Questions tagged [loop-gain]
The loop-gain tag has no usage guidance.
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Converter stability - bode plot
I referred this YouTube video for buck converter loop stability.
In the image we can see that the gain of the control loop we are talking about is Vout / Vin. Vout and Vin are both measured across ...
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1answer
52 views
Buck Converter Feedback Loop - Stability Criteria
Question 1 :
I am having some trouble understanding the Stability criteria of the Buck Converter Loop Stability which says, "The cross over frequency should be less than 1/8th or 1/10th of the ...
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1answer
75 views
Is it possible to find the voltage gain of the constant gain block of this control system?
I'm looking at the following circuit and I don't know how to find the gain of the constant gain block. The only information I'm given are the Bode plots (magnitude and phase) of the two transfer ...
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2answers
81 views
Differentiator cut-off frequency
A practical differentiator has two cut-off frequency but can no longer perform differentiaiton after first cut-off. After first cut-off there is a 20db per decade gain reduction but why does this ...
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1answer
84 views
Open-loop frequency response for an op-amp
I want to know what is the ideal circuit to measure open-loop frequency response and why?I have seen the typical open-loop frequency response curve for an op-amp a lot of times. But when I try to plot ...
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134 views
Gain error calculation for op-amp circuit in inverting configuration
For an inverting op-amp, calculate minimum gain that is required for the op-amp to achieve a gain error of 1%, if the required closed-loop DC gain is -9.
If the input resistance R1 is 1Kohm,find the ...
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3answers
77 views
What is the definition of “the loading of feedback network on the input/output side”?
There is the following example:
simulate this circuit – Schematic created using CircuitLab
Its loading of the feedback network on the input side is 40kohm in series. Its loading of feedback ...
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2answers
71 views
SMPS stability near and above switching frequency
I am simulating in LTSpice a switched-mode power supply (LT3845) at a frequency of 150kHz. I am measuring the loop gain at the feedback node according to this article.
The way I understand it, the ...
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90 views
Middlebrook's Method VS Tian's Method VS Middlebrook's General Feedback Theorem
For https://sites.google.com/site/frankwiedmann/loopgain , how is Tian's Method different compared to Middlebrook's Method and Middlebrook's General Feedback Theorem ?
Tian's Method
Middlebrook's ...
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1answer
238 views
Dominant Pole Approximation for loop gain
I have the following loop transfer function for a unity feedback system:
I understand that the \$(s+2)\$ term becomes \$2\$, but what happens to the \$s\$ ?
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2answers
72 views
4ma-20ma loop from 0.97V to 2.75V
I'm trying to develop a 4ma-20ma loop from an 0.97(4ma)-2.75(20ma) input.
This input comes from the output of a low pass filter of a PWM module.
I've searched the web quite thoroughly but all the ...
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0answers
59 views
The Effect of Input and Feedback Capacitors on an Operational Amplifier Open-Loop Gain Response
I have the following circuit that I want to compensate:
As shown from the step response, the circuit is unstable because of the high input capacitance (the green plot is without any additional input ...
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1answer
143 views
Improve the performance of Buck Converter Loop Stability
The following are the specifications of my Buck Converter.
Input Voltage Range : 6V to 16V
Output Voltage : 5V
Switching Freq : 2.1MHz
Output Inductor : 22uH
Minimum Load Current : 200mA
Typical ...
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1answer
209 views
Why 180 degrees of phase margin should be considered?
We've had one feedback on the phase difference, with the gain as 1. At 300 to 60 degrees, there's just a phase difference, and 61 to 299 degrees, the output is larger. Shouldn't we consider an angle ...
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1answer
173 views
How to measure amplifier circuit loop gain
In the last few days I've tried to understand how to get the loop gain (Aol*Ć) of my amplifier circuit.
I've received several suggestion.
I would like to hear your opinion on this matter.
Are these 2 ...
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1answer
106 views
Negative loop gain?
I was dimensioning the CE capacitor. To do that I was looking at some formulas seen in class. The formula (freq_C=1/(2*piReqC)) involves the knowledge of the loop gain (open loop gain*beta). It ...
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1answer
73 views
Calculating the loop gain for an ideal op-amp with time delay
In the circuit below, \$\tau \$ represents a time delay. We are using a first-order Pade approximation for this time delay. This means that \$\ e^{-s\tau} = (1-s\tau)/(1+s\tau)\$.
Below is the ...
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1answer
72 views
MATLAB model of an op-amp's Open Loop Gain
I'm using MATLAB to analyze an amplifier in a transimpedance circuit, and I'm a little lost in implementing the op amp's open-loop gain function with the following formula:
$$A(s) = A_{OL} \cdot \...
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1answer
180 views
Op-amp inductive output impedance oscillation
In Learning the Art of Electronics pg.362, the author presents an op-amp follower driving a long cable. I don't understand the explanation given for the cause of oscillation. In particular, the author ...
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2answers
91 views
Drawing the Bode Plot of a loop gain
Below I have the loop gain and the Bode plot for it, however I do not understand the phase plot. Why does it start at -90 degrees instead of 90 degrees since we have a zero? Please explain.
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70 views
Estimate the phase margin of a loop gain
The loop gain is \$10^6 /[(s+1)(1+s10^5)]\$. Below I have the Bode plot. How can I derive the unity gain frequency from the Bode Plot? How can I find the phase margin from the phase plot of the Bode?
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1answer
80 views
How to find voltage gain of below analog circuit without using Nodal analysis?
This is a theoretical circuit just made to see that if I can find voltage gain of circuit and it took me much calculations then anticipated to get voltage gain.I just want to find if there is better ...
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4answers
280 views
Negative feedback and op-amp poles
I'm trying to get a better understanding of frequency compensation for op-amps. I'll start off with a very specific questions and then get into the context. For a certain op-amp, open loop gain is ...
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2answers
334 views
What's the largest practical stable gain possible?
I have a feedback application to set the voltage of a node in which there is considerable loss in the loop. Simulations tell me that a differential voltage gain of 10000, 80dB (roughly about what you ...
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884 views
Stability at loop gain <1 and 180 degree phase shift
I'm reading this article about stability of OpAmps: Link to article.
It says: "If AĪ² is less than unity at the high frequencies where phase shift reaches 180Ā°, the high-frequency phase-shifted ...
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2answers
521 views
Amount of feedback in an amplifier
Sometimes I read things like "This amplifier has 30dB of feedback", what figure is that refering to? the \$\beta\$ of the amplifier? or the loop gain \$1+A\beta \$? or the negative feedback factor \$\...
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1answer
130 views
LT spice not predicting instability
I'm trying to make a linear regulator using an op amp and a pass transistor.
Here is the circuit:
From my limited understanding you can predict the stability of the circuit's feedback loop by ...
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1answer
282 views
General Feedback Theorem - Spice model and loop gain
I am implementing the General Feedback Theorem (Prof. Middlebrook, 2006) in LTSpice, using the example given in the Spice folder.
I used it to calculate the loop gain in a simple voltage follower ...
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1answer
119 views
Find optimal gains transfer function
I have the following closed loop transfer function:
$$G(s) = \frac{K(s^2 + 200^2)}{0.05s^3 + (1+K)s^2 + 200{K_v}s + 200K}$$
Where \$K\$ and \$K_v\$ are gains.
My goal is to find the optimal gains ...
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0answers
453 views
Why gain crossover frequency must be lower than phase crossover frequency for stable systems?
As far as I understood from here, phase margin means the amount of phase that needs to be added in order to coincide with 0 db gain and -180 degrees phase. Likewise for the gain margin it means means ...
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0answers
71 views
Why we can consider inductor as a current source when the current loop gain infinite?
I am reading about current-mode control from this site.
Please look at the excerpt below and especially the boldfaced part.
Question:
Why we can consider inductor as a current source when the ...
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2answers
181 views
Relation between crossover freqquency of loop gain and closed loop's corner frequency when feedback is not unity
In this Control Loop Cookbook from TI, in the red box the author said that the crossover frequency (indicated in figure 2) of the open-loop is same as 3dB down frequency or corner frequency of the ...
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4answers
534 views
Differential Amplifier's Unusual Behavior
This circuit is made of one differential BJT transistor pair and voltage-amplification stage with negative feedback.
simulate this circuit – Schematic created using CircuitLab
About this ...
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2answers
350 views
Benefits of higher order low pass in a negative feedback loop
Let \$G(s) = \frac{Y(s)}{R(s)}\$ be the loop gain of a negative feedback loop system.
For example a system like this, where Y(s) and R(s) are the laplace
transformed functions y(t) and r(t):
...
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1answer
83 views
Negative Feedback Portion of Amplifier
Here, in this question, I am referring to 3 stage Lin topology amplifier (differential stage - voltage amplification stage - output power stage + negative feedback).
I am wondering about portion of ...
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2answers
370 views
Loop bandwith and open-, closed- loop gain in ADIsimPLL
I'm want to use ADIsimPll to calculate the loop filter properties for a PLL I want to build. I read some things in the programs help topics which I find somehow strange. Maybe you guys can help me out ...
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1answer
75 views
Any transfer function has the same denominator 1+T
Sometime ago, I came across some theory saying that in a control loop any transfer functions such as ...
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5answers
668 views
Op Amp - Specifications are given for unity gain, how to choose another gain?
TL081 datasheet gives the following specifications:
How can I find the open loop amplification?
I know that the relationship between the differential voltage amplification and the (closed or opened?) ...
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1answer
593 views
Opamp Unity gain follower stability [closed]
Please give explanation considering opamp as ideal one as well as practical one.
Edit:
I got the explanation here. Decompensated Operational Amplifiers
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1answer
76 views
Open loop gain of op amp
I am not sure I am analyzing this correctly, but here's an attempt:
\$20lgA_o=92.0412dB\$, yielding \$A_o=40K\$
And
\$20lgA_o-20lg(1+100K/\omega_b)=40\$, yielding \$f_b=40.08Hz\$, hence \$f_t=f_bA_o=...
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2answers
4k views
calculate feedback factor in feedback circuit
Below is an exercise with solution about feedback network. I am wondering how the feedback factor is calculated like that. Could anyone shed some light on this?
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2answers
155 views
How do I calculate closed loop gain's deviation?
I have an exercise in my book that I am trying to solve:
How large must the magnitude of the loop gain, \$ |A \beta| \$ be in order
for the closed loop gain, \$ A_t \$, to deviate less than 0.1% ...
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1answer
351 views
Gain-Bandwidth of LTC6409
I am planning to use a differential amplifier, LTC6409 which is reported to have a GBW of 10GHz. I ran an AC analysis on LTSpice using the Spice model given here. I connected the negative input to ...
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3answers
1k views
Series-Shunt Feedback Amplifier Analysis - Finding Loop Gain
So I'm trying to solve a problem I found in a textbook (won't say which one so people can't use it to cheat on their hw) and I haven't been able to arrive at the answer found in the appendix. Please ...
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2answers
888 views
A question about loop gain, feedback and stability of an op amp circuit
I read this article on the feedback and stability concept of an op amp.
The article describes the relationship between open loop gain, closed loop gain, loop gain and some oscillation effects for the ...
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1answer
402 views
How to stabilize this op-amp/MOSFET control system
I am creating a constant-current source and am having troubles stabilizing the control loop around U3, this is the circuit:
This next picture represents the plot of loop-gain of U3:
I've tried ...
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3answers
501 views
Understanding the negative feedback and stablizing of a current limiter
Here's a current limiter circuit that uses the negative feedback to limit the current to a certain point.
simulate this circuit – Schematic created using CircuitLab
I have not studied feedback ...
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2answers
193 views
Open loop gain of non-inverting amplifier
How can I find open loop gain of this non inverting amplifier. I searched for answer but couldn't find the answer. Using formula Gain=1+R2/R1
I am not able to figure out values ofR2 and R1
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2answers
647 views
Positive feedback with constant transfer functions
If I have a control loop with positive feedback, the closed loop transfer function expression is: $$\frac{Y(s)}{X(s)} = \frac{P(s)}{1-P(s)H(s)} $$
where \$P(s)\$ is the direct loop transfer function ...
