Do we always need an emitter follower to drive a LATFET output stage??? - diyAudio
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Old 30th December 2006, 12:40 AM   #1
Leolabs is offline Leolabs  Malaysia
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Default Do we always need an emitter follower to drive a LATFET output stage???

Some just drive it from the collector of a BJT(either a CE or a cascode),enough to charge the gate capacitance???
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Old 30th December 2006, 10:20 AM   #2
lineup is offline lineup  Sweden
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Default Re: Do we always need an emitter follower to drive a LATFET output stage???

Quote:
Originally posted by Leolabs

Some just drive it from the collector of a BJT
.....
Is enough to charge the gate capacitance???
in many cases - yes
I could give you 10 really good hifi amplifiers
using only one of these output pairs:
2SK1058 - 2SJ162
2SK1530 - 2SJ201
outputs
... and driven by 'collectors' of VAS.

MATCHED PAIRS Lateral K1058 J162 - fair price from diyAudio.com member

Data and INFO of 2SK1058 2SJ162 - Renesas.com


Naturally we have to have not Too Low current to for drive ANY POWER MOSFET.
Does not matter if they are laterals.
If you use only < 5mA, you are not looking for hifi performance ..


Ask John Curl,
one of our most experienced members, when comes to amplifiers.
He will tell you this, like lineup tells you now:
=================
The Cure
for Capacitance
is Current
=================




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Old 30th December 2006, 10:50 AM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
the VAS is run in ClassA and is usually fed from the LTP.

At high frequencies, the load the LTP sees is the capacitance of the VAS//collector load.

At these same high frequencies the load the VAS sees is the capacitance of the output FET//CCS effective load.

The problem is that the LTP wants to see a small capacitance at the VAS and the VAS wants to see a small capacitance at the FETs.
This is a conflict and the usual solution is impedance conversion. That's another way of saying, provide more current.

Use an emitter follower and the problem is divided by the current gain through the EF.
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Old 30th December 2006, 11:02 AM   #4
sajti is offline sajti  Hungary
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I think, there is not necessary to use EF stage to drive MOSFETs. I made power amplifier with 4pairs of IRFP240/9240, which has no EF, but there is 100mA biased VAS stage, using MJE15032/33. Of course, You need darlington for the Vgs multiplier to handle this current, and EF between the input stage and the VAS, but this is not so critical.


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Old 30th December 2006, 09:16 PM   #5
suzyj is offline suzyj  Australia-Aboriginal
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One must remember that the input capacitance of a lateral FET is much lower than that of a vertical FET. This makes the drive requirements more relaxed.

Of course you still need a relatively low impedance driver, but there are plenty of topolologies that can do this without burning huge amounts of current and without using emitter followers.

A topology that I've had a lot of success with is that developed by David Tilbrook in the '80's, for the AEM6000 power amp. He uses a symmetrical differential VAS. In my take on this topology I burn just 6mA on each side of the differential VAS, and achieve 0.0048% THD (measured) at 10KHz (100W into 8 Ohms).

Cheers,

Suzy
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Old 31st December 2006, 08:28 AM   #6
AndrewT is offline AndrewT  Scotland
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Hi Suzy,
does that put an absolute limit on FET gate current of 6mA or 12 mA or does the VAS go into ClassAB?
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Old 1st January 2007, 10:35 PM   #7
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i noticed that little peak in the open loop response where the phase crosses the 180 degree mark, as well as your phase plot with feedback. rule of thumb is if you cross 180 with gain, it may oscillate, which is why it did when built (you still had gain at 1.1mhz, even after adding the bypass cap to the input). it would be interesting to see the gain/phase plot now that you added compensation to the VAS.

nice design...... i might try it myself.......
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Old 5th January 2011, 06:47 AM   #8
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Default Driving lateral MOSFETs

I agree with Suzyj's comment.

As long as you are driving a relatively small number of output devices, you can use the input capacitance of the output stage, in conjunction with the high output impedance of the voltage gain stage to set the dominant pole of the open-loop transfer function. You have to set it somewhere

Provided that you are not trying to use too much overall feedback, it works very well. In fact, I regard this as one of the advantages of MOSFETs over BJTs in the output.

Best wishes
Dave
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Old 5th January 2011, 07:39 AM   #9
sandyK is offline sandyK  Australia
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David Tilbrook ?
Now that's a name I haven't seen around for a fair while !
Thanks for those great designs of yours like the Ultra Fidelity preamp,5000 and 6000 etc. that so many of us constructed years ago . Actually, I know of a member who is still using his 6000 to drive the low end of his biamped Infinity speakers.
Regards
Alex

P.S.
A couple of years ago SuzyJ tested my old 5000 that had been rescued from a friend's garage. The channel where the electros hadn't dried out still tested .0062%

Last edited by sandyK; 5th January 2011 at 07:42 AM.
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Old 5th January 2011, 08:28 AM   #10
VHF man is offline VHF man  Australia
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Quote:
Originally Posted by sandyK View Post
David Tilbrook ?
Now that's a name I haven't seen around for a fair while !
Thanks for those great designs of yours like the Ultra Fidelity preamp,5000 and 6000 etc. that so many of us constructed years ago .

Hi Dave T and welcome to DIY.
I do still have an original AEM6000 that's fully operational and sounding as good as the day it was built - which is now > 22 years ago. A truly great design if I may say so.
And yes, the input C Vs VGS is reasonably linear on the old laterals - above 2V. Most amps these days tend to use miller compensation schemes which is really good for THD but at the expense of slew rate.

RRR Sandy K.
I also met Phil Wait on Sydney Harbour last week on NYE. I asked him about the ETI 470 design along with a few other projects that he was involved with back in the 70's. His advice today is to use one of the new LME 498xx chips - to drive lateral mosfets.
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