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Old 24th February 2004, 10:23 AM   #1
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Default The sound of VMOSFET

here is Soundstage's review of Ayre V5x, with Charles' commentary. Sounds like a pretty interesting amp. I also read somewhere else that the V5x uses current mirror and T-driver stage.

enjoy.

http://www.soundstage.com/revequip/ayre_v5x.htm

Charles, can you give some background on your comments about the VMOSFET? It is the first time I have heard of them.

=============from the Soundstage interview===========
But you're using MOSFETs, right?

"Nope."

But everybody agrees that MOSFETs are more like tubes and simply sound better. Don't they?

"You've got to be careful about making generalizations about MOSFETs because there are vertical MOSFETs and lateral MOSFETs and they don't behave exactly the same. Almost no one uses the Hitachi lateral MOSFETs, though -- and while the vertical ones are very linear in the lower regions, they have a wonky, non-linear input impedance that results in some really nasty high-frequency distortion characteristics. Well, nasty-sounding to me, at least."

============the end============================

I am curious if anyone else has the same observation.
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Old 24th February 2004, 11:33 AM   #2
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All power MOSFETs have a capacitive input impedance.

What is important however, is that this input impedance is comprised of Cgs which behaves pretty much as a simple capacitor (at least to a first order approximation), and Cgd which is the problem.

If you have a common-source stage the Miller feedback from drain to source via this capacitance causes the input impedance to dynamically change. This is especially noticable in SMPSUs where Vgs (when the gate is driven from a non-zero source impedance) shows a distinct 'plateau' due to the NFB from drain to gate. This reduces the switching speed of the FET quite noticeably.

Lateral power FETs show the problem to a lesser extent because Cds tends to be a lot lower than for vertical types.
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Old 24th February 2004, 12:58 PM   #3
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I guess my puzzle is to reconcile between the excellent reviews those Pass Labs amps got and Charles' take on the mosfets. Since Pass uses exclusively vmosfets, and the reviews for them seem to emphasise how good they sound.

I am at a loss as to how the two are miles apart.
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Old 24th February 2004, 01:11 PM   #4
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Quote:
Originally posted by Ouroboros


What is important however, is that this input impedance is comprised of Cgs which behaves pretty much as a simple capacitor (at least to a first order approximation), and Cgd which is the problem.

If you have a common-source stage the Miller feedback from drain to source via this capacitance causes the input impedance to dynamically change. This is especially noticable in SMPSUs where Vgs (when the gate is driven from a non-zero source impedance) shows a distinct 'plateau' due to the NFB from drain to gate. This reduces the switching speed of the FET quite noticeably.
But in the case of a output stage the mosfets are usually used as source folowers...so this parasitics capacity Cgd is not amplified by Miller effect as the stage doesn't have voltage gain..actually a litle loss..
The Cgs (Gate source) also become diminished by the bootstrap action of the folower...

In a nutshell ...in a mosfet folower the parasitics capacitys are less restrictive than in a commun source gain stage...
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Old 24th February 2004, 02:33 PM   #5
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In MOSFET's the reverse capacitance is really a problem, it looks small in datasheet but it's magnified when the Vds changes and is very noticeable even in a source follower, the Cgd impact is far greater because with an amplifier with for instance a rail voltage of +- 50 V the Cgd is working against a voltage change of up to 100 V swing related to the Gate comparing to the Cgs which is working against a voltage swing of just some volt.

From my experinces working with SMPS's is that the reverse capacitance effect can be seen very clearly although many SMPS IC's have a peak current drive capability around +- 1 A like UC3845 and even more and still the voltage plateau can be seen on the gate.

I tried to compare 2SK135 (Old classic Hitachi FET)with IRF630 (Obs, not 630N version, just 630 for clarity!) that seem to be quite close in performance, the datasheets are not really comparable because they use diffrent parameters when stating for instance the Crss but the 2SK135 have much smaller capacitance.

In the datasheet for IRF630 we can se that Qgs is 7 nC but for Qgd is clearly greater with 23 nC for a certain condition, so the conclusion is that the Cgd (or Miller capacitance) has a clear impact.

Edit: Vertical FET's have a distictive behaviour around the Vgs threshold and probably why Charlse Hansen don't like them, they are just hard to drive "linearly" and it's not so much depending on the topology, eg. CD or CS.

Maybe some input from EVA who sholud know this issue too.

Cheers!
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Old 24th February 2004, 03:01 PM   #6
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Default Mosfet= Hexfet Mosfet= Vmosfet

Millwood,

The IR MOSFET or HEXFET is has a hexagonal geometry so there are a bunch of the hexagonal cells in the devices in the MOSFET. The more cell the lower the on resistance and the higher the capacitance.

The V channel MOSFET was a Siliconix deal, where it is similar to a planner device with a V grove structure. Therefore, the VMOSFET is like a Big FET with a V channel. So you can't get a V grove MOSFET device with the kind of low on resistance like the IR HEXFET.
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Old 24th February 2004, 03:18 PM   #7
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Default Re: Mosfet= Hexfet Mosfet= Vmosfet

Quote:
Originally posted by jewilson
Millwood,

The IR MOSFET or HEXFET is has a hexagonal geometry so there are a bunch of the hexagonal cells in the devices in the MOSFET. The more cell the lower the on resistance and the higher the capacitance.

The V channel MOSFET was a Siliconix deal, where it is similar to a planner device with a V grove structure. Therefore, the VMOSFET is like a Big FET with a V channel. So you can't get a V grove MOSFET device with the kind of low on resistance like the IR HEXFET.


But the old Siliconix V and U grooves, the IR 'hexfet', the On-semi square cell, the Philips 'trench' structure, etc all have essentially vertical current flow between source and drain. The Hitachi (and Magnatec) audio FETs have horizontal current flow. The lateral geometry is what keeps the capacitance down.
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Old 24th February 2004, 03:32 PM   #8
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Ouroboros,

That all true, still there are major differences in the geometry of these devices. I don't pretent to know which one sounds the best, however the IR parts lead in sales in the major markets.

One thing I do like about the L MOSFET is the biasing.
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Old 24th February 2004, 03:38 PM   #9
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The VN13 and VP13 series were quite nice as the output devices for line-level circuitry, and also as drivers for power amp output stages. In many ways I liked them better than Hitachi's 2SK213/2SJ77 combo (lower capacitance, for example). Unfortunately the VN13 and VP13 have been out of production for years...

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Old 24th February 2004, 04:13 PM   #10
sam9 is offline sam9  United States
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"Almost no one uses the Hitachi lateral MOSFETs. . . ."

Uhh? Does that include 2SK1058/2SJ162 and the various BUZ Hitachi derived units from Magnatec?
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