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Old 18th December 2003, 07:55 PM   #601
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I've done some more thinking...

Quote:
I can make a similar argument in voltage as well: As the voltage on the base of the driver goes up, it raises the voltage on and current through the emitter resistor, such turning on the lower output BJT more.
Your error is that the emmitor of the driver is capable of controling the voltage of the lower BJT since it's output impedance is far greater than the base-impedance of the lower output BJT. In such a situation we talk of current drive, not voltage drive.

Quote:
As the emitter current goes up, voltage drop off the collector bootstrap goes up (think of it as a resistor for now). that means Vbe for the upper BJT goes down, turning OFF the upper BJT. Viola, you got a push-pull design.
Since the output impedance of the driver/bootstrap node is so much hihger that the upper-output-BJT, is does not determine te base voltage, but controles the current. Again current driven not voltage driven.

I fullt agree that it is Push-Pull.

I also fully agree with the fact that the MOSFET version works as a PP-class-A system. But it has becone an entirly different output stage, since in essence it works very different.

I still don't understand what consequenses there are for modifieing the upper-drive resistor. I hope my simplifier circuits can help with that.

Regards,
Thijs
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Old 18th December 2003, 07:59 PM   #602
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Quote:
One way to think about this is to assume very high gain BJT devices in the BJT version, and it will then become a "voltage" driven output stage->aka the mosfet version.
Ha, ha,ha.. LOL .. how many time are you going to make my head spin... I guess you're right again..
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Old 18th December 2003, 08:21 PM   #603
Geoff is offline Geoff  United Kingdom
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Millwood

Due to a significant intake of good malt whisky (or in US and Irish spelling, whiskey) I will have to delay replying to your comments on my post until tomorrow, but to quote (though I can't remember who) "I'll be back"

Geoff
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Old 18th December 2003, 08:28 PM   #604
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'k Geoff ,

I'am off to my Burgandy then..

Quote:
One way to think about this is to assume very high gain BJT devices in the BJT version, and it will then become a "voltage" driven output stage->aka the mosfet version.
Still thinking about it... still not sure if I actually agree..

g'night,
Thijs
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Old 18th December 2003, 08:35 PM   #605
paulb is offline paulb  Canada
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Quote:
Originally posted by Geoff
, but to quote (though I can't remember who) "I'll be back"

Geoff
Aha! Something I can help with. I think that was Ronald Reagan.
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Old 18th December 2003, 08:35 PM   #606
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Quote:
Originally posted by Geoff
Due to a significant intake of good malt whisky (or in US and Irish spelling, whiskey) I will have to delay replying to your comments on my post until tomorrow, but to quote (though I can't remember who) "I'll be back"

Geoff

take your time, geoff. we are in a holiday mode as well here.

to tschrama's point, the only requirement for a push-pull output stage to work is that when one output device blows, the other sucks (sounds a little weird, ).

Of course, they should be in complete sync. otherwise, you don't get the right output. What "triggers" the output devices is 2ndary.

What the driver does here is to generate two outputs, in perfect phase reversal so that it drives the push-pull output stage.

The bootstrap does is to enhance the gain of the upper device and enhance power output. You can prove this by letting the bottom device work like a current source (current sink more precisely).

a good way to understand this is to replace the output devices with controlled current sources (current controlled for BJTs and voltage controlled for MOSFETs).
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Old 18th December 2003, 10:18 PM   #607
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alternatively, try to think through how a push-pull stage in a BJT/MOSFET class AB amp work in relation to the drivers.

and if you bias them deavily enough, you can a class a amp.
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Old 18th December 2003, 10:47 PM   #608
mikelm is offline mikelm  England
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This current drive / voltage drive point seems just like a viewpoint thing, different ways of understanding the same situation.

My understanding is that generally a change in current is caused by a change in voltage

The idea to bootstrap the i/p tr collecter resister is neat.

I did not do exhaustive research on it but I think you may find that if you make the driver transistor a small mosfet and use a CCS as the i/p tr collector load you will notice that the distortion goes to much lower levels.

I tried it and like the sound

it may create slight stability issues

mike
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Old 18th December 2003, 10:55 PM   #609
mikelm is offline mikelm  England
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putting a CCS as driver tr load in my simulated cct increased 3rd HD a little and reduced 2nd HD a little.

phase margin seemed unchanged.

I think I will stay with a resistor.
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Old 19th December 2003, 03:06 AM   #610
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Quote:
Originally posted by mikelm
putting a CCS as driver tr load in my simulated cct increased 3rd HD a little and reduced 2nd HD a little.

phase margin seemed unchanged.

I think I will stay with a resistor.

that's consistent with my simulations as well. I don't fully understand why that's the case.
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