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Old 15th November 2009, 01:15 PM   #1
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Default Ib different in output-stage

Hi to All

I have a different consumption Ib PNP/NPN in my amplifier in the output stage, I can not know the reason.
Initially thought the VBE multiplier would cause, I shorting VBE multiplier, but the difference remained...later tried equal gains PNP/NPN (BF in simulator) but, difference continues

The figures can better understand the problem.

Thanks
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File Type: gif Plot.gif (10.4 KB, 68 views)
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Old 15th November 2009, 02:00 PM   #2
AndrewT is offline AndrewT  Scotland
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drivers Ib or output Ib?

Have you measured the gains of the transistors before you fitted them.
Did you measure the gains are the expected collector current that the amp would pass?
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Old 15th November 2009, 02:25 PM   #3
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Hi Andrew

Quote:
Originally Posted by AndrewT View Post
drivers Ib or output Ib?
Have you measured the gains of the transistors before you fitted them.
Did you measure the gains are the expected collector current that the amp would pass?
All three output stages are consuming PNP-NPN different, in relation to Ie the output stage is equal, but Ie in pre-drivers and drivers different.

See the simulation in Trs of output
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Old 15th November 2009, 02:27 PM   #4
AndrewT is offline AndrewT  Scotland
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Have you measured the real devices?
Have you measured the real voltages?
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Old 15th November 2009, 03:06 PM   #5
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Quote:
Originally Posted by AndrewT View Post
Have you measured the real devices?
Have you measured the real voltages?
No, would be a problem in the spice models?
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Old 15th November 2009, 03:11 PM   #6
AndrewT is offline AndrewT  Scotland
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we all know that models are only as good as your ability to check whether they do model real behaviour.
If you can't check that, then you should NEVER rely on a computer model, especially if someone's life depends on the answer.
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Old 15th November 2009, 04:08 PM   #7
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Hi

The problem was the models actually, used the Andy_C models (2s4793/1837 and MJL3281/1302), the difference between PNP/NPN this OK.

PS: In simulations that have different Ib, I used models Fairchild

Attached simulation Ib drivers

Thanks
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Old 15th November 2009, 04:34 PM   #8
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The base currents of the output transistors will be "mirror images" only if the output transistor are exact complements. Therfore, you should expect different base currents.
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