John Curl's Blowtorch preamplifier part II
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diyAudio Member

Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 6
Quote:
Well that's a nice story, but I have a 1954 electronics book that explains the concept of slew rate in amplifiers and how to design for high SR. Written by a US Naval Captain who taught at the Naval Tech University. What were you guys doing 20 years later???

So if you guys say things were not obvious probably means you read the wrong books

Anyway, we know that 'just degenerating the input stage' does NOT increase SR. Increasing the Iq in the input stage does. However, that might require a larger comp cap due to the increased transconductance, undoing the rise in SR you try to achieve. Fortunately, you could degenerate the input stage to lower transconductance again so you got your increased SR.

Agreed?

jan didden
__________________
/Yes! Its out: Linear Audio Vol 5!
I'm not an "accademic", just a plodder who loves a challenge - Ian Hegglun

Last edited by jan.didden; 28th May 2011 at 08:11 PM. Reason: sp

diyAudio Member

Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 6
Quote:
 Originally Posted by scott wurcer NO, NO, and NO they will have the same frequencies. There are people here I assume that are interested in some of the basic principles of engineering. John just ask directly THIS question to Ron Quan and his advisor Tom Lee (an old friend) they will set you straight. Volterra analysis extends to arbitrary non-linear systems even those with memory. The output with input at f1 and f2 can ONLY be at n*f1+-m*f2. Simple amplifiers do not fall into the non causal non time invariant category.
I'm not an expert on PIM, but I have always wondered how something that modulates the PHASE of a freq component could lead to additional harmonics?

jan didden
__________________
/Yes! Its out: Linear Audio Vol 5!
I'm not an "accademic", just a plodder who loves a challenge - Ian Hegglun

 28th May 2011, 08:12 PM #12543 diyAudio Member   Join Date: Jul 2003 Location: berkeley ca Jan, please look at Barrie Gilbert's paper for this info.
 28th May 2011, 08:32 PM #12544 diyAudio Member   Join Date: Jul 2003 Location: berkeley ca Moving on, Otala had published some theoretical papers starting in 1969 that promoted high open loop bandwidth and this tended to give high slew rate. BUT slew rate limiting is NOT TIM, it is the clipping that is caused by the TIM producing cap charging, the cap current being non-linear. In fact, a high slew rate op amp can have lots of TIM at much lower levels, IF a class B input stage is used. This is what Matti Otala wanted to include: The exceptions to the general rule, for example the uA741S, a high slew rate 741. Find one and measure it. You will find high TIM(SID) at moderate operating levels. So much for slew rate being the ultimate and complete measurement necessary. (more later)
diyAudio Member

Join Date: Sep 2006
Quote:
 Originally Posted by janneman Well that's a nice story, but I have a 1954 electronics book that explains the concept of slew rate in amplifiers and how to design for high SR. Written by a US Naval Captain who taught at the Naval Tech University. What were you guys doing 20 years later??? So if you guys say things were not obvious probably means you read the wrong books Anyway, we know that 'just degenerating the input stage' does NOT increase SR. Increasing the Iq in the input stage does. However, that might require a larger comp cap due to the increased transconductance, undoing the rise in SR you try to achieve. Fortunately, you could degenerate the input stage to lower transconductance again so you got your increased SR. Agreed? jan didden
Hi Jan,

Yes, slew rate and how to control it was fairly well-known for a long time. Unfortuantely, it was not considered adequately in the early power amplifier designs. Matti deserves credit for helping to shine the light on the need for high slew rate and good input stage linearity. As you point out, degenerating the input stage is a very effective and popular way to increase slew rate. The degeneration decreases the input stage transconductance without reduce input stage current capability. With reduced input stage transconductance, a smaller compensation capacitor is needed, which results in a higher slew rate. Input stage degeneration also linearises the input stage, greatly reducing "soft-TIM" (sub-slewing HF distortion).

Where Matti went off the tracks was in blaming high TIM on low open-loop bandwidth. It is very easy to show that when things are compared on the proper basis, apples to apples, low open-loop bandwidth does not decrease slew rate and does not increase TIM.

The important thing to keep in mind is that for a given loop gain at 20kHz, a smaller open-loop bandwidth simply corresponds to a higher feedback factor at low frequencies.

Cheers,
Bob

diyAudio Member

Join Date: Jan 2004
Location: cambridge ma
Quote:
 Originally Posted by john curl Jan, please look at Barrie Gilbert's paper for this info.
Sorry it's not there either, frankly I don't see what the fuss is about.
__________________
Clay is embedded in our subconscious. It has been there for at least 50,000 years.

diyAudio Member

Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 6
Quote:
 Originally Posted by john curl Jan, please look at Barrie Gilbert's paper for this info.
No I won't. SR is well understood and it has been explained again and again how Barrie viewed this, and your misunderstanding of it.

jan didden
__________________
/Yes! Its out: Linear Audio Vol 5!
I'm not an "accademic", just a plodder who loves a challenge - Ian Hegglun

diyAudio Member

Join Date: Sep 2006
Quote:
 Originally Posted by janneman I'm not an expert on PIM, but I have always wondered how something that modulates the PHASE of a freq component could lead to additional harmonics? jan didden
Hi Jan,

Every 6 months or so the PIM thing gets brought up again and everthing is thrashed out on both sides again. Those who want a good explanation of PIM, what causes it, and how NFB is involved, just need to go to my web site and read the JAES paper on PIM that I wrote. While Barrie Gilbert's long-subsequent paper is technically correct, it is really not a complete treatment. Moreover, John is putting words in Barrie's mouth when he asserts that Barrie's paper shows that low open-loop bandwidth exacerbates PIM.

The conclusions in my paper are:

1) PIM does indeed exist.

2) It is basically what has been known by video engineers forever as differential gain and phase.

3) One form of PIM is indeed "caused" by NFB and I show the math for predicting it.

4) the PIM generated by NFB depends on the closed loop bandwidth of the amplifier, NOT the open-loop bandwidth.

5) the PIM caused by negative feedback depends for its creation on the conversion of amplitude intermodulation distortion to phase intermadulation distortion. PIM from this mechanism cannot exist without substantial amounts of AIM (which is easily detectable).

6) Amplifiers without negative feedback have PIM as well, from numerous sources, only one of which is nonlinear junction capacitances.

7) Negative feedback actually REDUCES the PIM from #6.

In any case, those who want a fairly decent coverage of PIM should check out my paper, to at least understand the arguments. They certainly don't have to agree with me.

Cheers,
Bob

 28th May 2011, 09:24 PM #12549 diyAudio Member     Join Date: Jan 2004 Location: cambridge ma Please folks get the BS straight. PIM somehow magically creates distortion at impossible in-harmonic frequencies. The stuff that uccurs at the normal predictable frequencies is no problem. __________________ Clay is embedded in our subconscious. It has been there for at least 50,000 years.