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Old 1st August 2012, 12:05 PM   #101
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Looking at Mr. Pass's equations, for MOSFET with a high Z output stage (common source ?)

To lower Zout, the real options are:

- decrease the output source resistor
- increase F.B.
- or find a MOSFET with a low gm

Minimizing Zout is a real balancing act.



The "bricks" have a gm = 90 A/v,
So I'm thinking of going back to an Aleph style amp - but biasing it at only around 1 amp - but minimizing Zout.

I dropped the Aleph style amp only because of the power dissipation and heat.
Having a current source at each stage, has some great advantages.
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Old 1st August 2012, 12:12 PM   #102
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Originally Posted by Zen Mod View Post
without curves and specific data , it's inevitable that you must do some measuring

so - either measuring xconductance of mosfets (Papa's papers 0n FW site - Mosfet Testing ) or measuring output impedance directly
Yes, I was thinking of connecting some output resistors to the F5, measuring the change in Vout and then plot a load line.

But the Zout equations really help to understand the amp.
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Old 1st August 2012, 03:39 PM   #103
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You could also parallel more output transistors. However, lowering the current through each output transistor to keep the efficiency up also lowers the transconductance for each one, raising the total output impedance. No free lunch. I've been playing around with this idea myself recently. I suspect I'm not telling you anything you don't already know.

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Old 1st August 2012, 03:48 PM   #104
Zen Mod is offline Zen Mod  Serbia
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xconductance will go double with double outputs
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Old 1st August 2012, 04:55 PM   #105
gl is offline gl  United States
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Only if the current stays the same.

In the above post I said that if you reduced the current per transistor to gain efficiency then the transconductance would drop.

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Old 1st August 2012, 06:15 PM   #106
Zen Mod is offline Zen Mod  Serbia
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one step back , two in front

anyway - capacitance will rise it's ugly head then
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Old 1st August 2012, 06:31 PM   #107
gl is offline gl  United States
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Yup. No free lunch.
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Old 5th August 2012, 03:23 PM   #108
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one step back , two in front

anyway - capacitance will rise it's ugly head then
Had one step forward here.

Found this good article* which discusses HEXFET's in audio output stages.

There is a ton of good info in this article.

The author says because of HEXFET non linearities, a gate needs to be driven with as much current as possible to maximize linearity.

However, he doesn't look at Ciss - but instead looks at the Gate Charge.

He uses the equation I = 100 * Qg * f(max)

Turns on for a bread and butter IRF 240 Qg(max) = 70 nC
While for a Brick Qg (max) = 380 nC

This helps to explain why 3 jFET's have the poop to drive a Brick.


I would add that most datasheets show a Gate-Charge curve.
On the curve, there is a charge plateau, that is well below the max values.




* Using HEXFETs in High Fidelity Audio

http://www.irf.com/technical-info/appnotes/an-944.pdf
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Old 5th August 2012, 04:32 PM   #109
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Yup. No free lunch.
GL,

Recommend you print a copy of this - its quite a good article.

Using HEXFETs in High Fidelity Audio

He points out that for a MOSFET Rds(on) does increase with temperature.

However, Vgs(on) has a negative Temp co efficient -1mV/oC to -1.5mV/oC
and this is why MOSFET's are prone to thermal run away.

This explains why Mr. Pass put the thermistor in parallel with Vgs.

In fact, you'd think someone would make a component that has a complementary temp co efficient to Vgs


But the best comment from the article is Compromise will be almost essential
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Old 5th August 2012, 04:49 PM   #110
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All of that is more related to switching circuits.
As is Rds(on), relevant for switch mode stuff, but is of no consequence for conventional audio amps.
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