Phase shift issue in Dominant Pole Compensation strategy....

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Discussion Overview

The discussion revolves around the phase shift behavior in the context of Dominant Pole Compensation strategy in electronic circuits, particularly focusing on the relationship between frequency and phase shift. Participants explore theoretical aspects, practical implications, and the nuances of phase shift in relation to frequency changes, including the effects of parasitic capacitance and the role of poles in determining phase characteristics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants express confusion over the text's assertion that phase shift remains constant at 90 degrees while frequency increases, suggesting a paradox in understanding.
  • Others argue that phase shift does not always increase with frequency, providing examples of circuits where phase shift is constant or zero.
  • A participant mentions that parasitic effects, such as stray capacitance, contribute to phase shift, indicating that these effects are not accounted for in the text.
  • One participant clarifies that the dominant pole capacitor can provide a maximum phase shift of 90 degrees and that the loop gain must drop to 1 at a frequency where uncompensated poles contribute minimally to phase shift.
  • Another participant discusses the asymptotic nature of phase shift in relation to frequency, emphasizing that phase shift approaches 90 degrees but does not equal it, and that this understanding is reinforced through practical exercises.
  • Some participants explore the relationship between frequency and the occurrence of poles, suggesting that frequency indirectly influences phase shift by determining when poles are activated.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the interpretation of phase shift behavior with frequency. While some agree with the text's assertion of constant phase shift, others challenge this view, leading to an ongoing debate about the complexities of phase shift in electronic circuits.

Contextual Notes

Limitations include the simplifications made in the text regarding phase shift behavior and the neglect of parasitic effects, which may not hold true across all frequency ranges or circuit configurations.

  • #31
brainbaby said:
what I realized from your post 26 is that...in electronics one cannot be absolute...like there is nothing certain.
.
I don`agree. This conclusion cannot be drawn from my post#26

brainbaby said:
actually if something is true at a certain condition..doesn't necessarily mean that it would be true at other conditions as well...
...philosophical rule of thumb? Do you have an example?
 
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  • #32
LvW said:
I don`agree. This conclusion cannot be drawn from my post#26
sorry its a typo..I meant post 16 not 26...

LvW said:
...philosophical rule of thumb? Do you have an example?
earlier I use to think that roll off happens only at a rollover frequency...wp=1/RC...which I previously thought to be certain...but in your following quote you told me a fact that actually the decrease of amplitude takes place at very low frequency (f=1E-12 Hz) though hard to measure... so that's why I thought that one cannot be absolutely certain at all times..it mere conditional...(depends upon condition)
LvW said:
remember the simple RC lowpass with a pole at wp=1/RC. The decrease of amplitude with a corresponding phase shift will start already for f=1E-12 Hz (and even below).
 
  • #33
brainbaby said:
earlier I use to think that roll off happens only at a rollover frequency...wp=1/RC...which I previously thought to be certain...but in your following quote you told me a fact that actually the decrease of amplitude takes place at very low frequency (f=1E-12 Hz) though hard to measure... so that's why I thought that one cannot be absolutely certain at all times..it mere conditional...(depends upon condition)

I rather think, it depends not on certain conditions but on the degree of simplification one is able to accept.
To make it clear: At the pole frequency "starts" the roll-off of the asymptotic line that is used as an aid for constructing a much more realsitic curve of the phase response (which begins to deviate from the starting value much earlier).
 
  • #34
brainbaby said:
earlier I use to think that roll off happens only at a rollover frequency...wp=1/RC...which I previously thought to be certain...but in your following quote you told me a fact that actually the decrease of amplitude takes place at very low frequency (f=1E-12 Hz) though hard to measure...

that's the trouble with umpteen-digit calculators
by slide rule it's obvious from the t or srt scale when phase has got so close to its asymptote as to 'have arrived'
but the calculator goes out probably beyond twelve digits ,
one has to 'think analog' to realize the number has effectively quit changing.
 
  • #35
brainbaby said:
.in electronics one cannot be absolute...like there is nothing certain.
actually if something is true at a certain condition..doesn't necessarily mean that it would be true at other conditions as well...

I don`t know if I completely understood the meaning of these sentences - but It could be the truth replacing "certain" by "absolutely correct".
Let me explain: In electronics nothing is absolutely "correct" (formulas, explanations) because we always make simplifications and neglect some minor influences. This makes sense and is necessary - otherwise we would arrive at formulas which cannot be handled and evaluated. The most simple example is a resistive voltage divider which - of course - has some capacitive and inductive influences. However, as long as these influences are smaller than parts tolerances or other unwanted and/or unknown influences from other sources it makes much sense to treat the divider as "pure resistive".
However, there my be other situations ("conditions" as you say) where some simplifications are not allowed anymore.
Another example: We must know within which limits we can treat an operational amplifier as ideal. Otherwise, it can happen that an opamp-based circuit suddenly behaves unexpected or even begins to oscillate.
Summary: Each formula or function contains simplifications - and it is the task of a good engineer to know under which operating conditions these simplifications are allowed and will cause errors/deviations which are within acceptable limits.
 

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