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Old 23rd August 2013, 02:09 PM   #501
Bonsai is offline Bonsai  Taiwan
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Quote:
Originally Posted by Waly View Post
Disagreeing doesn't make your statement right . Take a look at the typical schematic posted here.

The first observation is that in this case the second gain stage is not necessary a TIS, since the input impedance of the emitter follower is much higher than the output impedance of the input stage (R3/2). So the open loop input stage voltage gain is essentially (R3/2)/(R8/2+R21||R33). R33 is the feedback resistor and, as you can easily see, the gain modulation by the feedback resistor happens through the R21||R33 (this is the RF/RG term in my previous post). Because of the large closed loop gain, 18ohm||560ohm doesn't make much of a difference, hence the "CFA" effects are largely diminished, in the 2-5% limits I already mentioned.

Again, this is about the input stage topology (LTP vs. diamond buffer, etc...) rather than any CFA effects which are essentially diminished by the large closed loop gains required for a power amp.
Changing the feedback resistor changes the gm, whether its 2-5 % or some other value, right?

Your observation is correct where the CFA loop gain is high, but it does not hold for cases where the loop gain bandwidth is very wide. Two good examples are the recent designs I published on the forum. The sx has a loop bandwidth of c. 50 kHz and he nx-Amp about 10kHz IIRC.

Secondly, you are basing your observation on a large closed loop gain - what happens at other gain settings?
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Last edited by Bonsai; 23rd August 2013 at 02:15 PM.
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Old 23rd August 2013, 02:24 PM   #502
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Default Manso's simple CFA with good performance?

Manso, referring to what Wahab thinks is your circuit in #478 ..

Can you explain the need for R36/37 & R8/14 in your CFA amp? I know they provide emitter degeneration but in a CFA, the 'feedback resistor' R21 already provides degeneration.

Or is this a peculiarity of Diamond i/ps? Bonsai?

I have a sneaking suspicion they set the current for the i/p devices.
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Old 23rd August 2013, 02:27 PM   #503
mcd99uk is offline mcd99uk  United Kingdom
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Quote:
Originally Posted by Bonsai View Post
No, I am not finding that additional shunt comp is needed in sims, or in practice (sx and nx amps). The 1 pF you see for the shunt cap in the circuit I posted is a place holder. I did some sims yesterday on this very issue, and will stick with Alex comp in my designs for now, in addition of course to exploring other comp options.
Can't access your site at present. Do your amps use "enhanced VAS"?

This CFA compensation stuff is what's really interesting me at present.
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Old 23rd August 2013, 02:52 PM   #504
Bonsai is offline Bonsai  Taiwan
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Quote:
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Can't access your site at present. Do your amps use "enhanced VAS"?

This CFA compensation stuff is what's really interesting me at present.
I am sorry, it's down due a problem with the the hosting company. It should be back up in the next 12 hours of so.
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Old 23rd August 2013, 03:15 PM   #505
mcd99uk is offline mcd99uk  United Kingdom
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Originally Posted by Bonsai View Post
I am sorry, it's down due a problem with the the hosting company. It should be back up in the next 12 hours of so.
No need to be sorry. Your site has been a valuable resource to me.

When its back up I'm going to see if I can figure out why alex comp works for your amps and not my one. There must be some subtleties I'm missing.
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Old 23rd August 2013, 03:21 PM   #506
Waly is offline Waly  United Kingdom
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Quote:
Originally Posted by Bonsai View Post
Secondly, you are basing your observation on a large closed loop gain - what happens at other gain settings?
I've already mentioned that the "CFA" behaviour in these topologies is largely diminished by the high closed loop gain (as usual in audio power amps, where closed loop gains are 20-26dB or higher). Definitely, at lower closed loop gain, this topology behaves as a textbook CFA. The lower the closed loop gain, the lower the feedback resistor, the lower the stability and the lower the tolerance to any capacitance at the inverting input. Hence, less tolerance for any capacitance at the inverting input (including a lead compensation cap).
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Old 23rd August 2013, 03:23 PM   #507
Bonsai is offline Bonsai  Taiwan
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You need to run the cap from the TIS output back to the feedback summing junction.

Start off by doing straight shunt comp, once it's stable, then change it to Alex comp. You can reduce the value of th cap substantially ( usually half the shunt comp value) and get the same BW performance - however, your PM and slew rate will be much better
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Old 23rd August 2013, 03:45 PM   #508
RNMarsh is offline RNMarsh  United States
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Originally Posted by kgrlee View Post
This is my CFA offering. It started of as something for Dave Zan .. then I realised that JLH had several designs of almost identical topology. LC's VSSA is also very similar.
Bring it up to this century... push-pull the input and direct couple it - get rid of the C3 and C4. Might try more linear output stage -- Szikai output stage. Then, for fun compare with jFET compl/push-pull input stage.

Thx-RNMarsh

Last edited by RNMarsh; 23rd August 2013 at 03:47 PM.
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Old 23rd August 2013, 03:49 PM   #509
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Quote:
Originally Posted by kgrlee View Post
This is my CFA offering. [...]
An interesting detail of your offering is that the quiescent current through the transistors to which feedback is applied is defined by a Contant Current Source, just as it is the case for Long Tail Pairs of a VFA. Subsequently the operating conditions of the whole circuit are well defined, despite the fact that at very low frequencies, there is no open loop gain obtained from the Input Stage.

In the CFA diamond circuit, the current through the transistors receiving feedback depends of a biasing network placed ahead of their base. Then the operating conditions may be less dependable and each circuit may ask for checking of the current through every active device.
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Old 23rd August 2013, 03:52 PM   #510
RNMarsh is offline RNMarsh  United States
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Good points.
Are we back to a rush to build too soon?

-RNM
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