Questions tagged [phase-margin]

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Opamp circuit stability (LTSpice)

I want to study the stability of my opamp circuit with LTSpice, I found this video as tutorial: tutorial on phase margin So, I did the simulation with closed and open loop as suggested and got the ...
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39 views

What is this functions Phase crossover frequency?

OLTF loop gain My approach What I have done wrong?
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0answers
32 views

How to find Phase crossover frequency?

I make the imaginary part of \$loop \ gain=0\$ to find phase crossover frequency. But in one video, the person told that this method can only be applied when Polar plot crosses the real axis and ...
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35 views

Gain margin using Root locus plot

How can I find Gain margin of any control system given its polar plot In schaum's series I have read that Gain Margin=Ku/Ki but in given figure the system never becomes unstable so shouldn't Gain ...
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1answer
87 views

Can a phase margin become bigger than 180° and is this bad?

I have a wrong mental model about phase margin in control theory. I see that a phase of -180° is bad. If my input is a sine wave and I have negative feedback, which has a -180° phase shift, the ...
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1answer
128 views

How can I export the frequency response from LTSPICE without phase wrapping?

After I plotted the frequency response of a circuit using LTSpice as shown: I exported the plots data as a .txt file in order to plot them after that using MATLAB, however, after I plotted the data ...
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62 views

Estimate Phase Margin in Spice

I am trying to determine the phase margin of my design and I have come across two methods for modeling the open loop gain in spice. The problem is they are yielding slightly different results. I have ...
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36 views

Voltage regulator feedback circuit and stability analysis?

I have been analyzing following buck-regulator circuit to calculate the phase-margin and gain-margin. Circuit Diagram: Buck regulator with input voltage = 12 V and output voltage = 3.3 V. Just as ...
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3answers
209 views

Effect of RHP zeros on stability

Why do zeros in the right half of the \$s\$ plane reduce stability (i.e., gain / phase margin)?
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1answer
102 views

According to the Nyquist criterion this system is stable but when I apply an impulse I get an unstable response, why?

I have this open loop transfer function and I have to study the stability of the closed loop transfer function using the Nyquist criterion. What I got is that the characteristic equation 1+GH(s)=0 has ...
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1answer
241 views

Example of stable non minimum phase system with negative gain or phase margin

I read in a particular book that the fact that gain margin and phase margin are positive implies stability is only true for minimum phase systems. But, for non-minimum phase systems, system can be ...
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1answer
81 views

Determining Phase Boost needed to Compensate SMPS

I'm attempting to find the Control-To-Output Bode Plot (open-loop) of a SMPS designed around the TPS54160 so that I can design the type II compensator. The first step in designing a compensator is ...
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1answer
162 views

Loop Gain and Phase Margin Correlation

I have found a definition of phase margin of amplifier system from Texas Instruments application report. This definition looks like this: $$\phi = tan^{-1} (A \beta)$$ where \$ A\$ is amplifiers open-...
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2answers
108 views

Can a system with more than 180 phase lag ever be stable?

For a system to be stable you must not reach a phase of 180 degrees before gain reaches 1 (for an ideal system). The layman explanation I was offered was that otherwise, your negative feedback becomes ...
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2answers
829 views

How to determine the phase margin and transfer function of this bode plot?

I have some struggles to find the transfer function of this plot. We got the hint, that the "breaks" of slope can be assumed to be on vertical and horizontal lines. Also we should pay attention on the ...
1
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1answer
64 views

Phase/Gain margin measurements for current mode boost regulator

I'm looking for where to place a sense resistor in the feedback path of a current mode boost regulator with a layout similar to the one in the picture. Should I place in on the FBL or FBH trace?
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66 views

Determining the phase margin of a negative series-shunt feedback circuit

I have been given a transfer function: \$H(s)={A_0\over (1+s/w_{p_1})(1+s/w_{p_2})} \$ With the condition that \$w_{p_1} << w_{p_2}\$ It's designed for this feedback circuit ...
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3answers
1k views

Controller to increase phase or gain margin

I can't find any good sources on this one. The only textbook I have says nothing about it. I've been restricted to the use of some formulas given by our teacher which I don't understand why they ...
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4answers
274 views

Capacitance on the output of an op amp

I have read a lot about capacitive loads on the output of an op-amp and the possibility of unwanted oscillations and instability. my knowledge on phase margin and frequency domain analysis is a little ...
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3answers
643 views

Phase margin measured at lowest positive-gain point, rather than 0dB point?

When discussing voltage regulators, one often hears that phase margin should be measured at the 0dB gain crossing. This makes intuitive sense in a simple system where the phase has only a negative ...
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1answer
292 views

Where break loop in Sallen-Key active filter to measure phase margin

I have designed the following Sallen-Key high pass filter (see image below). I would like to verify its stability using the phase margin of the open loop transfer function. Inserting a voltage source ...
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4answers
630 views

Intuitively, why does gain margin and phase margin infer instability in feedback system?

On MATLAB https://www.mathworks.com/help/control/ref/margin.html, gain margin and phase margin are additional gain in the system and additional delay in the phase of the system such that the system ...
5
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2answers
218 views

I know it has small phase margin.. but why is it oscillating not at rails?

I came across an issue at my workplace with the following circuit. I did a OL response and saw that the gain margin is 21 db and the phase margin is 14 deg. Clearly not so good. Note that the 680nF ...
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1answer
206 views

Harmonic distortion in sub threshold 2 stage opamp

I have designed an 2 stage opamp with all MOSFETs in subthreshold region as shown in the figure below, I have gain of 48 dB, GBW of 60KHz, Slew rate of 0.014 V/us and Power dissipation of just 0.6 ...
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2answers
7k views

Phase Margin and Damping Ratio Approximation

I have read in many texts that the closed loop system damping factor can be approximated as: \$\Phi_m= 100 * \zeta\$ With \$\Phi_m\$ as the phase margin and \$\zeta\$ as the damping ratio. The ...
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2answers
820 views

How do I calculate phase margin for a feedback amplifier

I'm learning analog design and can't seem to get a firm grasp on the concept of phase margin and wanted to ask a few questons: First Part: I understand that Phase margin is the difference between the ...
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4answers
3k views

Phase margin analysis of a [diy] PMOS LDO [with LTspice]

I'm somewhat familiar with using LTspice to do phase margin analysis for a classic i.e. obvious negative feedback path opamp circuit; in fact I've done that analysis successfully for fixing real opamp ...
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7answers
36k views

Gain margin and Phase Margin Physical Meaning

I have been trying to understand the physical concept of Gain and Phase Margin. What I understand about this is that a relative comparison around the critical point \$(-1,0)\$, which when converted ...
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2answers
628 views

Is the open loop phase response of an op amp the same as the closed loop phased response?

I am trying to do some calculation for my post lab and i dont understand why the lab manual only talks about open loop phase response. are these the same? I know the gain for open and closed loop ...
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1answer
2k views

Interpretation of mathematicial and corresponding physical meaning of gain and phase margin

In my textbook these are the definition of gain and phase margin: The gain margin is defined as the change in open-loop gain required to make the system unstable. The phase margin is ...
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2answers
3k views

Phase Margin Criterion

Why is 45 degrees considered minimum phase margin. Is it just for stability considerations or some other reasons too? Thanks.
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4answers
1k views

Can gain and phase margins be obtained from step responses?

We need to comply to specific gain and phase margins requirements whereas the controller has been tuned on-line without knowing the plant transfer function (in order to save time). The response does ...
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1answer
643 views

How do I decrease phase margin of an op-amp?

I have designed a CMOS op-amp circuit like figure The problem is the phase margin is 79. How can I decrease phase margin to 60?
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4answers
8k views

Stability and loop gain of negative feedback systems

When we look for stability of a feedback system, why do we only concentrate where loop gain is unity and check the phase margin there. The phase of the closed loop system might be passing through 180 ...
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3answers
314 views

Amplifier design: feedback with unclear poles

When designing a closed loop amplifier for audio the poles of the open-loop transfer function are usually only vaguely known. \$\frac{1}{(s-p_1)(s-p_2)...}\$ These poles can be modeled as \$RC\$ ...
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4answers
10k views

What does the “phase margin” parameter of an opamp imply?

I was reading datasheet of MCP6072. I saw a parameter namely "phase margin" (table 1-2, page 4). As far as I know, "phase margin" is a control engineering term, and implies the phase difference ...