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.
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.
mus said:
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.
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
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
cerrem said:
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
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.
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.
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.
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.
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.
I was considering using Fairchild's FQPF2N60, as it seems to have an even lower reverse transfer capacitance. Would this be a better choice than the IRFBC20? Also the FQPF2N80 has an 800V rating and the reverse transfer capacitance is almost as low as the 2N60.
What do you guys think of these?
What do you guys think of these?
David Davenport said:
Dave,
When it comes to electronics, I'm a "belt and suspenders" kind of dude. The drive capability of the 'X7 and its close relatives is so poor, the only combined MOSFET I'm comfortable with is the ZVN0545A. Its reverse transfer capacitance is all of 4 pF.
ZVN0545A data sheet here.
Hi SY, I just want to drive EL34s in pentode mode Class AB1 PP, without the risk of blocking distortion on transients. I'm not looking for Class AB2 (except maybe momentarily, to provide a bit of headroom during transients), because I don't believe there's anything to be gained from it and I doubt the EL34 grids would take kindly to prolonged grid current snyway. I could use 6SN7 cfs but I think MOSFET sfs should be a neater solution.
OK, that makes things a lot easier and very logical. 6L6-types do very well driven into AB2, but I've not had good results trying that with EL34.
In your app, then, source impedance is really not that important- pentodes have a nice low input capacitance (which is why I go the big grid resistor route instead of a follower). As a practical matter, I haven't had any undue problems with bandwidth driving IRF820 from 6SL7 with high-ish (200k) plate loads. If a device has lower capacitance than that, it's gravy.
You could drop just about anything in that spot and it will work fine. You could even lower the current, since the output stage is in AB1.
In your app, then, source impedance is really not that important- pentodes have a nice low input capacitance (which is why I go the big grid resistor route instead of a follower). As a practical matter, I haven't had any undue problems with bandwidth driving IRF820 from 6SL7 with high-ish (200k) plate loads. If a device has lower capacitance than that, it's gravy.
You could drop just about anything in that spot and it will work fine. You could even lower the current, since the output stage is in AB1.
Thanks SY, you must be telepathic! I was indeed thinking of a 6SL7 diff amp before the MOSFETs, despite ELI's warning about using wimpy tubes. I want another 6SL7 before that, as an LTP splitter input stage (with 6AU6 CCS in the tail). Actually, if I can find them at a reasonable price, I'd rather use the better-matched 6SU7.
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