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Old 25th April 2013, 02:47 AM   #131
The Dastardly Dad of Three
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Quote:
Originally Posted by lhquam View Post
As a result of exploring the idea of completely eliminating the source resistors and using cascoding, I now have a much greater respect for Nelson's F5-Turbo which has the effect of reducing the source resistance at high power levels, but has manageable thermal and DC bias stability. I might look at the notion of cascoding the F5-Turbo outputs.

Why not just bypass the source resistor with a capacitor (Kasey Bypass Kapacitor hahah) ? Seriously though this is an old idea but havent seen it applied recently (perhaps for good reason?)

The voltage rating drop across the resistor is small so we can use one of them 6.3V super low esr motherboard capacitors. I bought some and intend to try. Seems to me that that would give us DC bias stability benefit of source R, plus square law characteristics of no source R. Spice simulations gives very good results and i think its a very worthwhile thing to try for anyone with a stock F5.
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Old 25th April 2013, 02:51 AM   #132
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used organic polymers in recent amp. Cant say i heard them. DO yu want to hear diode or bypass cap. The diode does behave slighlty differently, doesnt it. It is more gradual than cap.
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Old 25th April 2013, 03:03 AM   #133
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Odd - i would have thought caps would be much more gradual than the turbo diode ?
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Old 25th April 2013, 05:47 AM   #134
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In order to not function as a high-pass filter, the capacitor would have to be on the order of 0.1F. ESR is also an issue.

Quote:
Originally Posted by kasey197 View Post
Why not just bypass the source resistor with a capacitor (Kasey Bypass Kapacitor hahah) ? Seriously though this is an old idea but havent seen it applied recently (perhaps for good reason?)

The voltage rating drop across the resistor is small so we can use one of them 6.3V super low esr motherboard capacitors. I bought some and intend to try. Seems to me that that would give us DC bias stability benefit of source R, plus square law characteristics of no source R. Spice simulations gives very good results and i think its a very worthwhile thing to try for anyone with a stock F5.
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Old 25th April 2013, 08:48 AM   #135
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i did some sims and move to another thread to discuss separately.
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Old 25th April 2013, 01:50 PM   #136
lhquam is offline lhquam  United States
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Originally Posted by kasey197 View Post
i did some sims and move to another thread to discuss separately.
Please leave a pointer to the other thread.
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Old 25th April 2013, 02:00 PM   #137
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Apologies .... Here it is

Bypass caps on source resistor (F5 example)
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Old 20th May 2013, 01:45 AM   #138
lhquam is offline lhquam  United States
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Quote:
Originally Posted by kasey197 View Post
Why not just bypass the source resistor with a capacitor (Kasey Bypass Kapacitor hahah) ? Seriously though this is an old idea but havent seen it applied recently (perhaps for good reason?)

The voltage rating drop across the resistor is small so we can use one of them 6.3V super low esr motherboard capacitors. I bought some and intend to try. Seems to me that that would give us DC bias stability benefit of source R, plus square law characteristics of no source R. Spice simulations gives very good results and i think its a very worthwhile thing to try for anyone with a stock F5.
I have some simulations of your suggestion of adding capacitors across the source resistors. Here are two plots of THD in the Balanced F5 using FQA28N15 and FQA36P15 VFETs at 10 watts output into 8 ohms with various values for the caps: 0pF, 0.015F, 0.047F, and 0.10F. The first plot is without cascoding the output FETs; the second plot is with cascoding. The series resistance of these caps was 0.015 ohms, taken from datasheets of available low-Z caps. Because of the balanced F5 even order harmonic cancellation, THD data is totally dominated by the 3rd harmonic.

Obviously, the simulations indicate a path to lowering 3rd (and other odd order) harmonics while preserving the thermal stability provided by the source resistor. The next step for me is to order appropriate capacitors and bench test this modification.
Attached Images
File Type: jpg F5bal-nocasc-10W-Cscaps-mix2.jpg (67.1 KB, 164 views)
File Type: jpg F5bal-casc-10W-Cscaps-mix2.jpg (65.8 KB, 146 views)
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Old 20th May 2013, 02:51 AM   #139
The Dastardly Dad of Three
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Absolutely rigorous analysis as always from you lhquam

The real world measurements may well be only half of what the sims suggest -
but the interesting thing in your simulations are the VERY significant difference in HF impact of the caps in the cascode vs non casc implementations.

What do you think thats attributable to, given the small difference (bw casc and no casc) for the no caps versions ?

Also I have some caps handy that might suit - if you pm me an address, i can drop them in the mail.
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Old 20th May 2013, 02:55 AM   #140
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also fwiw, i found that paralleling two low-z caps had better results than one larger cap of equivalent value (due to reduction in esr from leads etc).
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