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Interactive Audio Lesson
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Impact of Feedback on Input Resistance
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Today, let's discuss how a negative feedback system influences the input resistance of amplifiers. Can anyone tell me what we mean by the input resistance of a feedback amplifier?
Isn't it the resistance seen by the input signal at the amplifier's input port?
Exactly! The input resistance can be significantly affected by feedback mechanisms. Initially, in an ideal scenario, we set our feedback's input resistance to infinity. Why do you think we do that?
So that the feedback doesn't affect the input signal or load the source?
Right! And when we have finite resistances, we will see a change in this input resistance. The modified input resistance, denoted as R_in_f, can be calculated as R_in (1 + Ξ²A), right? What do the variables R, Ξ², and A represent?
R is the input resistance without feedback, Ξ² is the feedback ratio, and A is the loop gain!
Perfect! Now, let's summarize: the introduction of feedback can effectively increase our input resistance, which is critical in designing stable amplifiers.
Load Effects on Amplifiers
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Now let's discuss practical scenarios involving finite load resistance. What do we mean when we say load resistance?
It's the resistance connected to the output of the amplifier that affects its performance?
Correct! In our feedback systems, if we denote the load resistance as R_L, how do you think it affects the overall output?
It must change the voltage available at the output because some current would go through the load.
Exactly! Since R_L is finite, we have to adjust our expressions for gain and resistance. In practical cases, the gain becomes load-affected, noted as A'. Can anyone relate this back to how we calculate the input resistance?
We replace A with A' in our input resistance formula, right?
Spot on! The new expression for input resistance in practical amplifiers will reflect these changes. Always remember, understanding load effects is crucial.
Current Amplifiers and Feedback
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Let's now shift gears and discuss current amplifiers. How do you think feedback works differently in current amplifiers compared to voltage amplifiers?
Well, since the input and output signals are both current, and we want to avoid loading, we want the input resistance to be low.
That's correct! In current amplifiers with feedback, we aim to keep the input resistance ideally at zero. How does this influence our calculations for input resistance?
We ensure that the feedback current is amplified correctly without any loading effect, making R = β for the load!
Correct! When we calculate the input resistance for current amplifiers, it takes the form R_in = R(1 + Ξ²A) where A is the current gain. Always keep this in mind while analyzing feedback systems.
Introduction & Overview
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Quick Overview
Standard
The section explores the effects of feedback in practical scenarios where resistances are non-ideal, detailing how input and output resistances change due to feedback configurations in voltage and current amplifiers.
Detailed
Practical Cases with Finite Resistance
In this section, we delve into the influence of finite resistances on input and output resistances within feedback circuits of analog electronic systems. Initially, we consider an ideal scenario where the input resistance is infinite and the output resistance is zero. This allows us to examine properties like input resistance derived from voltage amplifiers subjected to feedback.
As we transition to practical cases, we investigate scenarios where load resistances are non-zero, which alter the voltage and current relationships within the circuit. The concept of load-affected gains emerges as a critical point, underscoring how these changes lead to variations in the feedback loop's overall gain, potentially impacting performance.
The section presents various configurations, including voltage amplifiers under shunt and series feedback, as well as current amplifiers, thus providing a comprehensive overview of their respective input and output resistances, adjusting for real-world applications. The importance of the desensitization factor arises repeatedly, illustrating how feedback can effectively alter these resistances, improving system stability and performance. Understanding these concepts is crucial for designing effective feedback systems in analog electronics.
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Introduction to Practical Cases with Finite Resistance
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Now, let we consider that in practical situation where definitely there may be a finite load R and due to which the voltage available here at this port it may not be same as internally developed voltage and in that situation what maybe the corresponding change. So, to start with let we consider R it is finite.
Detailed Explanation
In this section, the speaker introduces the concept that in real-world applications, loads (represented as resistance R) would not be infinite or zero as in ideal scenarios. This affects the voltage available at the amplifier's output. It reflects the need to consider actual component resistances in analyzing circuit performance.
Examples & Analogies
Imagine a water pipe system. In an ideal situation, if there are no blocks, water can flow freely through the pipes. However, if you were to place a blockage (like a finite resistance), the water pressure (analogous to voltage) drops at different points in the system because it's being partially used to push against the blockage.
Load Affected Gain and Its Calculation
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If we put say R here and then the voltage here v. In fact, this is same as v also. So, v = A v Γ Aβ² v where Aβ² is the load affected gain of the amplifier.
Detailed Explanation
The text explains that adding a load resistance (R) at the output of an amplifier affects the voltage gain. The actual gain (A') perceived is lower than the ideal due to the load, which is a common situation in circuit design. The speaker proposes calculating this 'load affected gain', denoting it as Aβ².
Examples & Analogies
Think of a speaker connected to a phone. The phoneβs output can drive the speaker effectively, but if you connect multiple speakers (loads), the sound output reduces (analogous to the gain dropping) because the phone has to share its power across multiple devices.
Feedback System Input Resistance with Finite Load
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So using this relationship and this relationship we can find that the corresponding input resistance of this circuit we call the say R in_f. So, it is remaining very similar to this equation except this A need to be changed by Aβ².
Detailed Explanation
This chunk elaborates on how to calculate the input resistance of the feedback system when a finite load is considered. It states that while the formula used for calculating input resistance remains similar, the voltage gain A must be adjusted to account for loading effects by substituting it with Aβ² (load affected gain).
Examples & Analogies
Imagine adjusting the flow of students through a crowded entrance. Initially, every child can enter freely (ideal situation). But as more children pile up outside (finite load), the flow needs to be managed differently, so we have to adapt our approach to ensure everyone gets through efficiently (similar to changing A to Aβ²).
Impact of Various Resistors on Feedback System
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So, let us say that this is having some finite value call R in_Ξ² and the corresponding voltage here getting developed across this R and R they are coming in parallel.
Detailed Explanation
This segment addresses how different resistances can modify the voltage. It highlights that if there are parallel resistors in the feedback network, the overall voltage and thereby the gain would change, which must be taken into account when designing and analyzing circuits.
Examples & Analogies
Think of two water tanks (resistors) being connected. The water level (voltage) in both tanks can affect each other due to the parallel connection. If one tank fills faster, it might influence the level in the other tank, similar to how resistor values affect voltage distribution across parallel resistors.
Real-World Implications of Feedback Systems
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So, here we do have a trans conductance amplifier and what do we have what we have here it is input it is of course, in the form of voltage and since it is trans conductance here the signal output signal it is current.
Detailed Explanation
The section explains the application of feedback systems in a transconductance amplifier scenario. It highlights that the input is voltage-based, while the output is current-based, which is essential in understanding how various feedback configurations are used in real electronic circuits.
Examples & Analogies
Imagine a smart thermostat that senses temperature (input as a voltage) and adjusts the heating system (output as current flow) accordingly. The sensor's reading impacts how much the heater will run, showcasing the importance of feedback in controlling systems.
Definitions & Key Concepts
Learn essential terms and foundational ideas that form the basis of the topic.
Key Concepts
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Desensitization Factor: The multiplier effect that feedback has on an amplifier's resistance values.
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Voltage Amplifier: An amplifier configuration where the input signal is voltage, and the output is also a voltage.
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Current Amplifier: An amplifier configuration where both input and output signals are currents.
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Load Resistance: The resistance connected to the output of an amplifier that affects its performance.
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Feedback Systems: Circuits that use a portion of the output to control the input, enhancing stability.
Examples & Real-Life Applications
See how the concepts apply in real-world scenarios to understand their practical implications.
Examples
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In an ideal voltage amplifier with feedback, the input resistance is profoundly increased due to feedback, illustrating how feedback stabilizes circuit performance.
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In a current amplifier scenario, keeping the input resistance at zero ensures maximum current flow into the circuit without loading, crucial for accurate feedback.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
π΅ Rhymes Time
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When feedback is near, input resistance grows, in amps it shows.
π Fascinating Stories
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Imagine a water system: as more pipes (feedback) are added, the overall pressure (resistance) increases at the tap (input).
π§ Other Memory Gems
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F.A.L.L. - Feedback Affects Load Resistance and Input.
π― Super Acronyms
R.A.F. - Resistance Amplified by Feedback.
Flash Cards
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Glossary of Terms
Review the Definitions for terms.
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Term: Input Resistance
Definition:
The resistance encountered by an input signal at the amplifier's input port.
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Term: Output Resistance
Definition:
The resistance seen by the load connected to the output of an amplifier.
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Term: Feedback Ratio (Ξ²)
Definition:
The fraction of the output voltage or current fed back to the input.
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Term: Loop Gain (A)
Definition:
The product of the gain of the amplifier and the feedback ratio, defining the overall gain of a feedback system.
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Term: Loadaffected Gain (A')
Definition:
The modified gain of the amplifier accounting for the presence of load resistance.