How does the output impedance of an opamp change with respect to gain?

[wayfaerer]

Hello everyone. I'm a little confused on this question as I've seen what appear to be two opposite views... though it's more likely I'm misinterpreting something.

At about 00:35 in this vid, the guy says the output impedance of an opamp decreases with gain:

However, in Reply #17 of this DIYstompboxes thread, the venerable PRR states of the TL072:

The closed-loop output impedance is the open-loop output impedance divided by the amount of gain applied to NFB.

The open-loop gain of a TL072 (any amplifier) varies with frequency (proof: no amplifier has gain at infinite frequency). General-purpose opamps have high DC gain which falls all across the audio band. But let us "pretend" that a TL072 has gain of 1,000 all across the audio band.

If we wire it unity gain, all 1,000 of gain is available to reduce errors, including output impedance. So the 300 ohms is divided by 1,000, the closed-loop output impedance is 0.3 ohms.

If we wire for closed-loop gain of 10, we need gain=10 for the closed-loop gain. The other 100 of open-loop gain is available to reduce errors. Now 300 ohms divided by 100 is 3 ohm output impedance.

And if we want to arrive at gain of 100, the output impedance is 30 ohms.

Output impedance rises with gain.

What confuses me about the quote is that the first sentence coincides with what the video says, while the last sentence states the opposite.

Am I missing something? Does the output impedance increase or decrease with gain?

[antonis]

Stuby PRR's answer more carefully..

hint: Discrimination between open-loop gain and closed-loop gain..

[wayfaerer]

I don't doubt PRR... just my interpretation.

That said, I still don't get it.

In the line "The closed-loop output impedance is the open-loop output impedance divided by the amount of gain applied to NFB," does "the amount of gain applied to NFB" mean closed-loop gain? I.e., the gain that we set with external resistors?

[antonis]

I might confuse you with terms of loop gain, open and closed ones but let's try..

Loop gain is the algebraic difference (in dBs) between open-loop gain and closed-loop gain..
In cardinal (absolute) numbers (e.g. 1000, 427, 10500) the difference turns into division..

So, for a 100 dB open-loop gain and 40 dB closed-loop gain, loop gain is 60dB..
100dB equals to 100000, 40dB equals to 100 and 60dB equals to 1000..

Now, for unity (closed loop) gain (100% feedback) all the open-loop gain is availiable..
For "some" other gain, availiable is open-loop divided by "some" , which is less than open-loop gain itself..

[PRR]

...does "the amount of gain applied to NFB" mean closed-loop gain?....

No.

[antonis]

I'm totally conviced that OP is more confused now than before posting his query.. 

[Clint Eastwood]

As I understand it, closed loop gain = amplifier gain with feedback, open loop gain = gain without feedback, the difference is the 'amount of  gain applied as feedback', also known as 'loop gain'.  And less feedback = more closed loop gain = higher output impedance. Correct me if I'm wrong, anyone..

[antonis]

Right, at a first glance..

https://books.google.gr/books?id=ziwRBwAAQBAJ&pg=PA323&lpg=PA323&dq=Acl+%3D+Aol/1+%2B+b+AOl&source=bl&ots=d4LX-4so_Q&sig=ACfU3U1kKlVKxkUORerpfwzjtBQKCLyXEg&hl=el&sa=X&ved=2ahUKEwjC9r20wqD2AhVEQvEDHe5DDCoQ6AF6BAgYEAM#v=onepage&q=Acl%20%3D%20Aol%2F1%20%2B%20b%20AOl&f=false

[PRR]

I don't know why possession of a TV camera makes one an expert.

[antonis]

I *think* the whole misconception is where feedback is identified as gain.. ( 00:35 )
(fully unacceptable for demo/educational/training videos..)

[wayfaerer]

Thanks for the clarifications... it makes more sense now.

By the way, I didn't mean for the question to pit one view against another. It's just that I had the question, did some research, and ended up with what seemed like conflicting information... so I posted the question and wanted to share where I was coming from in order to avoid a "Have you tried using the search box?" type of response (not that I see that around here much).

Thanks again.

[PRR]

Is there a way my explanation could be better?

[ElectricDruid]

Is there a way my explanation could be better?

You could persuade everyone else in the world to agree with you so as not to be contradicted?!?