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Op-Amp Feedback and Terminal Tracking
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Op-Amp Feedback and Terminal Tracking
Op-Amp Feedback and Terminal Tracking
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1
Question
Under what condition can the terminal voltages in an op-amp circuit with both feedback types be assumed to follow each other?
0:27
Answer
This condition is applicable only if the negative feedback loop gain is greater than the positive feedback loop gain.
2
Question
What determines the error signal in a negatively fed back system relative to the loop gain?
1:35
Answer
The error signal is a fraction of the input signal, specifically represented by \(rac{x}{1 + Aeta}\), where higher loop gain results in lower error.
3
Question
What is the stability status of a negative feedback system containing at most a single pole?
2:19
Answer
Such a system is unconditionally stable, regardless of the magnitude of the loop gain.
4
Question
Why is it advantageous to have a very high loop gain in a negative feedback system?
1:42
Answer
High loop gain reduces the error signal toward zero, causing the inverting and non-inverting terminals to reach near-identical potentials.
5
Question
What is the mathematical expression for the error signal in a positively fed back system?
3:02
Answer
The error signal is expressed as \(rac{x}{1 - Aeta}\), where the negative sign in the denominator represents the positive feedback.
6
Question
What is the primary stability requirement for the loop gain in a positively fed back system?
3:12
Answer
The loop gain \(Aeta\) must be strictly less than one to ensure the system remains stable and the output stays bounded.
7
Question
Why does the condition \(V_+ = V_-\). not hold for stable positive feedback systems?
3:58
Answer
Because stability requires \(Aeta < 1\), the error signal cannot reach zero, preventing the terminals from tracking each other.
8
Question
What does a loop gain of less than one guarantee for a positive feedback system with a bounded input?
5:22
Answer
It only guarantees that the output of the system will also be bounded.
9
Question
What specific feedback factor value is required for the error to go to zero in any feedback system?
2:58
Answer
The loop gain \(Aeta\) must reach infinity for the error signal to be exactly zero.
10
Question
What is the practical difficulty in achieving \(Aeta < 1\) with a single standard op-amp circuit?
4:44
Answer
Since op-amps typically have very high gain (A), the feedback factor \(eta\) must be extremely small (less than 1/A), which is not trivial to implement.
11
Question
In the provided example circuit, what is the gain of the non-inverting amplifier block?
6:20
Answer
The non-inverting amplifier block in the example is designed with a gain of two.
12
Question
How is the error voltage defined at the summing node in the example circuit using two equal resistors?
6:44
Answer
The error voltage is the average of the input signal and the fed-back quantity, expressed as \(rac{V_i - AV_o}{2}\).
13
Question
What is the purpose of placing an op-amp in the feedback path of the hypothetical example circuit?
9:49
Answer
It is used to construct a circuit with a very small gain, specifically equal to the reciprocal of the op-amp gain (1/A).
14
Question
What is the overall loop phase shift in the negative feedback circuit example described?
8:59
Answer
The overall loop undergoes a phase shift of 180 degrees, confirming it is a negative feedback loop.
15
Question
How does the closed-loop gain of the 'dotted box' sub-circuit relate to the op-amp gain A?
11:02
Answer
The gain from input to output of that sub-circuit is approximately \(rac{1}{1 + A}\).
16
Question
What is the estimated loop gain of the global positive feedback circuit when A is very large?
14:06
Answer
The loop gain is approximately one-half, calculated from \(rac{A}{2(1 + A)}\).
17
Question
What is the expected overall closed-loop gain for the positive feedback example provided?
16:40
Answer
The overall gain is approximately negative two, resulting from the combination of input sign and the factor of \(rac{1}{1 - 0.5}\).
18
Question
What happens to the terminal voltages of the op-amps in the global positive feedback example?
17:21
Answer
The inverting and non-inverting terminals do not track each other, and the condition \(V_+ = V_-\). is explicitly violated.
19
Question
How does the error magnitude between terminals in the positive feedback example relate to the gain A?
18:01
Answer
The error is huge and largely independent of the gain A, calculated as approximately \(2V_i\) in the specific case shown.
20
Question
What behavior should be expected if one simulates the positive feedback circuit with high finite gain and small inputs?
18:32
Answer
The circuit should yield a finite, stable output even though the op-amp internal signals are exceedingly large and out of phase.
21
Question
Why is \(Aeta\) automatically considered infinity in standard negative feedback op-amp analysis?
20:30
Answer
This assumption is made because open-loop op-amps typically have extremely high gains, driving the error toward zero in a closed negative loop.
22
Question
In a system with multiple loops, what dictates the overall tracking behavior of the op-amp terminals?
21:17
Answer
The terminals will only track each other if the global or overall feedback is negative.
23
Question
What is the relationship between stability and error signal magnitude in positive feedback?
3:52
Answer
Greater stability in positive feedback is achieved with smaller loop gains, which conversely leads to larger error signals.
24
Question
What is the effect of increasing loop gain A in the provided global positive feedback example?
21:53
Answer
Increasing the gain A will still result in a finite output, and the terminals will still fail to track each other.
25
Question
Define the term 'loop gain' in the context of the analyzed op-amp circuits.
2:06
Answer
Loop gain is the product of the amplifier's open-loop gain (A) and the feedback factor (\(eta\)).
26
Question
How does the stability of a system change if loop gain \(Aeta\) exceeds one in positive feedback?
3:21
Answer
The system becomes unstable, leading to an unbounded output if the poles move to the right-hand plane.
27
Question
In the example block diagram, what signal is appearing at the input of the final adder?
8:06
Answer
The signals appearing are the input signal multiplied by one-half and the fed-back signal \(rac{AV_o}{2}\).
28
Question
What is the mathematical definition of the error voltage at the op-amp input nodes in terms of loop gain?
1:49
Answer
The error is defined as the input signal divided by the quantity (1 + loop gain) for negative feedback.
29
Question
What happens to the virtual short assumption if an op-amp circuit is unstable?
3:32
Answer
The virtual short assumption is completely invalid as the terminal voltages will diverge rather than track each other.
30
Question
In the hypothetical circuit, what is the value of the output voltage \(V_o\). if gain A is assumed to be infinity?
10:27
Answer
The output voltage \(V_o\). will be exactly zero because the node is grounded through a high-gain negative feedback loop.