedback amplifier topologies and analysis. The learning outcome of this session, at the end of this session, students will be able to understand and analyze feedback amplifier topologies. So these are the contents of this session. So topologies of feedback amplifier, there are four basic amplifier types or topologies. Each of these is being approximated by the characteristic of an ideal control source. The four feedback topologies are as depending upon type of output quantity taken and the way in which the sampled output quantity given back to the input side of amplifier. Basically, there are four types of topologies or a feedback amplifier. So these are a voltage series or series shunt feedback amplifier, current series or series series feedback amplifier, current shunt or shunt series feedback amplifier and the fourth type is a voltage shunt feedback amplifier or shunt shunt feedback amplifier. So let us go for the first type of negative feedback amplifier that is a voltage series feedback amplifier. So, this figure shows a block diagram for a voltage series feedback amplifier. So, in this the output the quantity taken from the output side of this amplifier is a voltage and the first term of this name of this amplifier indicates that the quantity taken from output side of amplifier whether a voltage or current. So, in this case the output quantity taken from output side of amplifier is a voltage and the second term series stands for the feedback circuit provides that feedback signal that is proportional to the quantity taken from output of amplifier it is applied in series with the input signal existing at the input side of amplifier. So, for voltage series feedback amplifier. So, the input is effective input voltage for basic amplifier is V n and V s stands for external applied input signal voltage V s and this is a feedback circuit. So, it samples some part of output voltage and for that parallel connection is used at the output side of amplifier and the signal proportional to output voltage the signal proportional to output voltage it is that is designated as V f it is feedback to the input side of this amplifier. So, that feedback signal feedback voltage signal V f so, that is applied in series with the input signal V n V n stands for effective input voltage of amplifier and the feedback voltage signal V f. So, that is in that is coming in series with external applied input voltage signal source V s. So, as the feedback signal voltage signal is applied in series opposition in series opposition with the input voltage signal. So, both V f and V n are out of phase or of opposite polarity. So, feedback voltage signal opposes the input voltage signal existing at the input side of amplifier. So, due to this the effective input current the effective input current of this amplifier decreases. So, the input impedance input impedance of this voltage series feedback amplifier increases as effective input current decreases the input impedance of this amplifier increases. So, output is amplified voltage signal that is applied to load resistor R L. So, the voltage appearing across R L is a V out that is the output amplifier output voltage of overall amplifier. So, input voltage V n of the basic amplifier is a algebraic sum of the input signal V s and the feedback signal voltage V f. So, V f that is equal to beta times V out. So, beta stands for gain of feedback circuit or feedback factor. So, part of output voltage. So, that is V f it is mixed with V s and the resultant of this is applied to the input of basic amplifier. So, V o stands V o or V out stands for the output voltage of the amplifier. So, from this block diagram as input impedance increases at the same time as there is a parallel connection at the output side of this amplifier to take to part of output voltage. So, effective output impedance effective output impedance of this voltage series feedback amplifier decreases. So, from that block diagram of voltage series feedback amplifier the transfer ratio or gain of basic amplifier. So, that is designated as a that is the basic internal gain of gain or transfer ratio of amplifier. So, V n stands for effective input voltage of overall amplifier and V out is the out amplifier output voltage of amplifier. So, in that block diagram a stands for a ratio of output voltage to actual input voltage of a basic amplifier. So, that is a equal to a v designated as a v. So, it is a voltage gain V stands for voltage gain. So, a v stands for voltage gain of amplifier. So, transfer ratio or gain of overall feedback amplifier. So, considering that V s is a externally applied input voltage whereas, V f is output of feedback circuit that is feedback voltage signal and beta stands for feedback factor or gain of feedback circuit. So, that is beta equal to V f upon V out. So, overall voltage gain of voltage series feedback amplifier a V f. So, that is equal to V out upon input voltage V s external applied input voltage V s. The second type of topology that is voltage and feedback amplifier. So, in this voltage and feedback amplifier the quantity taken from output side of amplifier is again a voltage. So, for that there is a parallel connection or shunt connection. So, output of amplifier and the input side of a feedback circuit are connected in parallel and load resistor R L is coming in parallel to the input side of a feedback circuit. So, the signal proportional to output voltage the signal proportional to the output quantity sample that is the voltage. So, that is the current signal it is applied in parallel it is applied in parallel to the input signal current I s. So, the in where I s stands for the current resulting from externally applied input signal source. So, that current I s and the feedback current I f are in parallel or in shunt and I n is a effective input effective input current of a basic amplifier. So, as there is a parallel connection at the input side of amplifier as well as output side of amplifier the input impedance and output impedance of this voltage and feedback amplifier with feedback decreases both input impedance and output impedance are decreases. So, input is a current signal and output is a voltage signal. So, voltage shunt feedback amplifier provides an output voltage provides an output voltage view that is in proportion to the input current I s. So, input current I n that is effective input current of basic amplifier is the algebraic sum of I s and the output current of a feedback circuit that is I f and the feedback current I f. So, that is applied in parallel to the input signal source current I s. So, considering this block diagram of voltage shunt feedback amplifier the gain of basic amplifier. So, that is the designated a. So, that is nothing, but in this case for a voltage shunt feedback effective input current of amplifier is I n and output is a voltage voltage signal. So, forward transfer ratio or gain of this amplifier it is the ratio of output voltage to input current. So, that is equal to a. So, that is designated as R m. So, R m stands for trans resistance of amplifier. So, R m stands for trans resistance of this amplifier without feedback. So, transfer ratio or gain of this amplifier with feedback. So, that can be expressed as considering I s as external applied input current and I f is the output of feedback circuit. So, beta that is the feedback factor of feedback circuit. So, beta equal to I f upon V out. So, the trans resistance of overall voltage shunt feedback amplifier R m f designated as R m f. So, that is equal to ratio of output voltage V out to the input current I s. Now, third type of topology that is current series feedback amplifier. In current series the type of output quantity sampled from the output side of amplifier is a current. So, for current sampling the output side of amplifier and the input side of feedback circuit are connected in series. So, output load resistance coming in series with the input port or input side of a feedback circuit. So, output is a current signal and the input side of this amplifier. So, the feedback circuit provides a voltage signal that is proportional to the current sampled from output side of this amplifier. It is applied in series in series with the input voltage V s or in series with effective input voltage of this amplifier. So, this is nothing but a current series feedback amplified topologies. So, feedback voltage V f applied in series opposition with V s or a V in and the effective input voltage for amplifier is a V in. So, that is amplified at the output. So, output is a current and input is a voltage. So, this current series feedback amplifier acts as a trans conductance amplifier. So, trans conductance feedback amplifier provides an output current I o that is output current I out which is proportional to the input voltage V s. So, the feedback signal is the voltage V f which is added or mixed to the input externally applied input voltage source signal voltage V s at the input of this basic amplifier. So, from this block diagram we can express the gain of basic amplifier without feedback that is designated as G m. So, G m stands for a trans conductance trans conductance of amplifier. So, it is ratio of output current I out to input voltage V in and considering the negative feedback. So, the gain of this amplifier with negative feedback. So, that is a transfer ratio with feedback. So, that is expressed as G m f where G m f stands for transfer ratio or gain of this amplifier with feedback. It is a ratio of output current to the input voltage V s. So, fourth type of topology that is current shunt feedback amplifier. So, in this output quantity taken is a current. So, for that there is a series connection between output side of amplifier and the feedback circuit. So, load register R L is coming in series with the input side of feedback circuit and the feedback signal that is the current signal that is proportional to output current. So, it is applied in parallel in shunt with the input current I s resulting due to external applied input signal source. So, the current shunt feedback amplifier supplies an output current I out which is a proportional to the input current I in. So, this makes it as a current amplifier. So, current shunt feedback amplifier is a current amplifier. The feedback signal is a current I f and the input current of the basic amplifier is I in. So, I in equal to I s plus I f and the output current is I out. So, that is equal to the load current I l. So, from this block diagram we can express the gain without feedback and gain with feedback. So, gain without feedback that is the current gain of this amplifier ratio of output current to input current. So, designated as A i. A i stands for current gain of amplifier and current gain of amplifier with negative feedback. So, that is designated as A i f. So, it is a forward transfer ratio or current gain of this amplifier with feedback. So, that is equal to amplified output current I out upon the input external applied input signal source current I s. So, student can pause video here and think over this question and try to answer this question. Series feedback connection tends to increase tends to increase the input impedance of amplifier. Why? The answer is this is because the feedback signal is applied to input in series to oppose the external input signal applied causing the input current to decrease. So, input impedance of overall amplifier increases. So, this is the reference. Thank you.