IRFBC20 as source follower drivers for EL34s in AB1 PP - diyAudio
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Old 18th March 2008, 11:37 AM   #1
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Default IRFBC20 as source follower drivers for EL34s in AB1 PP

Father forgive me, for I have searched ... but couldn't find the answer.

I guess this question is for Eli, who has recently recommended the IRFBC20 for use as a source-follower driver, or for anyone else who knows. My queston is, what would be suitable source current and value of gate stopper resistor? TIA for any advice.
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Old 18th March 2008, 08:46 PM   #2
mus is offline mus  Canada
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Hi,

I'm also thinking to use this MOSFET source-follower driver for my parallel PP KT90. I have lots of IRF820a. Do they make big sonic difference comparing to IRFBC20?
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Old 18th March 2008, 09:07 PM   #3
EC8010 is offline EC8010  United Kingdom
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Quote:
Originally posted by mus
Do they make big sonic difference comparing to IRFBC20?
I wouldn't think so for one moment. Device to device variations in FETs can be as big as type differences!

Gate stopper: Depends a little on layout and a little on device. 4k7 should stop anything and there's no pressing reason to use a lower value.

The more current the merrier, and if you had a dedicated +40V, -60V supply, I'd say 10mA per FET would be ideal. You could probably get away with 2mA.
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Old 18th March 2008, 09:29 PM   #4
cerrem is offline cerrem  United States
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Both the IRFBC20 and the IRF820a have been given over to Vishay Silitronix..... So they are now making those parts...
IR handed over all the device dies that exceed 300V to Vishay...
One is 600V N-Channel FET and the other is 500V ....
The key is knowing what the gm is....since 1/gm is the output resistance of the Source.... Also the total Gate capacitance is critical when loading from the previous stage... you need to properly calculate the series gate resistor, "grid stop" , or you will get poor high frequency response...
So the 3 main things to know with these FETS is Vds, C, and gm...

Chris
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Old 18th March 2008, 10:11 PM   #5
cerrem is offline cerrem  United States
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Quote:
Originally posted by cerrem
Both the IRFBC20 and the IRF820a have been given over to Vishay Silitronix..... So they are now making those parts...
IR handed over all the device dies that exceed 300V to Vishay...
One is 600V N-Channel FET and the other is 500V ....
The key is knowing what the gm is....since 1/gm is the output resistance of the Source.... Also the total Gate capacitance is critical when loading from the previous stage... you need to properly calculate the series gate resistor, "grid stop" , or you will get poor high frequency response...
So the 3 main things to know with these FETS is Vds, C, and gm...

Chris

Both devices have the same gm of 1.4 Siemens.... which is basically 1.4 A/V .... in tube lingo it would be 1400mA/V which is pretty darn good....compared to an EF86 at roughly 2mA/V ...

Both devices have the same gate charges....
Both have about 350pF input capacitance....

One is 500Vds the other is 600Vds....both good for a driver..

The biggest difference is that the IRFDC20 only can hanle a MAX drain current of 200mA .... while the IRF820A can handle up to 1.6A ....so there is a big difference in power handling... The SOURCE loading will be significantly different.... to keep the device in safe operating region....given the same AC voltage swing....

Chris
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Old 18th March 2008, 11:36 PM   #6
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The actual transconductance (gfs) of a MOSFET is approx. = ID. The data sheet spec. for gfs is given at a high ID. Of course, parts makers try to look as good as possible.

What I like about the IRFBC20 is its low and stable reverse transfer capacitance. Reverse transfer capacitance is the critical parameter in voltage follower service. I would not try to drive an IRFBC20 with a 12AX7 or similar WIMPY type. Just about anything else is fine. The IRFBC20 data sheet is here.

Ray Moth asked for a recommendation for ID. As ID = 15 mA. yields a gfs of approx. 15 mA./V., that's my advice. An O/P impedance under 100 Ohms should be able to comfortably interface with a transmitting type operating with significant control grid current present.
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Old 19th March 2008, 07:46 AM   #7
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Well, thanks very much for all the input. One reason I asked about the drain current was because unless I buy an extra transformer (admittedly not a huge cost), I'm limited to a maximum of 4.5mA per drain. According to EC8010 "the more the merrier" although ha also said "you could probably get away with 2mA." So maybe 4.5mA would be suboptimal, but as long as it works OK I'd be happy.
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Old 19th March 2008, 09:11 AM   #8
Serge66 is offline Serge66  Switzerland
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Hi,
I tried once to implement a current source in the cathodes (tied together) of KT66. Not good. It was ok at small power, but not for higher output. As soon as the Class A was left, distortion become exceedingly high. It should work if you bias your tubes in Class A only.
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Old 19th March 2008, 10:05 AM   #9
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Hi Serge66,

My question was about using MOSFETs as drivers instead of cathode followers. What you're talking about is using a constant current sink (CCS) as a common cathode load for a pair of output tubes. As you say, that will only work for Class A because the CCS forces the total current through the pair of tubes to be always the same.

Class AB doesn't work that way, because each of the two tubes is meant to cut off during part of its negative-going cycle. That is contrary to the requirements of the CCS. What will happen is probably that the conducting tube will be forced to draw even more current to make up the difference, to compensate for the other tube not conducting at all while it is cut off. That would result in very poor linearity, in other words, severe distortion.
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Old 19th March 2008, 11:45 PM   #10
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Ray,

I don't think you will have a problem with ID = 4.5 mA. A driver whose O/P impedance is under 250 Ohms should be loafing, when working into EL34s. Even when triode wired, EL34 Miller capacitance is not huge.
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