Why is output impedance of op amp zero?

[mathman44]

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)

 

[schip666!]

Which opamp? The one's I spot checked are in the 100's of ohms...

 

[mmmboh]

Op amps in general have very low output impedance though..

 

[mathman44]

I would still consider 100 ohms to be small. Can anyone explain why output impedances of op-amps are as small as they are?

 

[mathman44]

Anyone?

 

[NobodySpecial]

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

 

[mathman44]

I mean "how" they're small (physically). Could you elaborate please? I can't seem to get any results from google on this topic.

 

[skeptic2]

Usually opamps use a lot of negative feedback. Negative feedback lowers the output impedance because it compensates for variations in the load.

 

[schip666!]

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...