The hypothetical op-amp., G1, (see attached), with a slew rate of 40V/uS, is connected to an output stage, G2, with a gain of 10, and the whole is enclosed in global feedback loop, giving an overall closed loop gain of 40.
It is straightfowardly demonstrated that the output slews at approx. 400V/us. This obviously assumes that the output stage possess a slew rate well in excess of this figure, otherwise the overall slew rate is then determined by the output stage.
This is almost invariably the case, and should therefore not be a concern.
It is straightfowardly demonstrated that the output slews at approx. 400V/us. This obviously assumes that the output stage possess a slew rate well in excess of this figure, otherwise the overall slew rate is then determined by the output stage.
This is almost invariably the case, and should therefore not be a concern.
Re: Slew rate etc
Hi Jan
,
O.K...Most designs of this type run the output stage in common emitter mode....this is essentially a bad idea, because the global feedback loop ceases to track the output when the amp. is driven into clip, ergo the output tends to latch up to one or other supply rail.
Moreover, output stage bias stability in some of these schemes is rather suspect......
A much better scheme, which preserves the advantages of an output stage with gain, without the latch-up problem, is one in which a push-pull complementary Wilson mirror, biased into class A, (a la Alexander amp.), is used to generate the output stage gain. The output stage proper can then be a traditional unity gain follower.
I do not however recommend the current feedback technique used in the Alexander amp.
...Oh...and the complementary Wilson mirror can be biased into class A with a negative feedback bias generator...with the Vbe multiplier used to bias just the output stage...
Another advantage with using an output stage with gain greater than unity, is that the main input gain stage runs on lower supply rails......and can thus be a discrete op. amp., with high quality, high ft transistors.
These tend to be available mainly in low-voltage specs....Non-dominant poles can be therefore be relegated well beyond the unity gain bandwidth of the amp., making the whole easier to stabilise.....
janneman said:
Hi Jan
,O.K...Most designs of this type run the output stage in common emitter mode....this is essentially a bad idea, because the global feedback loop ceases to track the output when the amp. is driven into clip, ergo the output tends to latch up to one or other supply rail.
Moreover, output stage bias stability in some of these schemes is rather suspect......
A much better scheme, which preserves the advantages of an output stage with gain, without the latch-up problem, is one in which a push-pull complementary Wilson mirror, biased into class A, (a la Alexander amp.), is used to generate the output stage gain. The output stage proper can then be a traditional unity gain follower.
I do not however recommend the current feedback technique used in the Alexander amp.
...Oh...and the complementary Wilson mirror can be biased into class A with a negative feedback bias generator...with the Vbe multiplier used to bias just the output stage...
Another advantage with using an output stage with gain greater than unity, is that the main input gain stage runs on lower supply rails......and can thus be a discrete op. amp., with high quality, high ft transistors.
These tend to be available mainly in low-voltage specs....Non-dominant poles can be therefore be relegated well beyond the unity gain bandwidth of the amp., making the whole easier to stabilise.....
PMA said:
by 'saturation' am i correct in assuming you mean clipping?and what exactly do you mean by 'limitation'?
Oh well...i'll just assume you mean slew rate limiting.....
Assuming single miller compensation for the first gain block, G1, it is evident that the compensation capacitor need swing only 4V to cause an ouput swing of 40V. Hence the increase in effective slew rate.
While of course in an ordinary unity gain output stage, the compensation capacitor has to swing the full 40V.
...and...
...more examples......download manuals and/or schematics in pdf:
http://www.a-and-t-labs.com/products.htm
...and then of course we have Hafler's designs:
http://www.hafler.com/home/
(click on 'library archive' )
...more examples......download manuals and/or schematics in pdf:
http://www.a-and-t-labs.com/products.htm
...and then of course we have Hafler's designs:
http://www.hafler.com/home/
(click on 'library archive' )
Walt Jung built an amplifier with gain in the output stage in this article:Walt Jung´s AD823/812 amplifier
I guess he knows what he is doing.
Nicke
I guess he knows what he is doing.
Nicke
Pardon the redundancy.....
But what is the point?
Slew rate is vastly overrated to begin with.
The easiest way to increase slew rate is degeneration in the input stage to lower its transconductance.
Something the Prof. Cherry doesn't believe in. Must be why he designs theoretical amps that are "impractical". (Your words, not mine. Mine are harsher.)
Jocko
But what is the point?
Slew rate is vastly overrated to begin with.
The easiest way to increase slew rate is degeneration in the input stage to lower its transconductance.
Something the Prof. Cherry doesn't believe in. Must be why he designs theoretical amps that are "impractical". (Your words, not mine. Mine are harsher.)
Jocko
Re: Pardon the redundancy.....
Correct.....
...up to a point...yes...Overdo it though, and you have intolerable offsets at output...more input noise...etc.....
.....Of course it would be idle to pretend a 400V/uS slew rate is desirable......you would have to generate a peak swing in excess of 3KV, (>560KW into 8Ohms), for this to be the case...........
However, i thought it might be a good idea to share one side effect of output stages with gain that you will not find written about anywhere.....at least as far as i know....
Moreover, the first gain stage encapsulating the main compensation network need not necessarily be an expensive discrete circuit, with sub-optimal, high-voltage transistors, (read mediocre Ft),.....a simple high quality, but comparatively inexpensive op-amp., (e.g: AD8610, OPA627, AD797), would be entirely acceptable.
...you would have to show that Cherry hasn't infact built his 'theoretical' designs....which is an entirely different order of things from being merely 'impractical'....... otherwise this is idle speculation.......
Jocko Homo said:
Correct.....
Jocko Homo said:
...up to a point...yes...Overdo it though, and you have intolerable offsets at output...more input noise...etc.....
.....Of course it would be idle to pretend a 400V/uS slew rate is desirable......you would have to generate a peak swing in excess of 3KV, (>560KW into 8Ohms), for this to be the case.......
....However, i thought it might be a good idea to share one side effect of output stages with gain that you will not find written about anywhere.....at least as far as i know....
Moreover, the first gain stage encapsulating the main compensation network need not necessarily be an expensive discrete circuit, with sub-optimal, high-voltage transistors, (read mediocre Ft),.....a simple high quality, but comparatively inexpensive op-amp., (e.g: AD8610, OPA627, AD797), would be entirely acceptable.
Jocko Homo said:
...you would have to show that Cherry hasn't infact built his 'theoretical' designs....which is an entirely different order of things from being merely 'impractical'....... otherwise this is idle speculation.......
Where to start.......
Maybe if you had stated that the purpose was to show how one can design with an op-amp, and then an output stage with gain; then maybe my inbox would not be full of confused questions directed at me (intead of you!).
And since you can do that, then you understand why Walt Jung is expected to design such objects.
(If it makes you feel any better, I have worked with a lot of the items you reference.)
Maybe you should more closely read the old stack of AES Journals that I suspect you might have. Marshall Leach ahs written articles explaining input degeneration, and if his advice is followed, it will not have the problems that you allude to.
And if you do read them, then you will see pompous twits like Prof. Cherry opine piously on how stupid Prof. Leach is.
No one that I know suggests using sub-optimal parts. That is your assumption.
Prof. Cherry claims to have built his amps, but then he uses terms like "non plus ultra" to descibe them. Yeah, they measure low distortion, and his awe struck studants may be agog at it, but does he make his living designing and building amps?
No, he doesn't. So his drivel is merely noise to my ears.
Jocko
Maybe if you had stated that the purpose was to show how one can design with an op-amp, and then an output stage with gain; then maybe my inbox would not be full of confused questions directed at me (intead of you!).
And since you can do that, then you understand why Walt Jung is expected to design such objects.
(If it makes you feel any better, I have worked with a lot of the items you reference.)
Maybe you should more closely read the old stack of AES Journals that I suspect you might have. Marshall Leach ahs written articles explaining input degeneration, and if his advice is followed, it will not have the problems that you allude to.
And if you do read them, then you will see pompous twits like Prof. Cherry opine piously on how stupid Prof. Leach is.
No one that I know suggests using sub-optimal parts. That is your assumption.
Prof. Cherry claims to have built his amps, but then he uses terms like "non plus ultra" to descibe them. Yeah, they measure low distortion, and his awe struck studants may be agog at it, but does he make his living designing and building amps?
No, he doesn't. So his drivel is merely noise to my ears.
Jocko
Arm chair engineering
First off, Slew rate:
From http://www.passdiy.com/pdf/a40.pdf by Nelson Pass
The combination produced phenomenal high frequency power which hit -25dB (reference peaks below 20kHz) at 50kHz. If the amplifier were clipped, I measured the 30V/us recovery, but with unclipped performance, the highest measured slew was
1.5V/us.
In other recordings and with other cartridges, .5V/us peak slew was more common and was generally associated with cymbals and synthesizers but not piano or vocal material. I found this interesting as I had always imagined higher slews than I found. It certainly makes one wonder whether the value of high
slew rate specifications isn't like damping factor, where very high values
show diminishing returns in sonic performance.
Second........
Walt Jung built an amplifier with gain in the output stage in this article: Walt Jung´s AD823/812 amplifier. This is not even a power amplifier but is a composite op amp preamp circuit for preamp line level use. Both the op amps use unity gain output stages if I am not mistaken.
Last and least.......
If people on going to provide links, please link what you are referring to so one doesn't have to wade through the web site to see what you found of interest For example:
http://www.hafler.com/techsupport/index.asp?ID=3 and tell us which of the couple dozen manuals you are referencing.
http://www.a-and-t-labs.com/K6_Sw_Amp/art_schematic/k6amp.pdf
(This amp has some design features that are a big enough liabilities to make the topology selection pretty irrelevant. This is based on actual experience and not schematic browsing by the way.)
The "I wrote a Spice model or drew a schematic" amplifier design school is now be reduced to include block diagrams! Everyone raise their hand that has actually built a real amplifier. I am reminded of the news "analysis" programs where a bunch a journalist sit around and speculate over issues that they have 1% of the information about.
No mention of the Nelson Pass designs which are some of the best amps with gain around the output stage?
First off, Slew rate:
From http://www.passdiy.com/pdf/a40.pdf by Nelson Pass
The combination produced phenomenal high frequency power which hit -25dB (reference peaks below 20kHz) at 50kHz. If the amplifier were clipped, I measured the 30V/us recovery, but with unclipped performance, the highest measured slew was
1.5V/us.
In other recordings and with other cartridges, .5V/us peak slew was more common and was generally associated with cymbals and synthesizers but not piano or vocal material. I found this interesting as I had always imagined higher slews than I found. It certainly makes one wonder whether the value of high
slew rate specifications isn't like damping factor, where very high values
show diminishing returns in sonic performance.
Second........
Walt Jung built an amplifier with gain in the output stage in this article: Walt Jung´s AD823/812 amplifier. This is not even a power amplifier but is a composite op amp preamp circuit for preamp line level use. Both the op amps use unity gain output stages if I am not mistaken.
Last and least.......
If people on going to provide links, please link what you are referring to so one doesn't have to wade through the web site to see what you found of interest For example:
http://www.hafler.com/techsupport/index.asp?ID=3 and tell us which of the couple dozen manuals you are referencing.
http://www.a-and-t-labs.com/K6_Sw_Amp/art_schematic/k6amp.pdf
(This amp has some design features that are a big enough liabilities to make the topology selection pretty irrelevant. This is based on actual experience and not schematic browsing by the way.)
The "I wrote a Spice model or drew a schematic" amplifier design school is now be reduced to include block diagrams! Everyone raise their hand that has actually built a real amplifier. I am reminded of the news "analysis" programs where a bunch a journalist sit around and speculate over issues that they have 1% of the information about.
No mention of the Nelson Pass designs which are some of the best amps with gain around the output stage?
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