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Old 29th April 2003, 12:52 PM   #1
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Thumbs up Output stages with gain Enhance Slew rate

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.
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Old 29th April 2003, 01:23 PM   #2
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Cool .....to continue...

...an interesting but rather impractical example of such a design, (by prof. E.M Cherry), is attached:
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Old 29th April 2003, 02:00 PM   #3
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Smile ...and...

...another, (slightly more practical), Australian design.....:
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Old 29th April 2003, 02:30 PM   #4
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Default Slew rate etc

Mikek,

Are you going to make a point, or did I already miss it?

Jan Didden
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Old 29th April 2003, 02:44 PM   #5
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Quote:
Are you going to make a point, or did I already miss it?
I think it is hidden in the title

Regards

Charles
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Old 29th April 2003, 02:45 PM   #6
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Default Re: Slew rate etc

Quote:
Originally posted by janneman
Mikek,

Are you going to make a point, or did I already miss it?

Jan Didden
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.....
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Old 29th April 2003, 02:56 PM   #7
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P.S: does anyone still want a copy of my electronics world paper published last year?...send me mail.
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Old 29th April 2003, 03:25 PM   #8
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Default output stuff

Of course I have a copy of your article.
Maybe I reply to your lengthy post later - got to run now.

Cheers,

Jan Didden
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Old 29th April 2003, 06:39 PM   #9
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Michael,

from practical point of view - what happens during saturation and limitation in some of the elements inside the NFB loop?

Pavel
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Old 29th April 2003, 08:13 PM   #10
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Quote:
Originally posted by PMA
Michael,

from practical point of view - what happens during saturation and limitation in some of the elements inside the NFB loop?

Pavel

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.
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