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Understanding Feedback in Amplifiers
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Welcome class! Today we'll discuss feedback in amplifier circuits. Can anyone tell me what feedback means in this context?
I think it means using some of the output to influence the input?
Exactly! We're going to focus on how it affects voltage gain and current gain. What do you think happens to these gains when feedback is applied?
Maybe they decrease?
Great insight! This phenomenon is known as 'desensitization'. Can anyone remember why we want to use feedback if it decreases gains?
To improve stability and control, right?
Absolutely right! Feedback improves performance stability. In summary, feedback can both decrease gains but enhance overall system stability.
Trans-conductance and Feedback
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Moving on, letβs talk about trans-conductance. Can anyone define it?
Isn't trans-conductance the ratio of current output to voltage input?
Exactly! So, when we apply feedback, how do we think trans-conductance influences the circuit?
It might decrease because of increased output resistance?
Right again! When we have higher resistance, it impacts the current gain. Recall that G, the trans-conductance, is important to understand the feedback effects. Can you all summarize what we've learned about it?
Trans-conductance helps us understand how feedback affects current and overall amplifier performance.
Well said! Now letβs remember that the impact of feedback can be quantitatively described by our equations.
Voltage Gain and Current Gain Changes
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To wrap up our discussion, let's examine the voltage and current gain specifically. How do we calculate voltage gain in a feedback circuit?
I think it involves G and the feedback circuit parametersβ¦
Correct! The voltage gain A = G Γ R, right? And after feedback is applied, the gain decreases by a factor we call desensitization factor. Can anyone state that factor?
Is it D = (1 + G'Ξ²)?
Exactly! And what does a similar analysis show concerning current gain?
It remains unchanged, yes?
Spot on! Remember, while voltage gain changes, current gain maintains under feedback. Letβs summarize the benefits of feedback once more at the end.
Introduction & Overview
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Quick Overview
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In this section, we explore the implications of feedback in amplifier circuits, particularly focusing on voltage gain and current gain. The concepts of input and output resistance are introduced, along with the effects of series-series feedback and desensitization factors.
Detailed
Detailed Summary
In this section of the chapter on Analog Electronic Circuits, we investigate the effects of feedback on voltage gain and current gain in amplifier circuits. Feedback can fundamentally change these gains, often described in terms of what's known as the desensitization factor, which denotes how feedback influences system parameters.
The discussion begins with an overview of the types of feedbackβspecifically series-series feedbackβwhere the input signal is voltage, and the output signal is current. The section delineates how voltage gain and current gain are modified under feedback conditions. The relationship between input and output resistance is also highlighted, demonstrating how the feedback loop can lead to increased resistance values, thereby affecting overall circuit performance.
The use of various examples, practical models, and equations reinforces the theoretical aspects by providing calculations related to trans-conductance, voltage gain, current gain, and trans-impedance. By the end of this section, readers should achieve a comprehensive understanding of how feedback configuration impacts the performance of amplifier circuits.
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Feedback Connection Effects on Gain
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In fact, we are making this G getting reduced by a factor of desensitization. So, this is getting decreased D = (1 + Gβ² Ξ² ) and it is (1 + g R ). And also we know that input resistance getting increased by this factor, output resistance it is also getting increased by the same factor D.
Detailed Explanation
In feedback circuits, when feedback is applied, several parameters change. Here, the transconductance G is reduced due to the desensitization factor, which is calculated using the formula D = (1 + Gβ² Ξ²) or D = (1 + g R). Both the input and output resistances of the circuit also increase by this same factor D, indicating that the overall performance and load characteristics of the circuit are affected by feedback.
Examples & Analogies
Consider a car with a turbocharger. When you apply feedback to the engine's performance, like adjusting the fuel efficiency, the engine becomes more fuel-efficient but may produce less power. Similarly, in feedback circuits, the adjustments lead to changes in overall resistance and gain.
Current Gain Observations
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If I want to see what kind of changes do you expect or do you see for a current gain then, we have to look into the expression of the current gain in terms of G. And this column gives us the corresponding expression. So, A it is G R.
Detailed Explanation
The current gain A in a feedback circuit is determined by multiplying the transconductance G with the resistance R. Notably, interestingly, although the transconductance G decreases as we apply feedback, the overall effect on the current gain remains unchanged due to a corresponding increase in resistance R. Thus, the product of these two effects results in no net change in current gain.
Examples & Analogies
Think of a baking recipe where adjusting the temperature affects the baking time. If you reduce the temperature slightly (akin to reducing G), you might need to increase the baking time (similar to increasing R) to achieve the same baked goods. Eventually, the end result remains as intended, mirroring how the current gain remains constant despite adjustments.
Voltage Gain Changes
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So, likewise, if I consider the if I consider the voltage gain and if I see the expression of the voltage gain from here which is G times R. So, A = G R and here again G it is decreased by desensitization factor on the other hand output resistance got increased by the same factor D.
Detailed Explanation
In a feedback mechanism, the formula for voltage gain A is A = G * R, where G is the transconductance. After feedback is applied, G decreases and the output resistance R increases proportionally by the same factor. As a result, while both terms change, they cancel each other out, leading to no effective change in voltage gain after the feedback application.
Examples & Analogies
Imagine a balanced scale. If you reduce the weight on one side (like reducing G), but then simultaneously add weight to the other side (similar to increasing R), the scale remains balanced. This situation encapsulates the balance of gains in a feedback circuit.
Trans-impedance Adjustments
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On the other hand, if I consider Z and its expression can be obtained from this column; namely, it is G R, trans-impedance = G R R.
Detailed Explanation
Trans-impedance (Z) represents how the output current converts to voltage. It is defined through Z = G * R * R, where G is transconductance. With feedback, while G decreases and the resistances increase, the overall effect shows that the trans-impedance increases due to the changes in R, suggesting a stronger relationship between output voltage and current post-feedback.
Examples & Analogies
Think of a garden hose connected to a water faucet. If the output flow reduces (like a decrease in G), but you add a sturdy nozzle (akin to increased R), the overall water pressure increases. Similarly, feedback enhances the performance of transimpedance in electronic circuits.
Summary of Effects due to Feedback
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So, in summary we can say that Gβ² it is also it can be well approximated by g. Input resistance of the circuit it is of course, r of the transistor and then R of the circuit main amplifier it is r.
Detailed Explanation
In conclusion, under feedback conditions, the approximation of Gβ² with g indicates the reinforced relationship between input and output characteristics, while the input resistance of the circuit ties closely with the transistorβs inherent resistance. This reinforcement outlines the principle of retaining system stability and how feedback influences overall performance.
Examples & Analogies
Consider a thermostat controlling room temperature. Just as the thermostat adjusts heating based on current temperature readings, feedback in electronic circuits stabilizes and optimizes performance by balancing output according to input characteristics.
Definitions & Key Concepts
Learn essential terms and foundational ideas that form the basis of the topic.
Key Concepts
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Voltage Gain: The output to input voltage ratio in amplified signals.
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Current Gain: The ratio of output current to input current that remains mostly unchanged under feedback.
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Trans-conductance: The current driven per unit input voltage change in an amplifier.
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Desensitization: A negative impact on gains due to feedback designed for stability improvements.
Examples & Real-Life Applications
See how the concepts apply in real-world scenarios to understand their practical implications.
Examples
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In a common-emitter amplifier, the feedback reduces voltage gain but stabilizes the circuit.
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A practical example shows how current gain remains unaltered while feedback modifies voltage and resistance values.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
π΅ Rhymes Time
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Feedback helps keep things in line, gains may drop, but stability will shine.
π Fascinating Stories
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Imagine a librarian who must keep the library quiet (stability) while whispering (feedback) to book readers. Some noise (gain) may reduce, but the library remains peaceful.
π§ Other Memory Gems
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To remember the key feedback effects, think 'Gains Down, Resistance Up, Stability Found'.
π― Super Acronyms
Remember 'F.I.G.S.' - Feedback Increases Gain Stability.
Flash Cards
Review key concepts with flashcards.
Glossary of Terms
Review the Definitions for terms.
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Term: Voltage Gain
Definition:
The ratio of output voltage to input voltage in a circuit, often influenced by feedback.
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Term: Current Gain
Definition:
The ratio of output current to input current in a circuit, remains constant under feedback in certain configurations.
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Term: Transconductance
Definition:
The measure of output current change per change in input voltage, important for characterizing amplifiers.
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Term: Desensitization Factor (D)
Definition:
A factor that represents how feedback can decrease voltage gain while enhancing system stability.
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Term: Feedback
Definition:
The process of feeding back a portion of the output signal to modify the input, affecting overall circuit performance.