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Old 9th May 2010, 12:36 PM   #1
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Default CFP output variation

As D. Self showed CFP output stage behaves better in the xover region than EF. I wonder if the circuit i propose will cancel the effect that output devices produce when they turn off. The current sources provide current that keeps the output transistors always on at a few mAs. Is it a good idea?
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Old 9th May 2010, 04:04 PM   #2
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Old 9th May 2010, 05:16 PM   #3
Mooly is offline Mooly  United Kingdom
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I can see what you are trying to do, but I don't think it will work quite as you intend. The whole point of a proper bias generator is to ensure the outputs don't reach cut off any way... what you are introducing is a current that will give unpredictable bias "within" the supposed Vbe generator/output loop.
T1 and T2 can only ever increase bias, they can not cut the outputs off or reduce the output conduction caused by your fixed current sources... so how do you set a stable operating point.

That's how it appears at first glance anyway
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Old 9th May 2010, 06:14 PM   #4
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Originally Posted by di$tortion View Post
The current sources provide current that keeps the output transistors always on at a few mAs. Is it a good idea?
Actually i very much doubt that 4mA would ever keep the output transistor conducting You see there is a 100 Ohm resistor accross from the base to emitter & this will need about 600mV to turn the transistor on, maybe a touch less which would require 6mA or there abouts.

As mooly says though this might mess up the biasing.

The problem with the CFP is that in a class B amplifier the drivers switch on & off & it's these that imo need to be kept conducting. The output transistors in an EF stage also switch on & off but the drivers normally don't.

I suggest you try simulating this as it's a simple idea i had recently & very similar to what you have done. All you need to do is move the constant currents from the base of the output transistors to the emitter of the driver transistors. This may well keep the drivers conducting, if it doesn't do that over the full cycle it'll certainly have the drivers turned on well before the output transistor needs to be on & it should keep the driver turned on well after the output transistor switches off.

4mA should be fine as the output transistors shouldn't conduct with 400mV on the bases

As i say though, it's just an idea & i haven't checked out it's efficacy.
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Old 9th May 2010, 06:24 PM   #5
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That is what i am trying to do. If the output transistors are biased by the ccs at around 10mA they will not cut off when the output voltage is negative in the class AB operation. The Vbe multi will then bias T3 and T4 in higher current to give class A operation for the first watts. The bias generator does not prevent the output transistors to cut off in class AB.
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Old 9th May 2010, 06:35 PM   #6
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The 4mA rating is the number LTSpice indicated for the d44h11. It will have to change for a "real" design with ON semiconductors.

I also though what you are proposing. I will simulate it. The whole idea is to use low power ccs to prevent the cut off instead of higher current sources connected from the output to the negative power supply.
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Old 9th May 2010, 06:58 PM   #7
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Originally Posted by di$tortion View Post
I also though what you are proposing. I will simulate it.
Let us know how you get on, obviously i for one am interested

Bests, Mark.
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