Why is the opamp output a voltage?

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  • #26
sophiecentaur
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It seems that Sophiecentaur did not noticed that your transistor is a FET and not a BJT.
You got me there. But the important thing was that what was shown was not an OP Amp and the device output impedance is high. The only way to produce a 'voltage source' with what is shown would be to have a very low Rd which would make the Voltage Gain and efficiency extremely Low.
 
  • #27
Fernando Perfumo
You got me there. But the important thing was that what was shown was not an OP Amp and the device output impedance is high. The only way to produce a 'voltage source' with what is shown would be to have a very low Rd which would make the Voltage Gain and efficiency extremely Low.
Of course, it is not an Operational amplifier. :)). Jadnull is I am convinced, a clever person, just he need to digest fully this things.
 
  • #28
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Not really. That is a very limited definition of 'Gain' and it may appear to confirm your view. But your 'Gain' is in fact 'Voltage Gain' (which is how you have defined it).
One would expect some enlightening power from the included >>that<< word there.

Yes, really: that gain what the topic is actually about is voltage gain.
Quite hard to find anything else regarding OpAmps. A type of 'current gain' is a special topic, but still exists: but to provide a power gain based on OpAmps would require a quite complex feedback circuit, possibly with a multiplier. Closest to this would be some measurement circuits I think, but those usually has no 'power' output.

Title of topic:
Why is the opamp output a voltage?
After that, the content of topic chewing on the fact that an output stage alone will always act as an output stage (alone).
 
  • #29
sophiecentaur
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Yes, really: that gain what the topic is actually about is voltage gain.
As the thread had included other transfer characteristics then the context was not as clear as you imply and the passage I challenged seemed to be excluding this.
I looked at this link for some support and they make it clear that the Gain can refer to the relationship between all combinations of input and output signals. Page 7 sets the context for OP Amp discussions. Signal Power often tends to be ignored or tacitly implied in the context of OpAmps.
However, even Voltage Gain value is not sufficient in many contexts. The Power gain for an amplifier with different input and output impedances is not equal to the Voltage gain. Perhaps it's my RF experience that makes me all too aware of this. A mains transformer with a ratio of 1:10 may produce an 'Voltage Amplification factor of 10 but it is in no way an 'Amplifier' of the Input Power. It always annoys me when the bones of the middle ear are said to "amplify" the sound received when they are, in fact, matching devices.
 
  • #30
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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.
 
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  • #31
sophiecentaur
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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.
 
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  • #32
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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.
 
  • #33
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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.
 
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  • #34
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This is the most typical op-amp, according to Wikipedia:
750px-OpAmpTransistorLevel_Colored_Labeled.svg.png

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.
 
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  • #36
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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).
 
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  • #37
berkeman
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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? :smile:
 
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  • #39
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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.)
 
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