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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 + βA), right? What do the variables R, β, and A represent?
R is the input resistance without feedback, β 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.
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.
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 = ∞ for the load!
Correct! When we calculate the input resistance for current amplifiers, it takes the form R_in = R(1 + βA) where A is the current gain. Always keep this in mind while analyzing feedback systems.
The multiplier effect that feedback has on an amplifier's resistance values.
An amplifier configuration where the input signal is voltage, and the output is also a voltage.
An amplifier configuration where both input and output signals are currents.
The resistance connected to the output of an amplifier that affects its performance.
Circuits that use a portion of the output to control the input, enhancing stability.
In an ideal voltage amplifier with feedback, the input resistance is profoundly increased due to feedback, illustrating how feedback stabilizes circuit performance.
In a current amplifier scenario, keeping the input resistance at zero ensures maximum current flow into the circuit without loading, crucial for accurate feedback.
When feedback is near, input resistance grows, in amps it shows.
Imagine a water system: as more pipes (feedback) are added, the overall pressure (resistance) increases at the tap (input).
F.A.L.L. - Feedback Affects Load Resistance and Input.
R.A.F. - Resistance Amplified by Feedback.
Term
What does A' represent?
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Term
How does finite load resistance affect input resistance?
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The resistance encountered by an input signal at the amplifier's input port.
The resistance seen by the load connected to the output of an amplifier.
The fraction of the output voltage or current fed back to the input.
The product of the gain of the amplifier and the feedback ratio, defining the overall gain of a feedback system.
The modified gain of the amplifier accounting for the presence of load resistance.
This section discusses the impact of finite resistance on input and output resistance in negative feedback systems.
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.
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.