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37 votes
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What does a Bode plot represent and what is a pole and zero of a Bode plot?

Bode plot Take a typical 2nd order low pass filter amplitude response bode plot: - 3-D picture introducing pole-zero diagram Here's the bigger picture of that response when combined with a pole ...
Andy aka's user avatar
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11 votes
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Why is frequency not measured in db in bode's plot?

Frequency is not a measure of signal amplitude or power hence, talking about "this or that" frequency in decibels makes no sense. If somebody says "increase the frequency by 3 dB"; ...
Andy aka's user avatar
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11 votes
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How can a Bode plot be like that?

How do I get the slope and intersection point? Start with a very simple example: - $$H(s) = \dfrac{1}{s}$$ When \$s = 1\$, the amplitude of the transfer function is clearly 1. When \$s = 0.1\$, the ...
Andy aka's user avatar
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10 votes
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What does actually the "unravel branch wrap" option do to the phase plot in LTspice?

It adds or subtracts multiples of \$2\pi\$ (360 degrees) to each point in the phase plot so that the phase is plotted continuously rather than having jumps in it. We know that physically the phase ...
Neil_UK's user avatar
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10 votes

Is -3dB bandwidth defined for any type of transfer function?

Is -3dB bandwidth defined for any type of transfer function? Maybe you do not realize what -3dB actually means? -3 dB means that the amplitude of the voltage (current as well) has dropped by a factor ...
Bimpelrekkie's user avatar
10 votes

Why are there ripples in the passband of some filters, like the Chebyshev filter?

Please note that this is a quite "normal" expected/desired behaviour. We have several different methods to approximate the ideal lowpass response (this would be "brickwall" ...
LvW's user avatar
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9 votes
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Why a low-pass filter applied to a square wave gives a weird wave?

There are many types of low pass filter. The one that gives your 'weird waveform' is the simplest 1st order RC filter. This passes the fundamental signal. However, it not only passes all the higher ...
Neil_UK's user avatar
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9 votes

Why do some op amps' datasheet specify phase margin at open loop?

Shouldn't it be just phase? I agree. It makes no sense to draw a graph like this and call it phase margin because, phase margin usually only occurs in one place i.e. where the open-loop gain is unity....
Andy aka's user avatar
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8 votes

Problem with Bode diagram in LTspice

The AC analysis is a linearized analysis. That means it simulates the small-signal behavior of each element at its operating point. In the case of the switch, that means it is held in either the on or ...
The Photon's user avatar
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8 votes

Why are there ripples in the passband of some filters, like the Chebyshev filter?

Higher order filters are "made" from building blocks of 2nd order filters and, to get a steeper response in the roll-off area (above \$f_c\$), quite often a 2nd order filter with a low Q ...
Andy aka's user avatar
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7 votes

Low Pass Filter Wrong Pole Location

It's just a conversion from radians to degrees. The expression for a capacitor's impedance uses units of radians/sec. for frequency. The simulator is reporting frequencies in Hertz. 10 rad/sec is 1....
Carl Raymond's user avatar
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7 votes
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How to sketch the Bode diagram of the output filter of a Buck converter?

Background Starting with your result and proceeding: $$\begin{align*} H\left(s\right)&=\frac{R}{R\,L\,C\,s^2+L\,s+R}\\\\ &=\frac{1}{L\,C\,s^2+\frac LR\,s+1}\\\\ &=\frac{\frac 1 {L\,C}}{s^2+...
jonk's user avatar
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7 votes

Pole zero plot of band reject filter

Perhaps this is where the confusion starts: 's' is complex (re + j*im, or sigma + jw), not just imaginary. Those two terms are often accidentally used interchangeably, and they shouldn't be. G is ...
P2000's user avatar
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7 votes

Why are there ripples in the passband of some filters, like the Chebyshev filter?

Filters lie in the domain of rational polynomials, that is, the transfer function is a ratio of the form: $$ H = \frac{P}{Q} = \frac{p_n \omega^n + p_{n-1} \omega^{n-1} + \ldots + p_1 \omega + p_0}{...
Tim Williams's user avatar
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7 votes

How to find the Transfer Function of this circuit?

You can easily determine the transfer function of this circuit by considering a simple \$RC\$ low-pass filter followed by a non-inverting structure described in this Wikipedia link. Because the op-amp ...
Verbal Kint's user avatar
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7 votes
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What does Gain mean when LTSpice plots Gain vs Frequency in AC analysis?

When you do an AC analysis and then measure the voltage at a point the result will be a plot of the voltage at that point vs. frequency. The default plot is in decibels referenced to 1 V, so if there ...
GodJihyo's user avatar
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7 votes
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Why do some op amps' datasheet specify phase margin at open loop?

Yes - it is a mistake. The wording just below the diagram ("Gain and phase vs. frequency") is correct. The phase margin (PM) is just one point of the phase response (it is app PM=60deg). ...
LvW's user avatar
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6 votes

What exactly is Pole Frequency in a Filter and how does it affect the frequency response?

1) Why does the magnitude Bode Plot of the response of a filter NOT approach Infinity at a pole? Try looking at this picture and recognize that poles may exist as infinities in the bode plot but ...
Andy aka's user avatar
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6 votes

Transfer function of electrical network

A quick look at the fast analytical techniques or FACTs gives you the transfer function of this guy in the blink of an eye. First, turn the stimulus off or reduce \$V_{in}\$ to 0 V: replace the source ...
Verbal Kint's user avatar
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6 votes

How are the -3dB point and the pole of an op-amp related?

Because a typical op-amp is dominant pole compensated, it has a single pole in the open-loop transfer function at low frequency, and a -20 dB/decade roll off. This leads to the concept of "gain-...
John D's user avatar
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6 votes
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LC filter out of the loop if crossover frequency is below resonant frequency?

I am glad you picked up this slide because it can generate confusion with a perfectly-stable converter. Actually, the step response of a switching converter operated in open- or closed-loop, depends ...
Verbal Kint's user avatar
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6 votes

Open loop gain of Op Amp 20 dB lower

That's because you have a DC feedback factor of 10. You're not measuring "open-loop" gain this way, you're measuring loop gain. $$ A\beta = 10^6\frac{1}{10}= 10^5 = 100dB $$
Designalog's user avatar
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5 votes
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Low Pass Filter Wrong Pole Location

Complex Number Diversion Complex numbers have both a Cartesian notation as well as two equivalent polar notations. The Cartesian notation, \$a+b\:i\$, is very hard to use in electronics but it is very ...
jonk's user avatar
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5 votes
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Are phase angles in filter transfer functions primarily for stability?

Different applications are sensitive to phase variation with frequency for different reasons. It can impact stability margin if the filter is in a closed-loop system. The total error between a source ...
TimWescott's user avatar
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5 votes

Zeros and poles in the following bode plot

How is it possible to make transfer function to become zero or infinity. You will only get zeros and poles in the bode plot if those zeros and poles lie on the \$j\omega\$ axis. The bode plot is ...
Andy aka's user avatar
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5 votes

Problem with Bode diagram in LTspice

The discussion is interesting an in particular, the (always) comprehensive reply given by a concerned citizen. However, it shows, in my opinion, that LTspice is not really suited for extracting the ac ...
Verbal Kint's user avatar
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5 votes

Pole zero plot of band reject filter

A little help: - If the TF denominator is this: \$(1 + as)(1 + bs)\$ Then \$(1 + as)\$ equates to zero when \$s = -\dfrac{1}{a}\$ And \$(1 + bs)\$ equates to zero when \$s = -\dfrac{1}{b}\$ If either ...
Andy aka's user avatar
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5 votes

Why is frequency not measured in db in bode's plot?

While drawing bode's graph paper where Y axis is gain given in db while the X axis is frequency given in absolute value but scale used is log. So we are plotting log response against log frequency. ...
Neil_UK's user avatar
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5 votes
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Bode Plot of Type 3 Compensator

Phase margin only applies to the open-loop transfer function of a closed-loop system. A compensator has a phase response, but phase margin doesn't make sense unless you add the transfer functions of ...
John D's user avatar
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