Why is the opamp output a voltage?

[sophiecentaur]

Hopeless academic overthinking on a clearly defined topic what's completely missing the very point at the end.

The OP stated a narrow focus question, and it was widened to a such useless scope that at the end it cannot be answered clearly - and it is still progressing further.

Yes, you have a point but it is also important to keep things clear and to avoid generalisations. You must be aware of the problems about using dB Gain figures that come about when people aren't aware of the pitfalls.
"Are those Voltage dBs or Power dBs?" is so often asked and that's why I am being picky. That's not "academic overthinking", it's avoiding elementary mistakes.

 

[LvW]

Hopeless academic overthinking on a clearly defined topic what's completely missing the very point at the end.
The OP stated a narrow focus question, and it was widened to a such useless scope that at the end it cannot be answered clearly - and it is still progressing further.

I agree with you.
Besides the fact that the OP has selected a wrong title, everythig he has stated was correct.
However, he did not explain the source of his his confusion. May be that`s the reason for the various answers.

 

[Rive]

Besides the fact that the OP has selected a wrong title

I think he choose the correct title, with a terrible example. For OpAmps the output stage defines only the operation limits/area: within those limits the output can be determined through the feedback. Any usage when the output (or something else) is a limiting factor instead of the feedback can be considered as faulty.
Since he did not know this, he thought that the answer will be related to the output circuitry. The answers followed this path and the topic started to diverge from the title and kept on giving more and more generalised answers, based on the commonly used output stages (and non-opamp related definitions).

That's how I see it.

 

[AlexCaledin]

This is the most typical op-amp, according to Wikipedia:

We see the input voltage difference converted into proportional current that is charging the 30 pF capacitor. The capacitor voltage is added to the (Q15 base potential which is almost the) "Vs-" potential and the result is applied to the input of the output voltage follower thus defining the output potential.

 

[berkeman]

charging the 30 pF capacitor.

It's not really "charging" that capacitor. That is likely the "dominant pole" setting capacitor for the opamp, which is needed for stability when the opamp is used in feedback configurations.

http://m.eet.com/media/1126333/266795-6666155_fig2.gif

 

[AlexCaledin]

It's not really "charging" that capacitor. That is likely the "dominant pole" . . .

- well, at least, when that pole is actually dominant - that is, when the Phase is approximately 90 degrees - the amplifier works practically according to "charging" the capacitor with high frequency (from several kHz to several MHz) current. Of course, at low frequencies a 30 pF capacitor can hardly define anything and the output is more like current (amplified by Q15, Q19 and Q14/Q20).

 

[berkeman]

well, at least, when that pole is actually dominant

Sorry, I'm not following. Do you routinely add a different dominant pole in your feedback opamp circuits? If you do, how do you manage to maintain a reasonable phase margin and ensure stability?

EDIT/ADD -- And can you list an opamp part number where you actually are required to add your own dominant pole to keep your feedback circuit stable?

 

[Tom.G]

EDIT/ADD -- And can you list an opamp part number where you actually are required to add your own dominant pole to keep your feedback circuit stable?

LM108
http://www.mit.edu/~6.301/LM108.pdf

 

[AlexCaledin]

Do you routinely add a different dominant pole in your feedback opamp circuits?

- sorry! What I tried to say was simply "when the process of amplification - not only stability - is actually determined by that pole", that is, when the frequency of the amplified signal is such as to make the open-loop phase shift be about 90 degrees.

No, I routinely choose opamps that are stable with simplest feedbacks. (I am thinking how to eliminate the crossover distortion in an audio amplifier, using actually two quite simple amplifiers working together, one of them forcing another, via a little transformer, to have non-distorted output current - it seems very promising when simulated. To find the principles of connecting two systems together, I have to keep each of them simple.)