Hump in Closed Loop Frequency Response -- Root Cause ? - diyAudio
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Old 10th January 2007, 08:57 AM   #1
EUVL is offline EUVL  Europe
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Default Hump in Closed Loop Frequency Response -- Root Cause ?

I am posting a general question and am hoping for some constructive ideas.

In the few Class-A power amps that I have built, typically an Aleph-X, the closed loop frequency response always seems to have a hump before the -6dB decay, indicating inductivity somewhere.
Of course one can damp this down by increasing the value of the capacitor across the feedback resistor.

But I wonder where the inductivity comes from and how it may be removed.

The frequency response as shown is with a speaker as load, but it does not change much even if the speaker is removed and replace with say a 100 ohm power resistor.
It also changes little even if one increases the values of the gate resistor of the output FETs.

Any comments as to what the root cause of the hump might be ?



Thanks,
Patrick
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Old 10th January 2007, 09:48 AM   #2
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it´s a typical phenomenon in feedbackloops. There is not necessarily a inductivity involved. Looks like a lack of phase margin (dt. 'Phasenreserve').
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Old 10th January 2007, 10:02 AM   #3
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So Jürgen, take Aleph-X as an example, how would you change the open loop characterisitcs to increase the phase margin ?


Patrick
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Old 10th January 2007, 10:25 AM   #4
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hehe, you got me.
I´m not familiar with that design. Check out for clean square wave response on reactive loads, long cabling etc.
Probably the small gain peaking won´t hurt.
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Jürgen
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Old 10th January 2007, 11:57 AM   #5
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While I take your point about phase margin, I am not sure whether I agree entirely that it has nothing to do with inductivity.

You can have too small phase margin and hence instability even without that hump. But the hump indicates excessive gain at high frequencies. In fact, if you take out the feedback cap entirely, the hump would be more prominent.

My question was not how to suppress that hump. One can just increase the feedbacl cap. Rather, I wonder what causes the hump in the first place, and whether it can be removed without adding a feedback cap. Feedback cap is the easy way out.


Patrick
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Old 10th January 2007, 07:51 PM   #6
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The hump means that you have a higher Q value to the rolloff
than you probably want, meaning there are at least 2 poles
of rolloff operating before you reach the unity gain point at high
frequencies.

There is nothing technically wrong with this, but as a rule we
like to remove it for sonic or at least aesthetic reasons. You
can do this by lowering the frequency of one of these poles
(that is to say, slow down some portion of the circuit) or by
raising the frequency of one of the poles (speeding up some
part of the circuit).

Usually we just trim the feedback cap, and that works fine.

I should add the a 500 KHz rolloff is quite fast, so you have
some bandwidth to burn IMHO.

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Old 11th January 2007, 06:15 AM   #7
EUVL is offline EUVL  Europe
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Thank for the advice. I shall look into it further.

I have tried the feedback cap route and ended up with 300kHz -3dB. Still very fast, but I take it as a challenge to cure the hump rather than correct for it with the feedback loop.

Pending further investigation. : )


Patrick
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Old 11th January 2007, 09:46 AM   #8
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Hi Patrick,

the other caps to look at are in the active current sources. I remember the same thing when building my Aleph-X´s. It must be somewhere at the beginning of the thread.

William
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Old 11th January 2007, 10:01 AM   #9
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William,

It is a general problem not specific to Aleph-X.

For example, my Xed Lindsley Hood MOSFET has a similar response, and it uses a phase splitter to drive the output FETs instead of the Aleph Current Multiplier.

Of course the phase splitter also has its own bandwidth, phase shifts, etc.


Patrick
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Old 11th January 2007, 10:56 AM   #10
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Quote:
Originally posted by EUVL
[snip] But the hump indicates excessive gain at high frequencies. In fact, if you take out the feedback cap entirely, the hump would be more prominent.[snip]Patrick

Another way to say this is that you have still a lot of gain when the phaseshift increases. Such a case means that the negative feedback tends to change to positive feedback which means increase of gain rather than decrease. As Nelson pointed out, you can decrease the phase shift, or increase the gain roll-of with frequency to get rid of it. Problem is often that these things are coupled, for instance if you increase the gain roll-off with a cap you probably at the same time increase phase shift so it doesn't get you very far.
Or you can increase the closed loop gain (less feedback).

Jan Didden
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