Solving OP AMP Structure Problems: Inverting & Non-Inverting Inputs

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Discussion Overview

The discussion revolves around understanding the internal structure of operational amplifiers (op amps), specifically identifying and justifying which inputs of the differential stage are inverting and non-inverting. The scope includes technical explanations and conceptual clarifications related to op amp behavior in response to input voltage changes.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about identifying inverting and non-inverting inputs and mentions reliance on common mode voltage values without understanding the concept.
  • Another participant asks if the original poster understands how BJTs work and how basic BJT amplifier configurations operate, indicating a foundational knowledge check.
  • A participant provides an analysis of a specific circuit, explaining that increasing the voltage at the inverting input leads to a drop in output voltage, thus identifying the input as inverting.
  • There is a clarification that if an increase in one input results in a decrease in output, that input is inverting, while an increase in output indicates a non-inverting input.
  • Agreement is reached on the interpretation of input-output relationships in op amps, confirming the earlier analysis.

Areas of Agreement / Disagreement

Participants generally agree on the relationship between input changes and output behavior in op amps, with some clarification needed on foundational concepts. However, the initial confusion about identifying inputs indicates that not all aspects of the discussion are resolved.

Contextual Notes

Limitations include the original poster's uncertainty about foundational concepts related to BJTs and op amp configurations, which may affect their understanding of the discussion.

Who May Find This Useful

Individuals studying operational amplifiers, electronics students, and those interested in BJT amplifier configurations may find this discussion relevant.

Drao92
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Hello everyone. I am studying internal structure of op amp and i don't know to solve some problems.
The problem is that i have to specify and justify which inputs of the differential stage are inverting and non-inverting input and i don't know how to do it. I tried to find the answer on google and on many many books but i didnt find anything to give me a clearly answer. Only thing i found is that we use the maximum and minimum value of common mode voltage but i didnt understand how.
I`ll also post some circuits if anybody can help me.
https://www.physicsforums.com/attachment.php?attachmentid=66211&stc=1&d=1391287797
https://www.physicsforums.com/attachment.php?attachmentid=66212&stc=1&d=1391287797
 

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First I must ask you a question, do understand how BJT work and also how the basic BJT amplifier configuration work?
Also can you tell me how the output voltage will behavior if you increase voltage at inverting input?
 
You can quite easy determine which input do what.
Let as exam this circuit.

https://www.physicsforums.com/attachment.php?attachmentid=66212&stc=1&d=1391287797

If we increase the voltage at Vin1 (Q1 base), Q1 will turn-on more, this means that the voltage at Q1 collector will drop. Because Q4 base is connect to Q1 collector. Q4 base voltage will also drop. This of course will open more Q4 transistor. So voltage at Q4 collector and Q7 base will rise.
This voltage rise at Q7 base will turn-on Q7 more. So the voltage at point A and at output will drop.
So form our analysis we can tell that Vin is inverting and Vi2 is non-inverting input.
 
Well, i got the idea. So if an increase in one of the input is droping the output it means it is inverting and if it increase the output it means it is non-inverting?
 
Drao92 said:
. So if an increase in one of the input is droping the output it means it is inverting and if it increase the output it means it is non-inverting?

attachment.php?attachmentid=46245&stc=1&d=1334484384.png


Yes, exactly as you said.
 
Thanks a lot!
 

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