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Old 21st September 2013, 01:39 AM   #1
danny92 is offline danny92  Portugal
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Default How to reduce THD20 in a lateral MOSFET amp?

Hi everyone,

I'm simulating some amplifiers based on Bob Cordell's designs, and I like this particular circuit (attached), but I wanted to achieve good distortion results before building this amp in real life, I have lots of BUZ900/905 lateral MOSFETs, and I want to use them in a low distortion, 4 ohm capable amplifier, but I'm not satisfied with this results that I've obtained, THD1 is less than 0.0002% (and for me is acceptable), but THD20 is very high, near 0.05%, and I can't accept that, for me an acceptable value for THD20 is less than 0.005% at "full power", for THD1 is less than 0.0005% at "full power". I don't know if this is possible for an amplifier with two BUZ900/905 output pairs.
Does anyone with experience in lateral MOSFETs could help me here?

Best regards,
Daniel Almeida
Attached Files
File Type: txt Cordell Models.txt (11.4 KB, 23 views)
File Type: asc Amp4_MOSFET_zobel_tmc4.asc (11.4 KB, 65 views)

Last edited by danny92; 21st September 2013 at 01:47 AM.
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Old 21st September 2013, 04:44 AM   #2
Ipanema is offline Ipanema  Malaysia
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what is the merit of measuring thd20 for 20khz application?
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Old 21st September 2013, 05:46 AM   #3
djoffe is offline djoffe  United States
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I see you're now using the improved compensation.

You can certainly drop R15 and R16 to improve the 20 kHz distortion.
20 kHz distortion drops to 0.006% with R15=R16=22 Ohms, with no signs of instability in the simulation.
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Old 21st September 2013, 08:22 AM   #4
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Less distortion at 20kHz frequency sine means that the amplifier becomes more susceptible to instability. It's a vicious circle. Currently an audio amplifier that has total THD less than 0.5% in the whole frequency range, we can say the amplifier is a pretty good, not perfect, but it's OK. What you want to do is not impossible but be careful with MOSFETs (to properly compensate the high frequency oscillations on the output MOSFET amplifier using "PI Zobel Network"). Then, to make stable amplifier have to limit, above all, reproduced frequency band.
But to tell you more details, post amplifier scheme in a readable format such as JPG or PDF.
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Last edited by donpetru; 21st September 2013 at 08:26 AM.
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Old 21st September 2013, 09:22 AM   #5
mcd99uk is offline mcd99uk  United Kingdom
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Increase R12, there's no need for such heavy biasing in the drivers. Put a small "speed up" cap across it of around 5x total MOSFET gate capacitance.

You gate zobel capacitance looks high. The order of 47pF - 100pF is more normal IMHO. The high gate zobel capacitance will be impacting phase margin. If you reduce the gate zobel capacitance you will be able to change the TMC values to reduce THD and keep the same stability margins.

Also, I see you have a cascoded VAS is this really necessary when using the KSC3503 transistor?

Last edited by mcd99uk; 21st September 2013 at 09:39 AM.
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Old 21st September 2013, 09:59 AM   #6
mcd99uk is offline mcd99uk  United Kingdom
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Q15, 16, 4 and 8 may be better being BC5x0c devices.
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Old 21st September 2013, 04:23 PM   #7
chicks is offline chicks  United States
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Might have a look at Renardson's design notes for his MJR7.

MJR7-Mk5 Mosfet Power Amplifier
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Old 22nd September 2013, 03:13 PM   #8
rsavas is offline rsavas  Canada
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Default But to tell you more details, post amplifier scheme in a readable format such as JPG

Quote:
But to tell you more details, post amplifier scheme in a readable format such as JPG or PDF.
1) I ran the sim of original schematic.
20-KHz Total Harmonic Distortion: 0.018930%
2) Changed R15-16 to 22 ohm, as suggested and as stated, 20K THD went down quite a bit,
20-KHz Total Harmonic Distortion: 0.005999%
Quite a reduction. I wonder why Bob C. thought that he needed that amount of emitter degeneration?
Attached Files
File Type: pdf Amp4_MOSFET_zobel_tmc4.pdf (14.6 KB, 34 views)

Last edited by rsavas; 22nd September 2013 at 03:28 PM.
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Old 22nd September 2013, 04:22 PM   #9
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@rsavas, try put MOSFETs in Sziklai configuration.
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Old 22nd September 2013, 06:00 PM   #10
rsavas is offline rsavas  Canada
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Quote:
@rsavas, try put MOSFETs in Sziklai configuration.
I would have to figure that one out!!
@donpetru, Do you not run LTspice youreself?then you can show me/us how it would be configured?
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