Questions tagged [phase-margin]

A stability metric which is the difference between the phase lag and -180 degrees for a circuit's output relative to its input at unity gain. Positive phase margin indicates stability, negative phase margin indicates that the circuit is not stable. Consider also using related tags such as "stability", "bode-plot", and "transfer-function".

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Confused regarding gain margin and stability

I plotted the bode plots of the transfer function: G(s)= 2.123×10^6 ((0.105s+1.01))/(s^2 (s+12566.37) ) It is as follows: As can be seen, it shows the gain margin as negative infinity, however loop ...
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What will be the values of phase crossover frequency when the gain margin is infinity?

$$G(s) = \frac{s+1}{s^2+2s+100}$$ For the given transfer function I am getting the gain margin equal to infinity in the matlab, but i am not getting the value of phase crossover frequency. What will ...
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Gain and phase margin of multiple feedback bandpass filter

I designed a multiple feedback bandpass filter using the Analog filter wizard. The design is purely education therefore I used ideal op-amp. Fc = 50KHz; DC gain = 6dB; Q = 5; I followed the ...
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How can gain and phase margin be used to assess stability of SMPCs as for most of the cases (except buck), they always have a Right Hand Zero?

I was going through this Operational Amplifiers, Theory and Practice by James Roberge. On Page 146, it is mentioned that for systems that have negative feedback at low or mid frequencies and that have ...
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Amplifier Stability - Observing phase margin

I am following a TI lab about opamp stability: "Precision Labs - Op amps: Stability". And I have some trouble to interpret phase margin from bode plot on an example. The transfer function (...
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Why is the phase margin more important than the gain margin?

I heard from some lecturers that the phase margin is more important than the gain margin. However, after searching online and in some control related textbooks I couldn't find any reference for such ...
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How to calculate the phase margin from Bode Plot if only the phase angle corresponds to the pole frequencies are given?

I have a Bode plot where only the phase angle corresponds to the two pole frequencies are given. The value of gain crossover frequency where the loop gain is unity is unknown. $$\mathrm{PM} = \phi - (-...
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What is the phase margin when there are multiple gain crossover frequencies

I am using LTPowerCAD to help design a buck converter circuit using the LT8645S. I would like to add a second stage LC filter at the output to achieve lower output ripple, but when I add the second ...
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Why phase margin considers only addition of negative phase?

When the phase margin of \$L(s)\$ is calculated, people calculate how much negative phase (or lag phase) can be added to the system \$L(jw)\$ until \$1+e^{-j\theta}L(jw)=0\$ for some frequency \$w\$. ...
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Please help me clear up this confusion regarding gain margin and phase margin and unity feedback

I understand the physical significance of gain and phase margin. However, I would like some clarification regarding the mathematical aspects. Videos such as the following: https://www.youtube.com/...
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Phase lower than -180 when reaching unity gain, is it unstable or can we substract 360?

I am getting a phase of -434 degrees when reaching unity gain (0 dB point). The phase curve starts at 0 degrees, thus, can we consider that -434= -434+360= -74 degrees and the phase margin is of 180-...
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How to find gain and phase margin from transfer function without Nyquist or Bode plot

Given transfer function L(s) as a ratio of polynomials of s, I know that we can find gain margin (GM) and phase margin (PM) by using a Nyquist or Bode plot. From a Nyquist plot: Find a point where ...
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Bode plot starting from 180?

This is an NMOS common source amplifier and I'm trying to calculate the Phase Margin, but it's starting from 180 instead of zero? dose it mean I have poles at the origin or it's very stable?
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unstable systems with positive phase/gain margin

Open loop Phase/Gain margins provide information on how much margin the closed loop system have until it becomes unstable. In other words, how much you can increase the effective loop gain and or ...
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20 dB/Decade - Justification

I've read that it is recommended to have 20 dB/Decade slope on the L(jw) magnitude plot of the Bode Diagram at gain crossover. This seems to imply a slope of -1 in dimensionless terms. However, what ...
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Infinite Phase Margin

I have been assigned a control systems problem, where I am supposed to stabilize a system using a controller technique from the frequency domain. When I implemented the system on Matlab and calculated ...
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How to quantify amplitude of oscillations in marginally stable systems?

A transfer function pole on the Imaginary axis indicates that the system is marginally stable which in time domain manifests as a sinusoidal motion with constant amplitude at the frequency of the pole....
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How to find gain and phase crossover from a Nyquist plot?

Note: I need to do this by hand which is why I'm struggling. I'm only using MATLAB here to show the nyquist plot, and I was also using it to try and check my answer. Edit: There are poles at -50, -10 ...
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Why is there no frequency compensation required in TI's TL494 example buck regulator design?

This example circuit is given in Texas Instrument's application note SLVA001: "Designing Switching Voltage Regulators With the TL494": They use a 140 μH inductor and a 220 μF electrolytic ...
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Optimum phase margin Controller Design

I am designing a PI voltage and current Controller (Cascaded Control for Buck Converter with LC filter) and I am confused about what is the perfect phase margin to choose for the application. I read ...
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About phase margin of inverting amplifiers

Phase margin is usually defined as how far is the phase of the loop gain from -180: $$PM = \angle \text({loop \space gain}) +180^{\circ} $$ Let's suppose that we want to compute the maximum phase ...
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Why do I need phase margin if I know the transfer function?

What is the point of examining the phase margin (or gain margin) for a closed-loop system if I can just solve for the transfer function. The transfer function will give any poles and zeros, which can ...
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Phase margin vs. damping ratio (non-unity feedback)

So there is a well known condition which relates the phase margin with the damping ratio for a unity feedback system: $$\Phi_m = \tan^{-1} \frac{2 \zeta}{\sqrt{-2 \zeta^2 + \sqrt{1+4\zeta^4}}}$$ ...
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Poles from Bode plot

How does one deduct the number of poles from a Bode plot? Is it further possible to decide where the poles are? I have not understood this concept even though there is so much focus on Bode plots in ...
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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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Phase margin for a system (design of PD controller)

I have a system with the following transfer function: \$G(s)=\cfrac{20}{s^2+s+20K_f}\$ where \$K_f=1.75\$ I want to use a PD controller and I want obtain a phase margin of 35 degrees. I need to ...
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Phase Crossover Frequency at 0 deg or -180 deg

I've learned in school to calculate gain margin by first finding the phase crossover frequency at -180 deg, but sometimes I see people calculate gain margin by finding the phase crossover frequency at ...
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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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Gain margin and phase margin in Nyquist plots

Finding gain and phase margins from Nyquist plots is basically finding the margin of change in gain and phase beyond which the plot encircles -1, respectively, with the assumption that the Nyquist ...
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Convert ringing frequency to phase margin for LDO stability

Based on ROHM document "Simple Test Method for Estimating the Stability of Linear Regulators", it showed that in page 2/4, the phase margin for Fig.5 Phase Margin = 0 deg, Fig.6 Phase Margin = 9.9 deg ...
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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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What is this functions Phase crossover frequency?

OLTF loop gain My approach What I have done wrong?
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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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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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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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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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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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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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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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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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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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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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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 ...
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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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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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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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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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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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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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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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