Hi everyone,
I'm testing in LTSpice several amplifiers from Bob Cordell's book to decide which one I prefer to build, as I don't have good models for BUZ901/906 pair (and I have lots of them), I'm using Cordell's 2SK1056 and 2SJ162 from Renesas, I've noticed that this amplifier that is the first on MOSFET amps chapter (11) has a very low performance in terms of THD, with TD1 of more than 0.01% and THD20 of more than 0.1%!!! I think that those are unacceptable values.
I've changed the values of the gate stopper resistors according to the respective pole formed in conjunction with Ciss and I've also changed the driver transistors to 2SC3503 and 2SA1381, that are faster than the MJE243/253 pair.
LTSpice circuit diagram attached,
Best regards,
Daniel
I'm testing in LTSpice several amplifiers from Bob Cordell's book to decide which one I prefer to build, as I don't have good models for BUZ901/906 pair (and I have lots of them), I'm using Cordell's 2SK1056 and 2SJ162 from Renesas, I've noticed that this amplifier that is the first on MOSFET amps chapter (11) has a very low performance in terms of THD, with TD1 of more than 0.01% and THD20 of more than 0.1%!!! I think that those are unacceptable values.
I've changed the values of the gate stopper resistors according to the respective pole formed in conjunction with Ciss and I've also changed the driver transistors to 2SC3503 and 2SA1381, that are faster than the MJE243/253 pair.
LTSpice circuit diagram attached,
Best regards,
Daniel
R15 and R16 are way too high. They could be as low as 50 Ohms each, maybe lower, and things still looked quite stable. (note that along the way I dropped RgateP and RgateN to 100 Ohms). With this change, the distortion drops to about 0.017%
Akitika GT-101 Audio Power Amplifier Kit
P.S. - I did make a few changes to the sim to help out the noise floor of the FFT.
Akitika GT-101 Audio Power Amplifier Kit
P.S. - I did make a few changes to the sim to help out the noise floor of the FFT.
Hello djoffe and thanks for your help,
I've reduced the Rgate resistors to lower values and I've added a Zobel network from gate to drain to dump oscillations, I don't want to change the 10:1 degeneration at the LTP (so I've mantained the 470 ohm resistors), I've instead reduced the CMiller capacitor to 12 pF, the THD20 is now lower between 0.02-0.06 %, however THD1 has a strange behaviour, it increases with the amplitude of the input signal, generally THD1 as THD20 is a little high at low amplitudes, then the distortion decreases as the input amplitude raises, and when the output devices are near saturation, the THD raises to very high values in this case with THD1 almost the opposite is happening.
Do you have any ideia why?
PS:
THD1 has values between 0.005-0.02 %
LTSpice circuit attached,
Best regards,
Daniel
I've reduced the Rgate resistors to lower values and I've added a Zobel network from gate to drain to dump oscillations, I don't want to change the 10:1 degeneration at the LTP (so I've mantained the 470 ohm resistors), I've instead reduced the CMiller capacitor to 12 pF, the THD20 is now lower between 0.02-0.06 %, however THD1 has a strange behaviour, it increases with the amplitude of the input signal, generally THD1 as THD20 is a little high at low amplitudes, then the distortion decreases as the input amplitude raises, and when the output devices are near saturation, the THD raises to very high values in this case with THD1 almost the opposite is happening.
Do you have any ideia why?
PS:
THD1 has values between 0.005-0.02 %
LTSpice circuit attached,
Best regards,
Daniel
Danny,
I did some sims and found:
1 kHz distortion increases with amplitude...that's a pretty typical result.
20 kHz distortion increases with amplitude, then starts to decrease.
That's typically a sign of crossover distortion.
You can do a lot to help that if you revise the compensation. See enclosed schematic, particularly in area of R1, C1, and C2. I also used some scripting to make things a bit more automatic.
There are other possibilities, but this might be a good step.
I did some sims and found:
1 kHz distortion increases with amplitude...that's a pretty typical result.
20 kHz distortion increases with amplitude, then starts to decrease.
That's typically a sign of crossover distortion.
You can do a lot to help that if you revise the compensation. See enclosed schematic, particularly in area of R1, C1, and C2. I also used some scripting to make things a bit more automatic.
There are other possibilities, but this might be a good step.
I often wonder how many of these simulations will end up to a real life made amplifier then again i wonder what the difference (s)is/are going to be when the simulations are implemented... That will be a complete act to do .
In my ears sounds like a flight simulator expert is asked to fly a real plane .... Can happen but its not going to be an easy task ...
In my ears sounds like a flight simulator expert is asked to fly a real plane .... Can happen but its not going to be an easy task ...
Hi everyone,
The problem was the 2SC3503/2SA1381 pair as driver transistors, with MJE243/MJE253 THD1 has a normal behaviour.
I've reduced CMiller to 15 pF to improve THD20 characteristics.
It's better to use TPC/TMC but I don't know much about that compensation methods.
I don't know if I should use the output triple.
Best regards,
Daniel Almeida
The problem was the 2SC3503/2SA1381 pair as driver transistors, with MJE243/MJE253 THD1 has a normal behaviour.
I've reduced CMiller to 15 pF to improve THD20 characteristics.
It's better to use TPC/TMC but I don't know much about that compensation methods.
I don't know if I should use the output triple.
Best regards,
Daniel Almeida
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