Open loop bandwidth in op-amp.

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cliffforrest

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Joined 2004
If the amplifier was perfect AND there was no internal frequency compensation then OLG would be constant at all frequencies from DC to light!

Precisely because of the internal compensation the OLG is NOT constant with frequency, which is what the graph is showing.

There is no point in thinking about the gain without the compensation cap since it is internal to the amp and cannot be removed.

Think about a series R and parallel C in (say) a crossover. As the frequency rises the impedance of the C falls and the "gain" reduces. It is the same efffect.
 
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RetroAudio

Member
Joined 2004
Beppe - I think perhaps you misread the 825 graphs,..or maybe I am (would not be the first time). The open loop BW looks slightly > 10khz with a gain lower than "normal". If you wanted to use it for a buffer I'm almost sure you could do a lot worse, as the external setup and power supply play a major role as well and should not be overlooked by any means. If they are not up to certain standards they will shoot down any opamp choice you may make.
 
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beppe61

Member
Joined 2005
cliff said:

If the amplifier was perfect AND there was no internal frequency compensation then OLG would be constant at all frequencies from DC to light!
Precisely because of the internal compensation the OLG is NOT constant with frequency, which is what the graph is showing.
There is no point in thinking about the gain without the compensation cap since it is internal to the amp and cannot be removed.
Think about a series R and parallel C in (say) a crossover.
As the frequency rises the impedance of the C falls and the "gain" reduces. It is the same efffect.
Click to expand...

I think I have understood: open loop gain is not constant with frequency.
Is it not a bad quality ? a sign of no linearity ?
The variation is remarkable I think.
Excuse me again but can I get the open loop bandwidth from the same graph ? or from what other graph ?
I am a little confused about how to get the value of this OLB.
I hope it were mentioned in the main figures more clearly.
The OLB should be a fixed value ?

Thanks again.
Kind regards,

beppe
 
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beppe61

Member
Joined 2005
RetroAudio said:
Beppe - I think perhaps you misread the 825 graphs,..or maybe I am (would not be the first time).
The open loop BW looks slightly > 10khz with a gain lower than "normal".
If you wanted to use it for a buffer I'm almost sure you could do a lot worse, as the external setup and power supply play a major role as well and should not be overlooked by any means.
If they are not up to certain standards they will shoot down any opamp choice you may make.
Click to expand...

The open loop BW looks slightly > 10khz
As you can very well understand I am quite confused about all these graphs and their interpretation.
Now, >10kHz seems to me a very good performance.
Actually I read some reports talking about the good audio quality of this op-amp.

with a gain lower than "normal"
when you say gain you mean open loop gain ?

the external setup and power supply play a major role as well and should not be overlooked by any means.
If they are not up to certain standards they will shoot down any opamp choice you may make
Please Sir wait a moment :xeye:
This could be LESSON TWO: POWER SUPPLIES !:D

Thank you very much indeed.
Kind regards,

beppe
 
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RetroAudio

Member
Joined 2004
Yes Beppe, I mean the normal for open loop gain. I thought that was the world which we were discussing. The reason I put normal in parenthesis is to make a point that most opamps are made with a higher gain is all,..you know,..smallish open loop BW and high gain.

The > 10khz performance is what it is,..neither good nor bad. Just up to the person to utilize it the way they need. As with all things audio related, most is up to the way we choose to use. I have used opamps with the narrow BW but high gain to very useful effect, so they have their place. That's why audio can be most frustrating and just have to know the application,..or think you know. And that's where we all get into trouble now, isn't it?

As for power supplies being lesson two, I have found everything is an ongoing schooling process. I have been at this many years now and have come to many realizations, both on my own and from others. You learn by doing, so do not be fearful of being your own person (which you seem to be doing..and I must commend you for it) and facing failure, for with failure will come success and further realizations in some form or another. It's also another reason just hanging out in these forums and having others do your work for you is merely an exercise in futility as you learn "not much". Ok, I'm done. This Zen crap is getting too much for even me....lol!
 
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beppe61

Member
Joined 2005
RetroAudio said:

Yes Beppe, I mean the normal for open loop gain.
I thought that was the world which we were discussing.
The reason I put normal in parenthesis is to make a point that most opamps are made with a higher gain is all,..you know,..smallish open loop BW and high gain.
The > 10khz performance is what it is,..neither good nor bad.
Just up to the person to utilize it the way they need.
As with all things audio related, most is up to the way we choose to use.
I have used opamps with the narrow BW but high gain to very useful effect, so they have their place.
That's why audio can be most frustrating and just have to know the application,..or think you know.
And that's where we all get into trouble now, isn't it?
As for power supplies being lesson two, I have found everything is an ongoing schooling process.
I have been at this many years now and have come to many realizations, both on my own and from others.
You learn by doing, so do not be fearful of being your own person (which you seem to be doing..and I must commend you for it) and facing failure, for with failure will come success and further realizations in some form or another.
It's also another reason just hanging out in these forums and having others do your work for you is merely an exercise in futility as you learn "not much". Ok, I'm done.
This Zen crap is getting too much for even me....lol!
Click to expand...

Thank you very much for all your kind, friendly and very helpful advice.
I will practice with some very basic circuits as a start then.

Kind regards,

beppe
 
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beppe61

Member
Joined 2005

Originally posted by RetroAudio
Beppe - Just make sure the opamp you choose is unity gain stable.
It should say that in the data sheets, or at least alude to that fact.
Many times an example circuit is even given showing the setup.
Click to expand...

Thank you very much again Sir.
I will look for that indication.
I think that after all a nice op-amp should be more than enough for my purpose.
Thank you very much again for your extremely kind and valuable help.
My best regards,

beppe
 
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beppe61

Member
Joined 2005
4fun said:
Hi Beppe,
More links :)
Douglas Self has som good opamp info on his site.
http://www.dself.dsl.pipex.com/ampins/webbop/opamp.htm
Click to expand...

Thank you very much Sir for the very interesting link.
By the way I notice that Mr Self gives as a evidence of the quality of an op-amp graphs of THD figures.
There is no discussion on open loop bandwidth.
I check that for the famous opa627, one of the best sounding op-amp around from what I have gathered.

Thanks again and kind regards,

beppe
 
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beppe61

Member
Joined 2005
Looking at some datasheets I have found the following figures:

OP-AMP OLG(dB) SLEW RATE (V/us)

AD825A 76(OLG) 130(SR)
OPA627 120(OLG) 55(SR)
AD811 400(SR)

Interestingly enough, I cannot find the Open Loop Gain figure for the AD811.
Maybe I have not read everyhting or well.
I would like to ask if everyone knows the typical OLG for the AD811, because I would like to assess if there is a relation between OLG and Slew rate (SR), as it seems that SR increases with OLG going down.
If this trend is general, I could conculde that the lower the OLG the higher the SR and the better the op-amp for audio purpose.

Thank you very much to everyone for the extremely kind patience and for the very valuable advice.

Kind regards,

beppe
 
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cliffforrest

Member
Joined 2004
The Ad811 is designed for video use, not audio.

With a 140Mhz bandwidth and 2,500 v/us slew rate it will be a difficult beast to tame in an audio circuit - oscillations, etc difficult to debug without good test equipment.

This high frequency performance is completely unecessary for audio.

Don't go there!

An F1 racing car is not good for trips to the supermarket! :D
 
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beppe61

Member
Joined 2005
cliffforrest said:

The Ad811 is designed for video use, not audio.
With a 140Mhz bandwidth and 2,500 v/us slew rate it will be a difficult beast to tame in an audio circuit - oscillations, etc difficult to debug without good test equipment.
This high frequency performance is completely unecessary for audio.
Don't go there!
An F1 racing car is not good for trips to the supermarket! :D
Click to expand...

Thank you very much Sir for your kind and valuable reply.
Actually I read that Jeff Rowland used it in its Synergy line stage to a good effect I understand.
On this basis I purchased two samples that are still in my closet waiting to be used.
Maybe they have found a way to solve the problems you mention.
Still I cannot find the open loop gain for this opamp.
For what is worth open loop gain is not always present in specifications.
Again I found this strange.

Kind regards,

beppe
 
darkfenriz

darkfenriz

Member
Joined 2004
I think, that no op-amp is designed for high open loop bandwidth, especially from Otala-generation reasons.
Flat OL bandwidth to some 20kHz is considered more like side effect in op-amps which are designed for good high frequency performance.
All in all ideal op-amp has infinite gain, so the more the better.
That's the philosophy behind the IC op-amp.

Adam
 
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beppe61

Member
Joined 2005
darkfenriz said:

I think, that no op-amp is designed for high open loop bandwidth, especially from Otala-generation reasons.
Flat OL bandwidth to some 20kHz is considered more like side effect in op-amps which are designed for good high frequency performance.
All in all ideal op-amp has infinite gain, so the more the better.
That's the philosophy behind the IC op-amp.
Adam
Click to expand...

Thank you Mr Adam for your kind and valuable opinion.
My reasoning is the following.
A famous designer states that a high open loop bandwidth is mandatory for good audio performance.
I take this as true.
I am now trying to select an op-amp (very convenient piece to use) that has a high open loop bandwidth.
And I am finding a lot of difficulties.
Maybe op-amps are not designed for audio purposes after all :confused:
But to assess this for me is really fundamental.
If things are this way, I will go straight away for a discrete solution, like for instance that very famous diamond buffer.
Lazy that I am I would prefer an op-amp after all.
But if this is not possible....

Thank you so much again.
Kind regards,

beppe
 
lineup

lineup

Member
Joined 2005
cliffforrest said:
The Ad811 is designed for video use, not audio.

With a 140Mhz bandwidth and 2,500 v/us slew rate it will be a difficult beast to tame in an audio circuit - oscillations, etc difficult to debug without good test equipment.

This high frequency performance is completely unecessary for audio.

Don't go there!

An F1 racing car is not good for trips to the supermarket! :D
Click to expand...

Very good information, cliffforrest :cool:

This is what I tell people here, too.

Because I have seen so many, even experienced Audio Enginners at our forum
- having long discussions about problems with Video Speed Op-amps
they are trying to use for Audio.

There are other parameters that are more important for Audio
Like:
- easy to use
- good for audio impedance sources
- stable at unity gain
- low distortion at audio frequency
- low noise within audio band

-------------------------------------------------------

There are a few operational amplifier chips
that have good performance in all these things.
They are generally very good for most audio applications.

This has made them popular among Audio DIY People:)

Here is my list of such useful op-amps, for audio.
I think some posters can add a few more.

* NE5534 - Best audio/$price ever in History of op-amps!!!
* TL07x - for the money, maybe most useful JFET input op-amp
* OPA134, OPA2134 - single & dual version .. The King of DIY JFET input OP


* OPA227, OPA228 - Great BJT input Audio Amplifier, maybe the best audio/$price

By reading this forum, and the reviews & reports of many DIY
I now favour this op-amp.
OPA227 single ( stable for Unity gain Gain=1 )
OPA2227, dual version

OPA228 single ( stable for Gain >=5, if you need higher gain than Unity )
OPA2228, dual version


:cool: If you http://www.diyaudio.com/forums/search.php
search forum for OPA228, OPA227, OPA2227, OPA2228
you will find opinions from members
on this very good AUDIO Operational Amplifier

This means, it was Designed for very Good performance
... at audio frequency.

As to what we know of Human Hearing this is:
20 - 18.000 hertz
This is also the usual limits of Modern Mainstream LOUDSPEAKERS
most people buy, build and use.

TI Home > Semiconductors > Amplifiers and Linear > Audio Amplifiers > OPA228, High Precision, Low Noise Operational Amplifiers

****************************************************


So OPA228
will be a good match for your Speakers.
But not for your Video Player.


If you are into VIDEO amplifiers
we have a good forum for this:

diyAudio Forums > Top >The Moving Image


Regards,
lineup
Lineup Audio Lab
 
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