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Old 13th August 2013, 01:21 PM   #21
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Quote:
Originally Posted by wahab View Post
If you drop this resistor you ll change the caracteristic
of the circuit , either it s a CFB and this will increase
the OLG or it s a VFA and this will change the feedback
loop phase/gain response.
Absolutly, wahab. That's why i proposed to look in closed loop for the bandwidths and phase curves at the input of the second stage ( just after input signal has been mixed with NFB one).
You can compare that way the frequencies where feedback ratio begin to decrease (open loop flat frequency limit as an increase of the response curve level) and what happens at the limit of the frequency range of the amplifier, without modifying their behaviors.
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Old 13th August 2013, 01:35 PM   #22
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by wahab View Post
I think Mikek is talking of global NFB and in this matter
he s right , you cant go over 30-35dB GNFB at 20Khz
without stability issues, of course the total feedback
including local loops can be much higher at said frequency.
OK, thanks
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Old 13th August 2013, 01:36 PM   #23
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Originally Posted by Bigun View Post
my first 2c:

1) It seems to me that both CFA and VFA can get the job done and the differences between them are only to be heard when comparing two different amplifiers (since they start with different topologies). Hence, more often than not, differences between them are difficult to isolate since there are many factors known and suspected to affect the sound of the result. Compensation is a key difference, as already mentioned above. There are many topological options that use CFA and many that use VFA which allow the designer to provide for certain performance parameters (e.g. PSRR, slew rate, harmonic profile, gain-bandwidth etc.) So I bring into doubt the usefulness of the question, which is better between CFA and VFA.

2) Without casting opinions about anybody in particular, I would imagine that a good results is achieved when a designer uses the tools he/she knows best, whether CFA or VFA, or indeed FETs vs BJTs, or SS vs Tubes - amplifier artists and engineers might do best when using certain approaches because this is where their talent and experience is.
This is a very good summary. The only problem is that it leaves not much to argue about, and arguing is the soull of this thread

jan
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Old 13th August 2013, 01:36 PM   #24
Bricolo is offline Bricolo  France
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Quote:
Originally Posted by wahab View Post
I think Mikek is talking of global NFB and in this matter
he s right , you cant go over 30-35dB GNFB at 20Khz
without stability issues, of course the total feedback
including local loops can be much higher at said frequency.
Are you saying, in other words, that one cannot build an amp with an UGBW (unity gain bandwidth) larger than 600kHz-1MHz without stability issues?


Imagine your amp is set as unity gain follower (other cases are worse), having 30dB GNFB at 20kHz means that its open loop gain at this frequency is 30dB. With a 6dB/octave roll-off, 0dB is achieved at 640kHz (the UGBW)

That doesn't sound that much to me, nor impossible without stability issues.
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Old 13th August 2013, 01:49 PM   #25
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Default Amplifier 101...

Some simplified schematics/block diagrams would be educational for many to show/explain the modes of operation/differences of CFA/VFA.

Dan.
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Old 13th August 2013, 01:54 PM   #26
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by Max Headroom View Post
Some simplified schematics/block diagrams would be educational for many to show/explain the modes of operation/differences of CFA/VFA.

Dan.
This is quite informative.
Attached Files
File Type: pdf Current vs Voltage Feedback OA-30.pdf (80.3 KB, 552 views)
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Old 13th August 2013, 02:04 PM   #27
Bonsai is offline Bonsai  Taiwan
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Quote:
Originally Posted by michaelkiwanuka View Post
Actually, MIC, or input inclusive compensation does not allow more major loop gain than ordinary Miller compensation.

Secondly, I suspect, but haven't examined this yet, that the compensation network that is required to make the input inclusive compensation loop stable may worsen PSRR dramatically.

My suspicions are founded in the fact that the similar shunt compensation network required to make the minor loop stable with Jolly-McCharles input cascode inclusive compensation ruins the PSRR.
I cannot comment on this Michael. You may well be correct - I would have to sim this to see.

Where's Harry?
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Old 13th August 2013, 02:08 PM   #28
Bonsai is offline Bonsai  Taiwan
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Originally Posted by Bob Cordell View Post
Thanks for opening up this thread, Bonsai. It is a good topic for discussion.

Cheers,
Bob
No problem. Hopefully we can keep it on track.
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Old 13th August 2013, 02:16 PM   #29
Bonsai is offline Bonsai  Taiwan
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Quote:
Originally Posted by wahab View Post
Whatever the topology , the higher the total gain the more
you ll eventualy have to "dump" high frequency gain either
using local linearizing loops or more brutaly with a shunt
at said frequencies.

TPC will get you to much higher than 30-35 dB at 20 kHz and that's one of the tricks to get single digit ppm in VFA's. Similarly, TMC can allow about 15 dB of additional feedback at 20k.

I believe the correct statement is you cannot get much more than 30 or 35 dB of feedback at 20 kHz with standard MC.

Mosfets will also allow the ULGF to be pushed up so you may be able to claw a few more dB there as well
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Old 13th August 2013, 04:43 PM   #30
Jay is offline Jay  Indonesia
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Quote:
Originally Posted by Esperado View Post
it seems i'm the only one, here, to had made a CFA version of a VFA amplifier in real world.

I proposed that an expert in simulation take the L.C. VSSA schematic and make a VFA of it, removing the F.B. resistances and adding an other input pair in LTP topology instead.
I agree with the idea but with different proposal considering that:

(1) You started with a bad cfa amp when you modded Crescendo into cfa.

(2) We cannot say that solid state is better than tube by trying to replace the transistors in a blameless amp with tubes. We treat components based on its character and design objectives.

(Instead of comparing measurement result we can otoh put the minimum standard measurement as the objective to achieve with both cfa and vfa and compare the result by listening).

With vssa i can see M400 or P101 (non symmetrical version) as the equivalent vfa. Here the vfa tend to have better bass and cfa better highs.

Now pay attention to ska gb150d. Cfp output is not suitable for fast amp, especially high input capacitance hexfet. This will be a great challenge to turn the dual ltp input to become a cf one.
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