FET compound pair? - 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 11th February 2002, 09:37 PM   #1
diyAudio Member
 
Join Date: Feb 2002
Location: Western Sydney
Question FET compound pair?

Hi Guys, another newbie question:
is it possible to make a compound pair of FETs a la Darlington, Tslaki (sp?) ? I think I've seen something similar using a BJT + a FET. Any help appreciated, cheers, Pete McK
  Reply With Quote
Old 11th February 2002, 10:28 PM   #2
diyAudio Member
 
traderbam's Avatar
 
Join Date: Jan 2002
Location: Earth
Sure it is.
But why would you want to?
  Reply With Quote
Old 12th February 2002, 03:38 AM   #3
diyAudio Member
 
Join Date: Feb 2002
Location: Western Sydney
Default fet compound

Hi Z in for guitar amp; lots of voltage gain...???
  Reply With Quote
Old 12th February 2002, 05:42 AM   #4
diyAudio Member
 
dshortt9's Avatar
 
Join Date: Apr 2001
Location: Milwaukee, Wisconsin
Default FET compound pair

Check this link:
http://www.ne.jp/asahi/evo/amp/J49K134/zu3.pdf

http://www.ne.jp/asahi/evo/amp/J49K134/report2.htm

and this:
Both use a compond FET output stage said to be very linear.
  Reply With Quote
Old 12th February 2002, 06:54 AM   #5
diyAudio Member
 
seangoesbonk's Avatar
 
Join Date: Jan 2002
Location: Vancouver, BC, Canada
Default Re: FET compound pair?

Quote:
Originally posted by PeteMcK
Hi Guys, another newbie question:
is it possible to make a compound pair of FETs a la Darlington, Tslaki (sp?) ? I think I've seen something similar using a BJT + a FET. Any help appreciated, cheers, Pete McK
I'm not sure if this answers your question, but a FET compound pair is called a Fetlington.
BJT==>Darlington
FET==>Fetlington
One example is the 2N7000, i believe...
  Reply With Quote
Old 12th February 2002, 11:06 AM   #6
AKSA is offline AKSA  Australia
diyAudio Member
 
Join Date: Sep 2001
Location: Melbourne, Australia
Smile Darlington FETs

Wow! A fetlington? Crazy term! I love it!

Doug Self has quite a bit to say about the CFP - or complementary feedback pair, often called a Sziklai pair.

FETs can be configured with a bipolar to form a CFP. This approach works extremely well, with a NPN driving a P-type FET from a positive rail, for example.

The advantage of this arrangement is that the Vbe of the driver is closely controlled and since it's parasitics are so low, there is no capacitive gate to drive. However, the self-oscillation tendency of FETs makes it a tad unstable unless the emitter of the bipolar carries a small resistor, typically 10R, to degenerate its gain and lessen the local feedback factor slightly.

A CFP can be configured for gain, as well. This reduces feedback factor, but costs linearity slightly. CFPs are very elegant, interesting circuit blocks, and the hybrid bipolar/FET version is one of the best.

Hugh

www.printedelectronics.com
  Reply With Quote
Old 12th February 2002, 10:13 PM   #7
jam is offline jam  United States
diyAudio Member
 
jam's Avatar
 
Join Date: May 2001
Location: Auburn, CA, USA
Hugh,

It seems like a good idea with Japanese mosfets, which seem to have a pretty consistant turn on voltage. It seems it would require some pretty heroic matching with International Rectifier devices.
Do you have any thoughts on the use of a CPF outside the the feedback loop?

Jam
  Reply With Quote
Old 13th February 2002, 12:22 AM   #8
jam is offline jam  United States
diyAudio Member
 
jam's Avatar
 
Join Date: May 2001
Location: Auburn, CA, USA
I ment CFP.

Jam
  Reply With Quote
Old 13th February 2002, 12:54 AM   #9
AKSA is offline AKSA  Australia
diyAudio Member
 
Join Date: Sep 2001
Location: Melbourne, Australia
Hi Jam,

CFPs outside the feedback loop. Hmmm. Yes, I have some experience, not too much.

To begin, I once built two identical amps with a CFP output stage on one and a conventional double emitter follower (DEF) on the other.

Same CD player, same music, same room, same speakers. Even same power supply.

The CFP, not using MOSFETs I will admit, was not musical. The DEF was. You could hear the person behind the voice.

I investigated on the test bench. On the negative rail CFP there was minor transient oscillation, despite a 10R output device base stopper. This oscillation caused loss of detail and considerable intermodulation. It was suitably scotched with a 100pF cap between base and collector of the driver, and the amp returned to the listening room.

Now the sound was completely dead. Lifeless. IOW, manacling the output stage to eliminate local oscillation completely destroys the music.

Next, I built a single ended hybrid CFP with a bipolar driver and an IRC MOSFET. No feedback. I drove it with a tube, ran 3A down the mosfets, and cooled it with a gargantuan heatsink (150W dissipation!). Vcc was 50V.

It sounded wonderful. Nelson would have liked it; the principles echo his approach. I called it the Glass Harmony.

One of the aspects of PP topology I considered was the crossover disjunction. With the CFP, the turnoff and turnon of the drivers is more abrupt (or cleaner, view it as you wish) than with DEF, which is slowed somewhat since there are two junctions controlling the event, rather than one. This makes the crossover disjunction narrower, typically 1.2V, but it does not necessarily make the switching spikes more benign than the DEF. Add to this the tendency towards instability, and the measures one must take to eradicate it, and, to me at least, the advantage is lost. Pity, it's an elegant configuration.

While high order switching spikes cannot effectively be smoothed with global negative feedback and one might be tempted to take the output stage outside the feedback loop as you suggest, this is really not practical with a PP topology because the high order switching spikes remain with or without feedback and inevitably cause listener fatigue. Of course, it goes without saying that with a Single Ended CFP topology this situation no longer prevails, and indeed, the Glass Harmony sounds so good it is more a musical instrument than an amplifier. But it uses too much energy, and I frown upon inefficiency. If only I could make a single ended push pull amplifier in AB! (Yes, I believe it can be done, and am presently chewing multiple pencils as we speak!)

There are other effects when taking the output stage out of the global feedback loop. I have tried this with the DEF configuration, and the effects are interesting:

1. A very twee, 2A3ish sound, fragile, refined, lacking in drive.
2. Wonderful sound stage due to taking the sluggish output stage outside the loop and thereby removing phase anomalies.
3. Extremely musical bass, owing to the H2 induced by some loss of damping factor. Very similar to a PP tube amplifier, actually.
4. The same old listener fatigue, unchanged.

I hope this gives you something to think on. This stuff is not easy.....

Cheers,

Hugh

www.printedelectronics.com
  Reply With Quote
Old 13th February 2002, 02:41 AM   #10
jam is offline jam  United States
diyAudio Member
 
jam's Avatar
 
Join Date: May 2001
Location: Auburn, CA, USA
hugh,

Thanks for your insight into the CFP. I had been trying to get it to work for seversl years with marginal results, I finally quit.
You certainly have answered some nagging questions that I had about the CFP.

Single ended push-pull AB? Now that sounds like a challange, if it can be done it certainly will solve a lot of problems we face.Would the bias have to track the input?

Cheers,
Jam
  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
Compound hornloudspeakers angeloitacare Multi-Way 39 11th September 2011 08:06 PM
Compound horns, anyone? brsanko Full Range 30 15th August 2011 02:15 AM
Hartley compound sub rdr Subwoofers 3 2nd October 2006 01:04 AM
sil pad 900s with or without compound? jeapel Pass Labs 3 6th March 2006 07:38 PM
Heatsink compound Thoth Solid State 11 1st January 2002 02:08 AM


New To Site? Need Help?

All times are GMT. The time now is 09:51 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