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Bob Cordell's Power amplifier book
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Old 11th December 2019, 04:04 PM   #9921
Bob Cordell is offline Bob Cordell  United States
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Quote:
Originally Posted by Fred Soop View Post
Bob,

Question regarding IPS degeneration, IPS current, and Cmiller. A common configuration seems to be Re=100 Ω, Cm=100 pF and I=1-4 mA. It's obvious that increasing Re allows for reducing Cm and it would seem that a higher current would also increase the slew rate, but both are at the expense of some increase in noise. I've seen Re as high as 470 Ω with Cm as low as 30 pF in some designs. That would include your figure 3.14, the high power version of the design evolution, (and figure 9.2, though IPS current is reduced to 1 mA). Why the change to that particular version after using 220 Ω and 62 pF in the versions just before it? Then, figure 4.1 shows 220 Ω and 27 pF.

What are the tradeoffs and how high could one reasonably go with Re to minimize the size of Cm? What is the optimum configuration, or is optimum really over a range of values? Also, is there any reason to change the degeneration resistor values in the current mirror?

Thanks.
Hi Fred,

There are many judgment calls in picking the IPS emitter degeneration and the optimums are quite broad. I usually set emitter degeneration at a factor of about 10. This means that the degeneration resistor is about 10X the value of re' of the transistor being degenerated. Put another way, approximately, 10X degeneration reduces the CE transconductance of a stage by a factor of about 10. Introducing degeneration does indeed increase input voltage noise, but the relative amount by which it increases it depends on the degree of contribution from other sources, such as the resistances in the base circuits. These noise issues are covered in the noise chapter in my book. For a given size of Cm, increased degeneration allows for increased LTP current, which increases slew rate. However, with a BJT input stage, there is a point at which the resulting input base current shot noise will increase overall noise if the impedance in the base circuits is high enough.

I often like to keep Cm in the range of 30 pF when using a 2T VAS, but this is again a broad optimum. I have at times gone lower and higher. Going significantly higher forces the VAS to supply more current at high frequencies to charge it, which is not necessarily desirable.

I also usually degenerate current mirrors by a factor of about 10, as discussed by Mark below. A current mirror with little or no degeneration will increase noise. Degenerating to the point where there is 200-300 mV across the degeneration resistor is usually a good choice, and usually corresponds to about 10X degeneration.

Cheers,
Bob
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Old 12th December 2019, 10:00 AM   #9922
jan.didden is offline jan.didden  Europe
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For those in Europe:

Designing Audio Power Amplifiers (2nd Edition) - Elektor

Jan
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Old 12th December 2019, 05:24 PM   #9923
Fred Soop is offline Fred Soop  United States
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Thanks, Bob. I think I'm finally getting this thing tuned in, though it appears that there may be some strange artifacts as a result of going out to a breadboard, rather than a perf board in the unit. I may make the changes on the installed board and see what happens.

I DO have the bias spreader tuned in quite well. The drivers are running at a high enough current that the speed up capacitor can be eliminated. This means that the drivers will heat up initially but will then remain at a relatively constant temperature. So, it is necessary to have 2 diode connected transistors, one sandwiched on a driver, the other clamped to a TO-3 output.

Temperature was stabilized, bias set to 24 mV. Unit then allowed to get cold overnight. On cold startup, bias was close to 24 mV within 10 seconds and tracked properly until temperatures stabilized. Next, the output was driven until the transistors were too hot to touch. Returned to idle and the bias returned to close to 24 mV within 5 seconds. I don't think we can get any better than that.

Cheers,
Fred
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Old 12th December 2019, 10:06 PM   #9924
PB2 is online now PB2  United States
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Quote:
Originally Posted by DouglasSelf View Post
Any non-linearity anywhere in the amplifier will cause the input currents to be non-linear, because the NFB loop forces the amplifier output to be linear.
There was an article by Taylor in Aug/Sept Wireless World 1977 that discussed the non-linear
input impedance of a diff pair that we discussed back in 2005. andy_c offers some insight in
this post:
Thoughts Concerning Cordell, Otala, and Gilbert papers

This is the reference, there was a part 2 in Sept:
"Distortion in low-noise amplifiers" by Eric F. Taylor, Wireless World August, 1977

I seem to recall that we had this scanned, not sure if it is posted anywhere. I have hard
copy somewhere around here from when I read it back in the 1970s.

Found it online: http://www.keith-snook.info/wireless...amplifiers.pdf

More Wireless World articles: ~ Scanned and cleaned up Wireless World Articles ~

Last edited by PB2; 12th December 2019 at 10:11 PM.
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