93.1.1. Introduction to Feedback System

So, yeah dear students, so welcome back after the short break and before the break we are talking about the change of input resistance of the different configuration. And whereas, we are going to talk about change in output resistance to start with let we consider it is a voltage amplifier and we want to see the change due to feedback connection.

In this introductory section, the lecturer welcomes students back and references previous discussions about changes in the input resistance of different configurations in electronic circuits. The focus now shifts to understanding how feedback connections affect output resistance in voltage amplifiers. Essentially, feedback is a process where a part of the output is fed back to the input to control the operation of the circuit, impacting its performance.

Think of feedback in a classroom setting. If a teacher asks students how well they understand the material, the students' responses (feedback) can help the teacher adjust their teaching methods to improve learning outcomes. Similarly, in electronic circuits, feedback helps improve performance by adjusting how the input responds to the output.

And since it is voltage amplifier as we have discussed, the circuit is given here. The configuration here it is referred as shunt series feedback or voltage series Feedback circuit.

Voltage amplifiers are circuits designed to increase the voltage of a signal. In this specific case, the configuration being discussed is known as shunt series feedback. This means that there is a feedback connection that samples the output voltage and feeds it back into the input in a way that can help stabilize or modify the gain of the amplifier. Understanding this configuration is essential for analyzing how the amplifier behaves with and without feedback.

Consider a thermostat in a heating system. It measures the current temperature (output) and adjusts the heating (input) accordingly to maintain the desired temperature setting. Likewise, a voltage amplifier uses feedback to manage its output more effectively.

To start with let we consider the feedback network it is ideal namely its input resistance, it is infinite and the output resistance as it is producing voltage, output resistance it is 0.

In an ideal feedback network, certain assumptions are made for simplicity. The input resistance being infinite means that the feedback network does not load down the previous stages of the circuit; it draws no current itself. An output resistance of zero indicates that the feedback source can adjust the output voltage without affecting the operation of the amplifier circuit. These ideal conditions make mathematical analysis easier and provide a baseline for understanding real-world applications.

Imagine a perfect sponge that can absorb an infinite amount of water (infinite input resistance) without getting full and releasing no water back (zero output resistance). This hypothetical sponge allows us to understand how feedback works without complicating factors.

However, for the forward amplifier we are considering finite input resistance and finite output resistance. We do have the voltage dependent voltage source A v , v it is appearing at the input of v in in the forward amplifier.

While ideal conditions help in theoretical understanding, actual amplifiers have finite input and output resistances. This means they do draw current and can affect circuit performance. The text discusses a situation where a voltage-dependent source is applied, and 'v' represents this voltage, which is relevant for stating the relationship between input and output when feedback is applied.

Consider a public fountain that can provide a certain amount of water (voltage) depending on how people use it (finite input and output resistance). If too many people try to fill their buckets at once, the fountain’s supply might not meet everyone’s expectations, just like a finite amplifier can't always respond perfectly to demands.

So, if you see if I consider this is short, then if I consider KCL in this mixer what we are getting it is v , input voltage of the amplifier it is ‒ v.

Using Kirchhoff's Current Law (KCL), the lecturer demonstrates how to calculate the input relationships in the circuit. By setting up the mixer's input, the effective input voltage is understood as the negative of the feedback voltage. This negative sign indicates that the feedback influences the input in the opposite direction, which is vital for designing stable feedback systems.

Imagine a seesaw where one side pushes down (input voltage) while the other side pushes up (feedback). The interplay between the two helps maintain balance, akin to how feedback influences amplifier input in a circuit.

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