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Old 12th June 2012, 05:30 AM   #121
dadod is online now dadod  Croatia
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Quote:
Originally Posted by AndrewT View Post
Emitter degeneration on which transistors? The pre-drivers, the drivers, or the outputs? Or all of them?
Sorry AndrewT I did no answer before. It is pre-drivers degeneration.
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Old 12th June 2012, 06:55 PM   #122
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Generally you degenerate the transistor who's B-E you are RC'ing, as too large a change in transconductance here will change the effect of the RC.
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Old 14th June 2012, 03:09 AM   #123
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Dadod, the 2SA970/C2240 models have very low Cje. Models with low Cje probably won't perform like the actual devices, and may be much faster even if the Ft measurement in correct. My models show 5pF, but this was obviously added on by me or someone else, and in reality it may be much larger. I would stick with models from Cordell or other members of the forums. You may try Cordell's BC56x0C or A1381/C3503 models instead, which are known to be accurate.
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Old 14th June 2012, 03:11 PM   #124
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Originally Posted by keantoken View Post
Dadod, the 2SA970/C2240 models have very low Cje. Models with low Cje probably won't perform like the actual devices, and may be much faster even if the Ft measurement in correct. My models show 5pF, but this was obviously added on by me or someone else, and in reality it may be much larger. I would stick with models from Cordell or other members of the forums. You may try Cordell's BC56x0C or A1381/C3503 models instead, which are known to be accurate.
My models for 2SA970/2SC2240 show 5pF too and I don't think it is to low. Cob for those transistors is 3 to 4 pF.
I would appreciate if you do some simulations with your models and suggest improvements. I noticed that you have some fresh ideas and it could be interesting to here it from you.
I simulated with Crdell's models and I got three times more THD20k, but stability is the same and I think 5ppm is still good result.
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Old 15th June 2012, 09:13 AM   #125
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Cje is usually several times higher than Cjc. Cje is the main reason that Ft is low at low currents and rises steadily with Ic. If Cje is too low, Ft at low currents will be way too high. If I could see an Ft vs. Ic chart for the A970/C2240 I could put in a reasonable value, but this information is not in the datasheets. The reason I think 5pF is a guess is that in the model, it is actually gives as "5pF" as opposed to Cjc which is in exponential form with numbers after the decimal, say "6.221e-11". Someone probably added it later so the models wouldn't be totally wrong, and due to the lack of numbers after the decimal it was probably a bad guess.

Consider these models for instance, which I modified:

Code:
.MODEL 2SC3423_k NPN (IS=10.000E-15 BF=156.29 VAF=100 IKF=72.247E-3 XTB=1.5 ISE=103.97E-15 NE=1.5392 BR=2.3164 VAR=100 IKR=10.000E-3 ISC=10.000E-15 NK=.4999 CJE=28p CJC=4.2p VJC=.35 MJC=.21196 TF=550p VTF=8.7934 ITF=114.51 TR=25.505E-6)
.MODEL 2SA1360_k PNP (IS=116.73E-15 BF=120 VAF=100 IKF=78.684E-3 ISE=297.72E-15 NE=1.5511 BR=.77363 VAR=100 IKR=1.0217 ISC=26.920E-15 NC=2.9970 NK=.49889 RC=4.5038 CJE=31p MJE=.33333 CJC=6.3p VJC=.35 MJC=.26272 TF=255p XTF=84.382 VTF=7.0050 ITF=.11443 TR=10.000E-9)
These are the 2SC3423/A1360, a PCT process Toshiba transistor like the 2SA970. They are generally held in high regard. The original models had 2pF for Cje! Thus, even at 1mA the models showed an Ft of 200MHz. To find the right Cje value I first took the correct value of Cjc from the datasheet, then I use Tf to adjust for proper Ft at 10mA (high current Ft). Then I adjusted Cje to put 500uA Ft at the right value (low current Ft, dominated by the lowered transconductance caused by Cje). I also changed some other parameters, not sure which.

Granted, these are 1W transistors and the 2SA970 is 300mW. But they are still pretty similar.

The BC550C/560C are still nearly unbeatable, except for Vceo. However this works to their advantage because they have very good low-Vce characteristics. They are also pretty common. Unfortunately using them in a high voltage amp like this could be difficult. I'm pretty sure they would do great justice in the current mirror, there is nothing better I've found for that Vce level (aside from the BC3x7, but these have worse general specs). If you bootstrap the VAS collector to the positive filtered supply, you can use the BC560C there and you eliminate a source of PSRR, as well as close the drive current loop through the VAS base, which could aid in stability.
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Old 15th June 2012, 04:45 PM   #126
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MPSA18
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Old 15th June 2012, 07:39 PM   #127
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Quote:
Originally Posted by homemodder View Post
MPSA18
I did a simulation with MPSA18, look here: Has anyone seen this front-end before?
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Old 16th June 2012, 09:55 AM   #128
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Dado a simulation and real life are a little different, but if the model for mpsa represents the real part the better. In the case where you have it in the LTP yes it will outperform the bc s and most other small signal transistors. Its very high beta helps offsets, PSRR and common mode rejection, in most examples I have found they also have lower capacitances than practically all small signal transistors bar RF transistors. Their noise could be a little higher but its not a big issue with power amps. A high VCEO would have been nice but there are ways around that as in your amp.
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Old 18th June 2012, 10:29 PM   #129
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Dado, there are no good models for the 2SC2240/A970. I've tried all I could find. I would suggest you simulate with something else, or your results won't be right.
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Old 19th June 2012, 05:00 AM   #130
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Originally Posted by keantoken View Post
Dado, there are no good models for the 2SC2240/A970. I've tried all I could find. I would suggest you simulate with something else, or your results won't be right.
Thanks keantoken,
Do you know good models for 2SC1845/2SA992?
Quote:
bootstrap the VAS collector to the positive filtered supply, you can use the BC560C there and you eliminate a source of PSRR, as well as close the drive current loop through the VAS base, which could aid in stability.
Could you show how to do this as I am not sure I understud it?
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