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Old 31st July 2007, 04:14 PM   #1
pooge is offline pooge  United States
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Default Doug Self editing errors

As many readers of Doug Self's articles and books are aware, the editing of Self's publications leaves a little to be desired...Ok, it sucks!

I thought I would start a thread as a common base to list these errors for Self and his publishers, in the oft chance that they would actually DO something about them, as many of the errors are carried over from his articles and prior editions.

This may also be a home base for gaining some clarity about some of his topics.

I hope that this can be a constructive dialog for the purposes expressed above, instead of yet another drawn out subjectivist bashing.
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Old 31st July 2007, 04:23 PM   #2
pooge is offline pooge  United States
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Default Vbe multiplier

I noticed inconsistencies between the "Thermal Dynamics in Audio Power" article in 10/96 issue of Electronics World, and the "Audio Power Amplifier Design Handbook", 4th ed.

First, I noticed the same editing errors in each. Table 1 of the article and Table 13.4 of the 4th edition of the book show inconsistent values from the Figures. The values of R1 and R2 appear reversed for the EF, and R3 appears to be reversed between the EF and CFP. I assume the figures are correct.

But what is more important is that in the article, the output of the Vbe in the article is taken at the lower end of R3, as shown in (A) of the attached figure, while in the book, it is taken at the upper end of R3, as shown in (B) of the attachment. I am not sure which one is correct.

I'm guessing that the book is the correct one, because he says that the intent of R3 is to subtract a correction voltage due to current changes.

However, in (C) of the attachment, Self shows a two transistor version of Figure 13.32 of his book, where the Vbe output is again taken at the top of the resistor.

Can anyone shed any light on this issue?


Does anyone have any practical experience with this two transistor circuit regarding oscillation problems or other consequences? Would R3 also be 1k for an EF output stage?
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Old 31st July 2007, 05:07 PM   #3
pooge is offline pooge  United States
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Default Two-pole compensation

Another question I have about Self's book is his two-pole compensation diagram of Figure 7.1d on page 188 of the 4th ed.
It is reproduced in (A) of the attached figure.

He shows one end of Rp attached to the negative rail. He also states "at HF, Cp2 has low impedance and allows Pr to directly load the VAS collector to ground..." on page 193.

Thus, it would appear that Rp should be attached to ground as shown in (B) of the attachment, rather than the voltage rail as shown in (A).

Moreover, one of Self's references to two-pole compensation here shows the resistor connected to ground.

So am I correct that (B) is the correct way to do this?

This would appear to be a good way to extend the open-loop bandwidth. Does anyone have any experience with this as far as stability issues or other disadvantages? It there a way of damping the rise in open-loop at high frequencies?
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Old 31st July 2007, 06:31 PM   #4
teemuk is offline teemuk  Finland
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Pooge,

Regarding your first question (VBE-multipliers), B is the correct configuration. That VBE-multiplier topology has three connection nodes in oppose to two. You can simulate all three configurations (conventional and the ones without extra resistor and transistor) and see how they differ from each other if you feed the collectors with a steadily rising current. The two-transistor version, with the added resistor, stays very linear after a certain point but is in my experience just too unstable.
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Old 31st July 2007, 06:53 PM   #5
pooge is offline pooge  United States
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Thanks.

If (B) is correct, and is used in a complimentary VAS, what would be the effect of R4 on the signal? R4 is between the output of the upper VAS and the upper bias voltage. There is no corresponding resistor between the lower bias voltage and the lower VAS output.

Does this matter to the signals, or should another like resistor be placed in the lower half?
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Old 31st July 2007, 09:52 PM   #6
teemuk is offline teemuk  Finland
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No need for another resistor. The whole VBE-multiplier as is can safely be in between the complementary VAS stages - it's the injection points to drivers/output transistors you need to worry about. You want to create a steady bias voltage between the bases of the drivers/output transistors - not in between the VAS stages. The schematic diagrams of Self's book show pretty clearly the take-off points for the bias voltage. I think this is pretty self-explanatory once you think it for a moment.

The effects of using both VBE-multiplier circuit variations are explained quite well in Chapter 12 of Self's Audio Power Amplifier Design Handbook, Third Edition. See "Current compensation", pages 362 - 364. Especially examine the figure 12.33 of the concerned book. Also, you can run the SPICE simulation I mentioned and see the effects yourself. IMO, you can visualize how the circuits work much better if you simulate them. Self's book just tells why they do what they do.
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Old 1st August 2007, 12:06 AM   #7
GK is offline GK  Australia
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Default Re: Two-pole compensation

Quote:
Originally posted by pooge
Another question I have about Self's book is his two-pole compensation diagram of Figure 7.1d on page 188 of the 4th ed.
It is reproduced in (A) of the attached figure.

He shows one end of Rp attached to the negative rail. He also states "at HF, Cp2 has low impedance and allows Pr to directly load the VAS collector to ground..." on page 193.

Thus, it would appear that Rp should be attached to ground as shown in (B) of the attachment, rather than the voltage rail as shown in (A).

Moreover, one of Self's references to two-pole compensation here shows the resistor connected to ground.

So am I correct that (B) is the correct way to do this?


Either way would work. By "ground" he would mean "AC ground". The supply rails are at "AC ground" beacause they are bypassed.


Cheers,
Glen
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Old 1st August 2007, 12:12 AM   #8
pooge is offline pooge  United States
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Default Re: Re: Two-pole compensation

Quote:
Originally posted by G.Kleinschmidt


Either way would work. By "ground" he would mean "AC ground". The supply rails are at "AC ground" beacause they are bypassed.

Cheers,
Glen
Thanks. I was thinking he should have said signal common when connecting to the rail, since it is a common emitter. But your analysis makes sense.
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Old 1st August 2007, 12:23 AM   #9
sam9 is offline sam9  United States
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Regarding the Vbe configurations:

I've used both B and C as opposed to just simulating them with an EF output section. Both function. My instrumentation isn't sufficient (hobby budget) to unambiguously say one or the other results in lower THD or better thermal tracking. C is harder to set but not impossible; you just need patience.

This is not a definitive evaluation but rather a recounting of practical experience of a hobbyist with limit resources.
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Old 1st August 2007, 12:28 AM   #10
sam9 is offline sam9  United States
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Regarding two pole compensation:

I believe A is correct. Maybe B will work as well. The affect of A was within my means to detect and it definitely reduces THD at higher frequencies. Choosing the wrong value of Rp can cause some oscillation problems but I found you have quite a bit of latitude which may depend on the rest of the amp.
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