Output stage device types. Yes or no question. - Page 3 - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 31st October 2008, 12:55 PM   #21
diyAudio Member
 
jacco vermeulen's Avatar
 
Join Date: Oct 2004
Location: At the sea front, Rotterdam or Curaçao
Send a message via Yahoo to jacco vermeulen
Quote:
Originally posted by janneman
the Rdson, about 0.01 ohms
0.01 ??

Did someone say decent complementary ?
Cost difference between a 4-device vertical MOSFET output stage and an 8-device Lateral Mosfet one is like $20-$25, makes 50 bucks difference for stereo.
Which is Hogwash, compared to the total cost of the powersupply and (proper) amp case.
Attached Images
File Type: jpg latte2.jpg (57.7 KB, 404 views)
__________________
The buck stops Here
  Reply With Quote
Old 31st October 2008, 02:11 PM   #22
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 7
Quote:
Originally posted by jacco vermeulen


0.01 ??
[snip]

Hi Jacco,

He banged the gate with 9V he said, so he was switching them fully on. Vds close to zero.

Jan Didden
__________________
If you don't change your beliefs, your life will be like this forever. Is that good news? - W. S. Maugham
Check out Linear Audio!
  Reply With Quote
Old 31st October 2008, 02:15 PM   #23
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Thanks for all the comments,
Jacco, cost is a factor for sure. A double die lateral is around £12 GPB. So four of those £50 say all in. The verticals do have an advantage.

Tempco issues. The verticals are critical on biasing if you want to run at reasonable quiescent currents. I found 120 ma enough, but the challenge is getting this value from cold and maintaining it through all conditions

Rds on. What is the rds of a fully saturated lateral ( single die ) I would have guessed at not much lower than 1 ohm , says he looking at the data sheet. Not specified as such, can be as high as 12 volts at max drain current. Calculator time -- 1.7 ohms.
That's why the verticals can bite ! They can become nearly a piece of wire when saturated , not a good choice of words that is it.

Gate zobels -- values, connections -- dunno, have to experiment on that one. To be honest I have found them (verticals) brilliant as regards stability etc. Must be doing something wrong
  Reply With Quote
Old 31st October 2008, 02:35 PM   #24
diyAudio Member
 
megajocke's Avatar
 
Join Date: Jan 2003
Quote:
Originally posted by janneman



Hi Jacco,

He banged the gate with 9V he said, so he was switching them fully on. Vds close to zero.

Jan Didden
Rdson of the laterals is in the 1-2 ohm range when fully saturated - but the current when they desaturate is low with 9V gate bias. With 9V on the gate the initial current is about 10-15 amps with a stone cold fet but decreases quickly as the 500W+ dissipation heats the die pretty fast. The power supply did not current limit and sag during the test and there is a lot of output capacitance in it. Vds was 50V during the whole test.

The transconductance/resistance change causes the current to drop fast. About 6A after a couple tenths of a second which fell to about 2A after a few seconds. A while after that the transistor got hot enough for the internal zeners to start clamping the gate at a lower than usual voltage and if the current into the gate terminal exceeded about 10mA they latched up like thyristors dropping drain current to about 0.2A.

If the current into the gate terminal was kept low enough to not make the zeners latch up the transistor would respond to decreasing Vds with increasing Ids to keep the same temperature and power dissipation!
  Reply With Quote
Old 31st October 2008, 02:39 PM   #25
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Hi megajocke,
That sounds reasonable. 2SK1058 at max Idss, the device can have as a worst case 12 volts Vsd.
Which of course reduces efficiency.
Attached Images
File Type: jpg fet.jpg (56.5 KB, 344 views)
  Reply With Quote
Old 31st October 2008, 02:54 PM   #26
diyAudio Member
 
jacco vermeulen's Avatar
 
Join Date: Oct 2004
Location: At the sea front, Rotterdam or Curaçao
Send a message via Yahoo to jacco vermeulen
This here gathering is DIY and not commercial enterprising, right ?

Hop-shopping across the frontier buys single-die lateral MOSFETs for as low as $3/pc +taxes.
Around the corner boutique parlors easily charge $5 for a vertical MOSFET, and without a guarantee that you'll receive all same batch items !

A decent output stage requires at least 4 output devices for thermal efficiency reasons, no matter the type of output device.
It doesn't take 3 to 4 times as many Lateral MOSFETs to achieve comparable transconductance numbers as a Verti-M output stage.
Read what EUVL had to say about Gm numbers of Lateral/Vertical parts, and about not to read datasheets as the book of psalm.

Lateral MOSFETs are not expensive, they cost less than 25 years ago.
And 25 years ago, a Vertical MOSFET cost more than a Lateral one on this continent.
If someone isn't building crab amps, he'd have to buy more Vertical output devices than the required number he needs if he's going through the regular vendor channels.
Tolerances of Vertical parts are larger than those of (Japanese) Laterals.
If cost is an issue, then you'd also have to add the paid VA that is lost because of the difference in Gate voltage threshold to the balance.
__________________
The buck stops Here
  Reply With Quote
Old 31st October 2008, 05:08 PM   #27
CBS240 is offline CBS240  United States
diyAudio Member
 
CBS240's Avatar
 
Join Date: Aug 2005
Location: K-town
Quote:
Originally posted by unclejed613
unlike tubes, gates should never draw any current. unless they're shorted to the channel or they have internal zener protection. with RF there's also the possibility of RF current into the gate capacitance.


You bet there will be gate current required to drive the mosfets at RF. It requires a bit of energy to push and pull the majority carriers into and out of the channel at such speeds.


Quote:
VFETs actually have a positive tempco only up to a point (up to about 100mA or so, sometimes as high as 1 amp) after that it turns negative like L-MOSFETs. that explains why they behave better with higher bias current.
Yes as pointed out, this is backwards, some verticals like hexfets have a zero temp co at many amps. For example, a 47 amp hexfet transistor zero temp co may be ~30A. A little bit high for a bias current I would think.

The change in transconductance wrt Vgs and temperature is another motivating factor for error correction.

If used properly, I agree with Bob, they just can't be beat, but there are certainly some drawbacks one must deal with.
__________________
All the trouble I've ever been in started out as fun......
  Reply With Quote
Old 1st November 2008, 11:52 AM   #28
Banned
 
Join Date: Apr 2005
Location: Stockholm
Hi,
In enhancement types, the control electrode is isolated from the current conducting channel, resulting in practically zero gate current and infinite input impedance. The on-state drive current is zero as well.
MOSFETs have a resistive nature, RdsOn of Hitachi devices is 1Ohm @ Ta=25°C, promoting linearity. MOSFETs exhibit a positive temperature coefficient of 0.7%/°C to 1%/°C (- 2.2mV/°C for p-n junction), acting as a negative feedback in parallel operation. RdsOn increases, Gm decreases with increasing temperature.
The lateral structure is not efficient. The price of efficiency is always sound quality.
  Reply With Quote
Old 1st November 2008, 12:08 PM   #29
CBS240 is offline CBS240  United States
diyAudio Member
 
CBS240's Avatar
 
Join Date: Aug 2005
Location: K-town
Quote:
Hi,
In enhancement types, the control electrode is isolated from the current conducting channel, resulting in practically zero gate current and infinite input impedance. The on-state drive current is zero as well.
True....at 0 Hz.

change in Vgs across the input capacitance requires AC current to drive because Z=1/(2pi*F*C) Also there is Cdg in there too...also not very linear wrt Vds.
__________________
All the trouble I've ever been in started out as fun......
  Reply With Quote
Old 1st November 2008, 06:36 PM   #30
Banned
 
Join Date: Apr 2005
Location: Stockholm
CBS240,
well, to my knowledge, no one has been calling for more driving current than I have.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
cascode output stage what device? Stefanoo Pass Labs 24 27th February 2009 03:35 PM


New To Site? Need Help?

All times are GMT. The time now is 11:42 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2