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
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
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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.
But to tell you more details, post amplifier scheme in a readable format such as JPG or PDF.
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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?
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?
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But to tell you more details, post amplifier scheme in a readable format such as JPG
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?
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?
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Unfortunately, I have not installed LTspice but it's not bad to try a simulation in Multisim (to compare the results). So, try use Multisim to compare the results.
Connect the MOSFET's as are connected power BJT's (Q4 and Q5) in the image below:
Connect the MOSFET's as are connected power BJT's (Q4 and Q5) in the image below:
An externally hosted image should be here but it was not working when we last tested it.
Yes understand the CFP structure for bjt, I was not sure about using MOSFET as drain follower.
I have Bob Cordell & Doug Self''s books, so need to sim DS blameless PA that uses a CFP bjt to get to this stage first.
This thread is to improve the submitted MOSFET o/p design.
No Mutisim for me, this thread started off using ltspice, so we should stick with it.
I have Bob Cordell & Doug Self''s books, so need to sim DS blameless PA that uses a CFP bjt to get to this stage first.
This thread is to improve the submitted MOSFET o/p design.
No Mutisim for me, this thread started off using ltspice, so we should stick with it.
ridiculous 20K thd is just an exercise in learning circuit design. It is one of the better tests to run on a amp, especially in sim. The idea is to make a simple PA without great component complexity & cost.
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