Why is output impedance of op amp zero?

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Discussion Overview

The discussion centers around the output impedance of operational amplifiers (op-amps), specifically exploring why it is considered to be low. Participants are examining both the physical characteristics and the operational principles that contribute to this property, with a focus on theoretical and conceptual understanding.

Discussion Character

  • Homework-related
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant asks why the output impedance of op-amps is so small, indicating a desire for a deeper understanding of the physical basis.
  • Another participant notes that specific op-amps can have output impedances in the hundreds of ohms, suggesting variability among different models.
  • It is mentioned that op-amps generally have low output impedance, which is a desirable characteristic for driving loads.
  • A participant explains that low output impedance allows op-amps to source significant current to drive large loads, such as loudspeakers or motors.
  • One participant seeks clarification on the physical mechanisms behind low output impedance, prompting further elaboration.
  • Another participant attributes the low output impedance to the use of negative feedback, which compensates for load variations.
  • There is a discussion about the internal structure of op-amps, with reference to the 741 op-amp's output stage and its use of transistors to manage current supply.

Areas of Agreement / Disagreement

Participants generally agree that op-amps have low output impedance and that this is beneficial for driving loads. However, there is no consensus on the specific physical mechanisms or characteristics that lead to this property, as some participants seek more detailed explanations.

Contextual Notes

The discussion includes varying interpretations of "small" in terms of output impedance, with some participants focusing on the resistance values while others consider the operational implications. There is also a reference to specific op-amp models, which may indicate that the characteristics can vary across different types.

mathman44
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Homework Statement



Hi. Just like the title says, why is the output impedance of an op-amp so small?

(as in physically, I understand why a low output impedance is a desirable characteristic)
 
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Which opamp? The one's I spot checked are in the 100's of ohms...
 
Op amps in general have very low output impedance though..
 
I would still consider 100 ohms to be small. Can anyone explain why output impedances of op-amps are as small as they are?
 
Anyone?
 
Do you mean why are they small? So you can source a lot of current from them and so drive large loads, such as loudspeakers/motors/long cable runs.

Or How are they small? It's based on having a small resistance transistor (eg a FET) in the output
 
I mean "how" they're small (physically). Could you elaborate please? I can't seem to get any results from google on this topic.
 
Usually opamps use a lot of negative feedback. Negative feedback lowers the output impedance because it compensates for variations in the load.
 
I'm still a little unclear on the actual question. Basically the output impedance is low because that's what amplifiers do...they "convert" a small signal into a larger one and I suppose the ratio of input to output impedance could be one measure of such.

By "physically small" I'm guessing you don't mean inches and microns, but "resistance" wise. If that's the case, all(?) opamps have an output stage made of a couple of relatively large transistors that can source a (relatively) large amount of current.

For instance look at the internal schematic for the 741 opamp on page 4 of this datasheet:

http://www.national.com/ds/LM/LM741.pdf

On the right side is the output pin connected to two transistors (Q14, Q20) through 25 ohm resistors. These transistors are "turned on" by the preceding circuitry and supply power to whatever is connected to the output...
 
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