Input and Output Impedance of an Opamp Circuit

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Discussion Overview

The discussion revolves around the input and output impedance of an operational amplifier (opamp) circuit, specifically focusing on a voltage follower configuration. Participants explore the implications of using an ideal opamp versus a real opamp, and the effects of negative feedback on impedance values.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants assert that for an ideal opamp, the input impedance is infinite and the output impedance is zero.
  • Others question whether the opamp in the circuit is ideal or if a datasheet for a real part should be referenced to determine the impedances.
  • A participant emphasizes the need to distinguish between the opamp itself and the amplifier circuit created with it, noting that negative feedback significantly alters input and output resistances.
  • There is a discussion about the voltage difference between the inverting and non-inverting terminals, with one participant suggesting it would be approximately zero.
  • Some participants propose that the input impedance could be modeled as the input resistance multiplied by the open-loop gain of the opamp, while others express confusion about how the output impedance can be low.
  • One participant mentions that the input impedance of the opamp is high and questions the implications of this when considering the circuit configuration.
  • There is a debate about the resistance between the input signal source and ground, with conflicting views on how this relates to the input impedance of the circuit.
  • Some participants refer to external resources to support their claims about input impedance, while others point out the absence of specific resistors in the provided circuit diagram.
  • Clarifications are made regarding the assumptions about resistors in the circuit, with one participant suggesting that R1 could be considered negligible.

Areas of Agreement / Disagreement

Participants express differing views on the input and output impedance values, with no consensus reached on how to interpret the circuit configuration or the implications of using an ideal versus a real opamp.

Contextual Notes

Participants note the lack of clarity in the circuit diagram and the absence of specific resistors, which complicates the discussion about impedance values. There are also unresolved questions about the assumptions made regarding ideal conditions versus practical implementations.

jaus tail
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Homework Statement
What is input and output impedance of opamp in circuit
Relevant Equations
Input impedance is infinity, output impedance is 0
244354

I know for ideal opamp the input Z is infinite, and output Z is 0, but not sure about this opamp.
 
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jaus tail said:
Problem Statement: What is input and output impedance of opamp in circuit
Relevant Equations: Input impedance is infinity, output impedance is 0

View attachment 244354
I know for ideal opamp the input Z is infinite, and output Z is 0, but not sure about this opamp.
The picture is very hard to see. Is is connected as a simple follower?

And are you asked to assume an ideal opamp, or are you using a datasheet for a real part to find those impedances?
 
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Yes there is a voltage follower. There isn't any data sheet. Just the figure n we have to write the input and output impedance.
 
jaus tai - you clearly have to discriminate between (a) an opamp and (b) an amplifier circuit that is build using an opamp as a core.
Your circuit is not a pure (naked) opamp but a "circuit" (an amplifier) because we have negative feedback, which DRASTICALLY changes the input as well as output resistances.

Just a basic question (which may help you to find the answer for the input resistance): As long as the opamp is operated within its linear transfer characteristic and with a finite output voltage smaller than the supply voltage, what do you think could be the approximate diff. voltage between the inv. and the non-inv. input terminals (µV, mV, V )?
 
Approximately the voltage difference would be 0 between the inverting and non-inverting terminal
How do I find the input and output impedance since there's no resistance vin circuit. No feedback resistance too. Even that's a simple short circuit.
 
From this link I guess the input impedance would be very high. It'd be like Rin times the Open loop gain. and Output impedance is low. I don't understand how the output impedance would be low.
https://www.electronics-tutorials.ws/opamp/opamp_3.html
Ref:
Electronics-Tutorials-Opamp
 
When the voltage difference between both inputs is nearly zero (for the model of an IDEAL opamp it is identical to zero), what then is the resistance between the input signal source and ground?
 
LvW said:
When the voltage difference between both inputs is nearly zero (for the model of an IDEAL opamp it is identical to zero), what then is the resistance between the input signal source and ground?
The resistance between input signal source and ground would be same as resistance between signal source and input of opamp, which is zero here. Now since the non-inverting terminal is grounded, then the inverting terminal is also grounded, so the entire input signal shorts to the ground?? But the input impedance of opamp is very high.

I went through this link that says the input impedance is R1.
https://www.electronics-notes.com/a...rational-amplifier-op-amp/input-impedance.php
Ref: Opamp Input Impedance
There is an inverting opamp circuit with imput Z = R1.
 
jaus tail said:
But the input impedance of opamp is very high.
Yes, the input impedance of the op-amp itself is high... and it is connected in parallel with the output impedance which is low (almost Zero).

So what is the the parallel impedance of a high value resistor paralleled by a Zero Ohm resistor?
 
  • #10
Tom.G said:
Yes, the input impedance of the op-amp itself is high... and it is connected in parallel with the output impedance which is low (almost Zero).

So what is the the parallel impedance of a high value resistor paralleled by a Zero Ohm resistor?
Zero ohm resistor. So the input impedance of circuit is 0 ohm and the output impedance is also 0 ohm.
 
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  • #11
jaus tail said:
I went through this link that says the input impedance is R1.
https://www.electronics-notes.com/a...rational-amplifier-op-amp/input-impedance.php
Ref: Opamp Input Impedance
There is an inverting opamp circuit with imput Z = R1.

But in your circuit there is no resistor R1. If there would be one, why didn`t you show it?
In this case, the answer would be different, of course.
Rule: A good and correct answer requires a good, clear and correct question.
 
  • #12
LvW said:
But in your circuit there is no resistor R1. If there would be one, why didn`t you show it?
In this case, the answer would be different, of course.
Rule: A good and correct answer requires a good, clear and correct question.
R1 is not in the question. I couldn't find the circuit in question, anywhere in the book about opamp. So I searched for input resistance of opamp and the link came up.
We can like assume R1 to be wire resistance and then consider it is equal to negligible i.e 0.
The original question is the one that I have pasted on the first post. No R1.
 
  • #13
Sorry - I have responded to your post #8 in which you have mentioned R1.
I will not repeat such an error...sorry.
I withdraw...
 
  • #14
LvW said:
Sorry - I have responded to your post #8 in which you have mentioned R1.
I will not repeat such an error...sorry.
I withdraw...
It's all right. No problem...
 

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