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Old 14th May 2016, 03:49 AM   #1
gorge is offline gorge  Australia
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Default no mention of active loads in Bob Cordell's book

Do you find it strange that there's not a single mention of the benefit of having an active load in his book?
An active load essentially means more gain and the benefit is huge in that you can trade gain for negative feedback. Maybe I'm searching using the wrong keyword?
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Old 14th May 2016, 05:18 AM   #2
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Dynamic (active) load variants are:
- Constant current source (CCS);
- Current mirror (CM);
- Bootstrap.

All of them are covered in the book.
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Old 14th May 2016, 07:35 AM   #3
gorge is offline gorge  Australia
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Bootstrapping isn't as good though because you can't have as much negative feedback for a given gain. This is because you don't have a very high impedance load in your VAS which is essentially what a current mirror behaves as.

And having a current mirror in itself doesn't necessarily mean that you're using it as a dynamic load. Does he discuss current mirrors when used in this way?
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Old 14th May 2016, 09:50 AM   #4
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If you want to discuss Bob Cordell's book or take issue with his treatment of topics and terminology, the best person to ask is Bob, in his book thread, where I'm sure he has an alert set for activity. I think it's reasonable to give him the opportunity of reply there. Given the many ways to describe and apply the same circuits and functional blocks in electronics, it's not surprising that they can be defined and named differently too.

I don't think starting a string of short Q&A threads is going to get the help you are seemingly asking for though and you could roll all the "but why" questions into one thread if you pick a considered title. Anyway, Bob's isn't a book that starts at basic audio electronics, more like a qualified technician level. You have to start at your own level.
Bob Cordell's Power amplifier book
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Old 14th May 2016, 12:58 PM   #5
AKSA is offline AKSA  Australia
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Gorge,

I regret the earlier thread and did not intend to encourage criticism. However, I think you were a bit impatient, and this I construed as unreasonable. However, it's over.........

Bootstraps can be useful. Because of the large, imperfect cap, they lose effectiveness at around 100KHz, and this is very useful because it loads up the voltage amplifier and quickly reduces loop, making it very easy to compensate the amp - that is, use a smaller lag compensation cap. In turn, this can improve the sound of the amp by improving slew rate.

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Old 14th May 2016, 01:55 PM   #6
gorge is offline gorge  Australia
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yeah, actually I misunderstood the definition of bootstrapping. It sounds like a difficult technique to get right due to the potential for positive feedback to cause instability.
I don't know why you wouldn't just use a current mirror for an active load as a way of increasing gain.

Also, by having positive feedback wouldnt that mean that you can't have global negative feedback? So that's another disadvantage.
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Old 14th May 2016, 03:57 PM   #7
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Quote:
Originally Posted by gorge View Post
yeah, actually I misunderstood the definition of bootstrapping. It sounds like a difficult technique to get right due to the potential for positive feedback to cause instability.
I don't know why you wouldn't just use a current mirror for an active load as a way of increasing gain.
Why do you think we don't use it this way? We do.

Quote:
Originally Posted by gorge View Post
Also, by having positive feedback wouldnt that mean that you can't have global negative feedback? So that's another disadvantage.
Why do you think so? Bootstrapping the VAS load from the amplifier's output is a very easy yet efficient technique, used by many designers. It actually improves stability due to the reasons just mentioned by Hugh.
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