Predicting AC open-loop gain for an internally compensated op-amp

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Discussion Overview

The discussion revolves around predicting the AC open-loop gain of an internally compensated operational amplifier (op-amp) at a frequency of 1kHz, given its DC open-loop gain and AC open-loop gain at 10kHz. Participants explore concepts related to gain rolloff, poles, and frequency compensation in op-amps.

Discussion Character

  • Homework-related
  • Technical explanation
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • Some participants note that the gain decreases as frequency increases beyond a certain breakpoint, suggesting that the gain at 1kHz should be larger than 40dB.
  • Questions are raised about the typical number of poles in an internally compensated op-amp and their locations, indicating a need for clarity on these concepts.
  • There is uncertainty expressed regarding the terminology of "poles" and "rolloff," with participants seeking definitions and explanations.
  • One participant mentions the importance of understanding dominant pole compensation and provides links to resources for further learning.
  • Another participant suggests that answering questions about poles and rolloff will lead to a more quantitative answer regarding the expected gain.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the expected gain at 1kHz, and there are multiple competing views regarding the concepts of poles and rolloff. The discussion remains unresolved with respect to the exact gain value.

Contextual Notes

Participants express uncertainty about the definitions of key terms and the implications of the gain rolloff, indicating that assumptions about the behavior of the op-amp may not be fully articulated.

Who May Find This Useful

This discussion may be useful for students studying analog electronics, particularly those interested in operational amplifier behavior and frequency compensation techniques.

adamaero
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Homework Statement


An internally compensated op amp has a DC open-loop gain of 1,000,000 (V/V) and an ac open-loop gain of 40dB at 10kHz. Provide the expected gain in dB at 1kHz...

Homework Equations


Please help, I don't know what to use for an exact answer.

The Attempt at a Solution


Beyond the break-point, the gain goes down as frequency goes up. So I just picked the value larger than 40dB (since the frequency decreased).
 
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adamaero said:

Homework Statement


An internally compensated op amp has a DC open-loop gain of 1,000,000 (V/V) and an ac open-loop gain of 40dB at 10kHz. Provide the expected gain in dB at 1kHz...

Homework Equations


Please help, I don't know what to use for an exact answer.

The Attempt at a Solution


Beyond the break-point, the gain goes down as frequency goes up. So I just picked the value larger than 40dB (since the frequency decreased).
How many poles does an internally compensated opamp typically have, and where located? So what would you expect the rolloff to be in the 10kHz to 1kHz region?
 
berkeman said:
How many poles does an internally compensated opamp typically have, and where located?
What? Is there another word for poles or rolloff? I don't know what these mean.
Is my intuitive answer usually how this problem would be answered?
 
adamaero said:
What? Is there another word for poles or rolloff? I don't know what these mean.
Is my intuitive answer usually how this problem would be answered?
What class are you in? What textbook are you using? What have you found when you use the very helpful Internet to search for information on Dominant Pole Compensation for Opamps?
 
302 Analog electronics
https://www.uwplatt.edu/files/collaborative-engineering/Documents/ELECTRICAL%20ENGINEERING%20TECHNICAL%20ELECTIVES.4.14-2_0.pdf

Microelectronics 4th edition by Neamen

Thank you for giving me a precise phase to search and learn about:
https://en.wikipedia.org/wiki/Operational_amplifier#Frequency_compensation
half-power frequency = 3dB down frequency vs the unity gain frequency...(Note to self - Lab Design project #1 III. Measurements, Amplifier gain)
http://www.ti.com/lit/an/sloa020a/sloa020a.pdf#page=19
http://web.mit.edu/klund/www/papers/ACC04_opcomp.pdf

I have to go, but I will come back and read more about this...
 
adamaero said:
ac open-loop gain of 40dB at 10kHz. Provide the expected gain in dB at 1kHz...
adamaero said:
So I just picked the value larger than 40dB
adamaero said:
Is my intuitive answer usually how this problem would be answered?
You are heading in the right direction, but just saying the gain is "greater" is not an answer to the question
berkeman said:
How many poles does an internally compensated opamp typically have, and where located? So what would you expect the rolloff to be in the 10kHz to 1kHz region?
Answering those questions will give you the quantitative answer... :smile:
 

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