Miller Compensation Capacitor vs Slew Rate - diyAudio
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Old 13th April 2013, 03:42 PM   #1
danny92 is offline danny92  Portugal
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Default Miller Compensation Capacitor vs Slew Rate

Hi everyone,

I'm projecting a MOSFET output power amplifier based on a design I've seen in Bob Cordell's book "Designing Audio Power Amplifiers", but I've a problem, it seems that the quiescent current passing thru the MOSFET output devices are shifting the phase of the closed loop to dangerous levels, so I'd to increase the value of the Miller Compensation Capacitor to high levels, but this leads to another problem, because the slew rate it's reduced. In simulations I've made in TINA something strange are happening because when I have a closed loop gain of about 5-10 V/V the harmonic distortion in the FFT for 20 kHZ is below 0.5 %, for higher gain levels are enormous, higher than 1% !!!

I don't know if this is a common problem in MOSFET amplifiers or Bob Cordell's designs, but I've tried with other designs and the result is similar, could be a problem with TINA? If so, how can I fix that? It's better to use another simulator?

PS: The VI limiter circuit, and the clamping diodes aren't causing any strange effect, without it the result is the same.

Thank you very much for your attention
Best regards,
Daniel Almeida
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Last edited by danny92; 13th April 2013 at 03:44 PM.
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Old 13th April 2013, 04:02 PM   #2
lineup is offline lineup  Sweden
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The only strange I can see is R49 47k from Snk to ground.
But this should not be a problem.
Not such problem as you tell about.

For higher gain the THD should be higher.
But more than 1% is not okay.
Do you think you have good models for all your transistors?
You have tried to use other MOSFET?
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Old 13th April 2013, 04:05 PM   #3
danny92 is offline danny92  Portugal
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Thank you very much for your fast reppy, which was very helpful.

I'll try to use the pair of IRFPs and then I'll tell you the results, the THD it's only higher than 1% for gains above 15 V/V.

Best regards,
Daniel Almeida
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Old 13th April 2013, 04:18 PM   #4
danny92 is offline danny92  Portugal
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I've tried with the pair of IRFPs and the result it's even worse, I've to use a 220 pF capacitor and even with low voltage gains (10 V/V) the harmonic distortion is higher than 1%.
This effect of phase shift due to quiescent current passing thru the output devices is common?
Because of that I've to increase te Miller capacitor, decreasing the slew rate, another way that works without reducing the slew rate so much consists in trying to put random resisistor values in series with the Miller compensation capacitor to add a zero until phase margin of at least 45║ is reached, but this is a strange procedure and I don't know if it works in real life, I've never ever seen a working desin of audio power amplifier with zer-pole compensation.

Please can you help me here, does anyone have tried to do a similar project?

Best regards,
Daniel Almeida

Last edited by danny92; 13th April 2013 at 04:22 PM.
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Old 13th April 2013, 05:08 PM   #5
godfrey is online now godfrey  South Africa
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There's something odd here. 100pF should be big enough. With 470 Ohm emitter resistors in the input stage and a gain of 10, the unity loop gain frequency is only about 300KHz.

Are you sure the output devices are biased properly? You could check the quiescent voltage across the 0.33 Ohm emitter resistors.

I don't understand what you mean by "phase shift due to quiescent current passing thru the output devices". Quiescent current doesn't cause phase shift. Lack of quiescent current may cause problems though.

Last edited by godfrey; 13th April 2013 at 05:12 PM.
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Old 13th April 2013, 05:13 PM   #6
danny92 is offline danny92  Portugal
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Thank you very much for repplying godfrey,
I've tested with quiescent voltages between 24 mV and 50 mV in the source resistors, 70mA-150mA bias current.
Problem with TINA?

Best regards,
Daniel Almeida
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Old 13th April 2013, 06:29 PM   #7
godfrey is online now godfrey  South Africa
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Possibly. Depends if the MOSFET models are any good.
There are some decent SPICE models on Bob Cordell's website here: CordellAudio.com - Home
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Old 13th April 2013, 06:53 PM   #8
danny92 is offline danny92  Portugal
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Right, thank you
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Old 13th April 2013, 07:37 PM   #9
mcd99uk is offline mcd99uk  United Kingdom
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I found with my single attempt at designing the best miller compensation scheme seems to be the miller input variety detailed in Bob's book.
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Old 13th April 2013, 09:29 PM   #10
jxdking is offline jxdking  China
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I doubt that R39 and R40 are too large. 33 Ohm should be fine.
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