 Hello everyone. In this session, we are going to learn about feedback amplifier concept. Learning.com of this session, at the end of this session, students will be able to understand a basic feedback amplifier concept and types of feedbacks. So, these are the contents of this session. So, introduction to feedback amplifier. Now, let us start with the basic definition of feedback amplifier and what is the feedback? Feedback is defined as the process in which a portion of output signal, some portion of output signal of amplifier is fed back to the input side of amplifier to produce effective input signal for amplifier. So, that is used to control that is used to control the output signal of amplifier and many characteristic of amplifier. So, in feedback amplifier, the output quantity, either voltage signal or current signal, so that is sampled by means of suitable sampling network and it is fed back to the input side of amplifier using feedback network. So, output of feedback network, so that is a fraction of output signal, a signal proportional to the signal sampled from output of amplifier. So, that is combined with the external signal, that is externally applied signal source signal. So, through a mixer network and applied into the basic amplifier. So, after mixing of feedback signal with the external applied input signal source, the effective input signal is applied to basic amplifier. Now, let us go for conceptual development through block diagrams. The block diagram of a basic feedback amplifier consists of five basic elements. These basic elements of a feedback amplifier are input signal source, comparator or a mixer network at the input side of a basic amplifier. Then the third component is a basic amplifier, then a sampling network and finally, the feedback circuit. Now, this figure shows block diagram of basic feedback amplifier. So, it consists of basic amplifier. So, at the input side of this basic amplifier, there is a input signal source. So, that is connected with the mixer network or comparator. So, to which the feedback circuit is connected, the feedback circuit is connected in between the output of basic amplifier and the input of basic amplifier. So, using a comparator or a mixer network. So, for a mixer network, the input is one of the input is from input signal source, that is externally applied and another signal from a feedback circuit. So, feedback circuit takes some part of output signal, either a voltage signal or current signal using a sampler or a sampling network and it is a feedback to the input side of amplifier. So, here VIN is the effective input voltage of a basic amplifier and IN is the effective input current of a basic amplifier whose voltage gain is designated as A or AV and IN is the output current and V is the output voltage of a basic amplifier and after a sampling network, so output load resistance RL is connected. So, voltage appearing across load register RL, so that is the amplified output voltage of this overall amplifier V out and current flowing through this load RL. So, that is the output current of amplifier that is equal to a load current. So, output of basic amplifier is fed back to the input side of basic amplifier using a feedback circuit or feedback network. So, VF stands for the output voltage of feedback circuit and IF stands for the output current of a feedback circuit. The input for feedback circuit is the output voltage of a basic amplifier that is V out and the forward transfer ratio or gain of this feedback circuit also called as a feedback factor. So, that is designated as a beta. So, beta stands for the gain of a feedback circuit. So, this figure shows a basic feedback amplifier block diagram. Now, let us discuss each block of this block diagram one by one starting from input signal source. Input signal source which provides either a voltage V s or current I s. The signal source is either a voltage source V s or in that is in series with its internal resistance R s or a current source whose output current is I s that is in parallel with its internal resistance R s and second component element of this feedback amplifier that is a comparator or a mixer network. The two networks use for mixing of the input signal from input signal source and the feedback signal coming from the feedback circuit. So, that is shown in figure 2. The circuit shown in figure 2 a is a series connection and it is used to mix the signal voltage V s and the feedback signal voltage V f from feedback circuit. So, resultant of V s input signal voltage V s and feedback voltage V f is V i or V in that is applied to a basic amplifier. Shand connection another possibility for mixer network or comparator. So, that uses a shand connection or parallel connection. So, that is shown in figure 2 b in which the source current I s and feedback current I f from feedback circuit are mixed and the resultant of this I s and I f that is a effective input current I i is applied to basic amplifier. So, this figure 2 shows series connection at the input side of basic amplifier in which the output voltage of feedback circuit that is V f. So, it is combined or mixed with the voltage from external signal source V s it is applied in series. So, V s or a effective input voltage of basic amplifier and the voltage signal from feedback circuit are coming in series. So, there is a series connection at the input side of basic amplifier whenever a voltage signal that is proportional to output quantity is applied in series with the input signal of amplifier. Now, the second figure shows whenever the output of feedback circuit is a current signal I f. So, that is proportional to the quantity sampled at the output of amplifier. So, it is applied in parallel or instant with the input current from input signal source I s and the effective input current for amplifier I i is a resultant of I f and I s. So, basic amplifier next element of this feedback amplifier is a basic amplifier. The basic amplifier is the most important part of a feedback amplifier. The circuit amplifies the effective input signal either a voltage signal or current signal. So, that results from a comparison or mixing in the mixer network to control the output signal. So, basic amplifier amplifies the effective input signal and provides output voltage or a current signal. So, that is controlled by means of the effective input signal. Now, next element of feedback amplifier that is a sampling network. The sampling network are of two types similar to mixer network namely voltage sampling and current sampling. So, depending upon the type of electrical quantity taken from output of basic amplifier. So, there are two types of sampling networks. One is a voltage sampling another is a current sampling. So, as shown in figure 3 a the output voltage is sampled and part of output voltage is taken and the output port of feedback network sampled and the output port of the feedback network. So, in figure 3 b the output current is sampled. The part of output current of the output basic amplifier is sampled and the output port of a feedback network is connected in series with load. So, whereas in case of voltage sampling. So, output voltage is sampled and output input port of feedback circuit it is coming in parallel with load resistor RL. For current sampling the output load resistance is coming in series with the input port of a feedback circuit. So, this is called as a current sampling. Now, this figure 3 a and b shows the voltage sampling and current sampling at the output of basic amplifier. So, for voltage sampling the output voltage is taken from the basic amplifier output and it is given to the feedback circuit. So, there is a parallel connection or standard connection. So, the input side of feedback network and load are coming in parallel as per as whenever current is sampled from the output of basic amplifier the load RL and the input side of feedback network are coming in a series. Now, next element of feedback amplifier that is the feedback circuit. The feedback network or feedback circuit is generally a passive 2 port network and consist of resistor, inductor and capacitors. Mostly in amplifier the feedback network or feedback circuit consist of resistors. Its function is to transfer a signal that is proportional to part of output electrical quantity either a voltage or current signal back to the input side of amplifier. Now, let us go for the expression for a gain of feedback amplifier. So, considering the voltage amplifier let the voltage gain of basic amplifier is a b and overall voltage gain of feedback amplifier is a designated as a b f and beta stands for feedback factor or gain of feedback circuit and v f is output voltage of feedback circuit and v s is a external input voltage. So, v out is a final output voltage of overall amplifier. So, from block diagram the effective input of amplifier gain. So, that is equal to v s plus v f. For v f we can write v f equal to beta that is beta is nothing but a feedback factor not a current gain of transistor in common emitter circuit configuration. So, v f equal to beta into v out and so v n equal to v s plus beta into v out and v out equal to a v into v n. So, after simplification v out equal to a v voltage gain of basic amplifier multiplied by external input voltage v s plus beta multiplied by a v into v out. So, after simplification so we are getting the expression for a voltage gain for overall feedback amplifier. So, that is a ratio of final output voltage to external applied input voltage v s. So, that is equal to a v upon 1 minus beta a v. So, voltage gain overall voltage gain of feedback amplifier a designated as a v f. So, that is equal to a v that is a voltage gain of basic amplifier divided by 1 minus beta a v. So, basically there are two types of feedbacks positive feedback and negative feedback. So, in case of positive feedback both effective input voltage of amplifier and feedback voltage are in phase and feedback factor is positive. So, gain with feedback for a positive feedback amplifier a v f equal to a v upon 1 minus beta into a v. Now for a positive feedback voltage gain with feedback is greater than a voltage gain without feedback. So, positive or regenerative feedback in which the overall gain of amplifier is increased a positive feedback it is always used in oscillators. The second type of feedback is a negative feedback. In negative feedback the input voltage signal and the feedback voltage signal are out of phase by 180 degree. They are of opposite polarity. So, feedback factor is negative. So, gain of overall amplifier with feedback a v f is equal to a v upon 1 plus beta a v. So, in this case the overall voltage gain of this negative feedback amplifier is less than voltage gain of basic amplifier. So, that is without feedback. So, negative feedback is used in amplifier to make operating point of a transistor insensitive to both manufacturing variations as well as surrounding temperature. Now, student can pause video here and think over this question and try to answer this question. Which type of feedback is used in oscillator circuit and why? So, the type of feedback used in oscillator circuit is a positive feedback because positive feedback increases the effective input signal of a amplifier. So, this positive feedback is useful in increasing the strength of initial input signal of the amplifier. So, that is required in oscillator circuits. So, this is reference. Thank you.