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Old 1st April 2013, 06:30 PM   #761
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Quote:
Originally Posted by magnoman View Post
Slew rate: Nothings free.

Don't forget CFAs, rather, understand their subtleties as one day a requirement will come along where such a cfa will be superior.

My 2 cents.

Thanks
-Antonio
Out of curiosity, I did a comparative simulation of a VFA and a so called 'CFA' input stage. Provided that the CFA stage is correctly configured and provided that the trannies are truly complementary (not possible in real life), the CFA stage produced 2000 times less distortion.

One more 2 cents.

Cheers,
E.
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Old 1st April 2013, 06:43 PM   #762
nattawa is offline nattawa  Canada
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Quote:
Originally Posted by Edmond Stuart View Post
Out of curiosity, I did a comparative simulation of a VFA and a so called 'CFA' input stage. Provided that the CFA stage is correctly configured and provided that the trannies are truly complementary (not possible in real life), the CFA stage produced 2000 times less distortion.

One more 2 cents.

Cheers,
E.
Good to know that! However, since a truly complementary is non-existed in real life a study of the distortion behavior over trannie mismatching between VFA and so-called CFA might give more insight. Thanks for the two cents, unfortunately in good old Canada, the pennies has recently been discontinued of being a legal tender, no longer can one cash a penny here in real life any more, though we do see it in papers such as our bills and government notice about taxes we owe .
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Old 1st April 2013, 07:14 PM   #763
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With unmatched transistors the difference is far less spectacular: only about four times better than VFA.

Cheers,
E.
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goed verliezen dan dooft het licht…(H.M. van Randwijk)
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Old 1st April 2013, 08:39 PM   #764
dadod is online now dadod  Croatia
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Quote:
Originally Posted by Edmond Stuart View Post
Out of curiosity, I did a comparative simulation of a VFA and a so called 'CFA' input stage. Provided that the CFA stage is correctly configured and provided that the trannies are truly complementary (not possible in real life), the CFA stage produced 2000 times less distortion.

One more 2 cents.

Cheers,
E.
Hi Edmond, could you show simulated amps circuit? In my simulations I've got just opposite, must be that I do something wrong.
BR Damir
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Old 1st April 2013, 08:49 PM   #765
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Okay

Cheers,
E.
Attached Images
File Type: png CFAvsVFA.png (28.1 KB, 77 views)
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Old 1st April 2013, 09:04 PM   #766
dadod is online now dadod  Croatia
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Quote:
Originally Posted by Edmond Stuart View Post
Okay

Cheers,
E.
This is not fair comparison. As CFA you use current conveyor and ideal output stage(as in VFA), but a current conveyor is very load sensitive and with real output stage VFA will produce lower distortion(at least in all my simulation attempts).
BR Damir
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Old 1st April 2013, 09:06 PM   #767
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Quote:
Originally Posted by Edmond Stuart View Post
Out of curiosity, I did a comparative simulation of a VFA and a so called 'CFA' input stage. Provided that the CFA stage is correctly configured and provided that the trannies are truly complementary (not possible in real life), the CFA stage produced 2000 times less distortion.
Yes, but with less open loop gain available, significant nonlinearities elsewhere in the path really kills. Really evident making changing an ideal voltage follower have a nonlinear term.

As far as mismatching I typically start with an ideal transistor set of parameters ie IS={IS} BF={BF} CJE={CJE}, common between npn and pnp transistors except that one set has an additional multiplier M so these parameters look like IS={M*IS}, BF={M*BF} etc. Typically will add reality to parameters one or a few at a time, then add mismatching.
I was actually surprised how much mismatch was allowed before reaching the same level of distortion.

I've been fasinated by the subtle differences in the i-v curves of the input transistors between cfa and vfa or voltage steering versus current steering.

Thanks
-Antonio
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Old 1st April 2013, 09:12 PM   #768
wahab is offline wahab  Algeria
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Quote:
Originally Posted by Edmond Stuart View Post
the CFA stage produced 2000 times less distortion.

.
Three orders of magnitude is a lot, too much in fact.

Once you have extracted the available gain NFB is the only thing
that will reduce non linearities , so either there s tremendously
higher OLG , or way better linearity to start with , wich is not
possible given that it s the same active components.
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Old 1st April 2013, 09:24 PM   #769
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Ideally you can get real close to 0 nonlinearity with optimal degeneration of the cfa emitters.

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-Antonio
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Old 1st April 2013, 09:30 PM   #770
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Exactly (that is, under ideal conditions)
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