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Old 8th January 2006, 12:28 PM   #11
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Hello,

thanks for your very helpful comments!

I´ve made some changes of topology, I like Hugh´s Idea. Now it´s a really bootstapping amplifier...

But I see the problems in the cascoded driver stage "The Saint" mentioned. Zeners must have half the rail voltage and limit output swing to half rail voltage, too ?!?

Sorry for my questions, I´m just a beginner...

greets
Peter

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Old 8th January 2006, 12:48 PM   #12
lineup is offline lineup  Sweden
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You started with quite a nice circuit, for a beginner.

You moved supply rail filters, C1 R3 C4 R4, one step too far.
Should be to the right of Voltage Amp Stage, VAS.
Transistor Q16 is doing the voltage amplifiation.

R10 and R19 belongs to output stage drivers.
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Old 8th January 2006, 02:36 PM   #13
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for the mosfet amp you have (hopefully) Rout about 1/gm open loop, reduced for closed loop.

the major downside to all of this is the output swing

not sure what Q10 or Q11 do.

Mos devices may not have a low Vgs-sat, especially at high currents. if Vth is 2V, Vgs for 4A might be 4V, depending on the device construction. this might limit output swing. at some point you wonder if it wouldn't make more sense just to move to BJTs in class A if you want low efficiency. of course class A idles a lot hotter.
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Old 8th January 2006, 04:23 PM   #14
nikwal is offline nikwal  Sweden
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Quote:
This cascoded transistor are in my view a very bad idea , because as the output mosfets don't have voltage gain , you loose in the cascoded stage , precious volts for full voltage swing.
Well ..does'nt the entire prestage need a higher voltage anyway, so what you are saying is.. That the prestages will end up too high in voltage to get the correct voltage swing?
And C9,R11,R17 how is that working compared to a real current source?
And remember, a classAB amp will play in classA on low powers :-D, otherwise it would be called classB, hehe..
And dont worry about the damping, it will be enough..
You might want to try a 2uF capacitor on the output and see how it will handle that(or not)..
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Old 8th January 2006, 08:33 PM   #15
ilimzn is offline ilimzn  Croatia
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Quote:
Originally posted by AKSA
Anthony's comments are all on the money.......
However, another option to preserve your neat constant voltage driver.
The neat thing about the constant voltage driver is that it largely removes the driver transistors from thermal stability considerations, making dissipation on them more constant on account of constant voltage. They can be kept on the board and off heatsinks, which may prove important as follows:

In order to get the idle current stable without source resistors, you can use either a single pair of lateral MOSFETs on the output (unfortunately you get higher rail), call stis option (L) or a single pair of vertical mosfets with a Vgs multiplier to set the bias current, that would be option (V).

In both cases the bias setting string (currrently your BJT and resistors) needs to have two diodes in series with it, in thermal contact with the driver stage (just the current gain transistors in the cascode). This is where having them on the board and off the heatsink helps a lot.

For option (L) you only need a variable resistor instead of the Vbe multiplier, assuming you set Ibias to >=100mA. For option (V) you need a Vgs multiplier. Basically, replace your BJT in the bias generator with a small MOSFET, like IRF510 for instance. Resistors around it will need recalculating. This MOSFET needs to be located on the same heatsink as the output pairs. I have used this approach with verticals using no source resistors to keep gm high, and it works very well. Thermal coupling is excellent without the need for exotic mounting arrangements due to the IRF510 TO220 case with it's big metal tab.

Oh, and another thing: even though IRF9140 are getting difficult to find, you might want to use them as your Pch MOSFET and IRF240 as the Nch. This is about as good as you can get regarding vertical MOSFET complementarity, which is slightly more important in a design with only one pair of MOSFETs and without source resistors, than just using the 'traditional' complementaries, which really are not. They also have a signifficantly higher power dissipation rating than laterals, and cost a lot less, while providing about 2x the max current capacity compared to single die laterals, and limiting rail loss - again important if you use only one pair.
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Old 9th January 2006, 07:57 PM   #16
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Quote:
Originally posted by AKSA
Split the two Rs supplying the bases of the 'cascoding' transistors. Make them equal, and run two caps of 220uF from the midpoint back to the output rail.

This double bootstrap will enable you to swing the cascode bases well beyond the rail, giving very high rail efficiency despite using mosfets.

This is only true in the negative voltage swing , but in the positive voltage swing , it will be limited by the voltage of the positive rail ( the voltage of the VAS transistor and his CE saturation voltage more the Gate Source voltage of the mosfets ,as the output stage has no voltage gain).
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Old 9th January 2006, 08:32 PM   #17
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as for the source resistors: actually you get a higher damping factor with them, cause of a slight current feedback loop that forms.. they also take care of thermal stability issues and identical load distribution to your mosfets..
why use irfp140 and not irfp240+irfp9240?..
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Old 9th January 2006, 08:41 PM   #18
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Quote:
Originally posted by CrazyChipMan
@tube_dude
what are lateral mosfets ?
Hi Crazychipman

Lateral Mosfets have a lateral construction of the Channel, but what made them diferent from the vertical are their electrical characteristics .

The following advantages are offered by the lateral construction:

- It has a very low zero-temperature coefficient drain current ( usually near 100 mA and the vertical 3 A )

-Lower threshold voltage than vertical.

-The Gate-to-Drain capacitance is low ant that offers the advantage for high frequency operation.


Now the disadvantages:

-Less tranconductance than the vertical.

-Price is higher and availabality is less than the vertical.

-Higher Rds(on) than the vertical.

Cheers
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Old 9th January 2006, 09:00 PM   #19
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Quote:
Originally posted by costiss
as for the source resistors: actually you get a higher damping factor with them, cause of a slight current feedback loop that forms..
Hi costiss

Can you please explain how the source resistors that enlarge the open loop output impedance and diminish the output mosfets transconductance , increase the damping factor?
Thanks!
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Old 9th January 2006, 10:30 PM   #20
ilimzn is offline ilimzn  Croatia
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Yes, that was just what I was about to ask
Seeing that the output impedance is 1/gm and the source resistors are in series with that...

Why use IRFP9140 instead of 9240? Very simple - check out the data sheet. Compared to 240, 9240 has half the current handling, about 60% transconductance. Hardly what I would call complementary, so I would ask why use the 9240 as complement to 240?
Then check the same for 9140 and you will find that they are a MUCH better mach, 9140 has half the breakdown voltage (not important if + to - rails is below it), but practically the same current handling, transconductance and gate capacitance, (keep in mind it gm is normally given for different operating points on different parts so you need to compare Id vs Vgs graphs, which unfortunately are on different scales).
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