I was going to put together a preamp made of a 6922 folowed by a beefy cathode follower, so that it will drive a MOSFET in a source follower setup. The 6922 will simply be battery biased with close to 3V, and be a gain stage, since a source follower has none.
I know I can't have too high an output impedance, for the input capacitance of a source follower is rather high, but my question is, what is the output impedance of a 6922 in such a circuit? Will I even need the cathode folower to drive the poweramp?
I know I can't have too high an output impedance, for the input capacitance of a source follower is rather high, but my question is, what is the output impedance of a 6922 in such a circuit? Will I even need the cathode folower to drive the poweramp?
guys,
I thought that in the source follower the input capacitance didn't matter much at all.
In fact, I would argue that the 6922 should be able to drive the mosfet gate directly w/o the cathode follower. I was looking into this a while back and I found there are some nice high voltage mosfets that would be just peachy in a preamp.
I thought that in the source follower the input capacitance didn't matter much at all.
In fact, I would argue that the 6922 should be able to drive the mosfet gate directly w/o the cathode follower. I was looking into this a while back and I found there are some nice high voltage mosfets that would be just peachy in a preamp.
Well, the source folower that it would drive wouldn't be part of the preamp, the source follower is the poweramp, an IRFP044, so the capacitance is rather high...
SY, the cathode of the 6922 will be DC biased at 3V, by battery, rp in parallel with the load resistor, that I can work with, thanks. It's too late to check the rp, gotta get some sleep, but it would be real nice not to need the cathode follower...
Thanks again.
SY, the cathode of the 6922 will be DC biased at 3V, by battery, rp in parallel with the load resistor, that I can work with, thanks. It's too late to check the rp, gotta get some sleep, but it would be real nice not to need the cathode follower...
Thanks again.
SY said:
The Crss is very very small, maybe 1/10 th of the Ciss (total input capacitance) so I don't see the problem.
And I thought I was being real original but someone had already built something similar apparently with hi-power mosfets as well.
http://www.geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm
IME if you run a mosfet hot enough it turns into a tube (soundwise)
Well, the MOSFET I'm dealing with has a typical Crss of 200nF (so the IRF datasheet says). The Ciss is listed as 2500nF, so it is indeed under 10%, but 200nF still seems high to me. I see how a lower output impedance of the previous stage would result in high-frequency roll-off, but not sure exactly how to quatisize it (is that even a word?).
"Quantify" is the word you're looking for. Anyway, you can see the effect of Crss by using f3 = 1/(2piRC), where R is the source impedance of the driver stage and C is the reverse transfer capacitance.
Depending on what you're trying to drive, grataku's point may be valid- for example, an IRF820 (which is one I use a lot) has only 10 pf drain-to-gate C. With 20Kohm driving it, the f3 is somewhere in the AM radio band.
Depending on what you're trying to drive, grataku's point may be valid- for example, an IRF820 (which is one I use a lot) has only 10 pf drain-to-gate C. With 20Kohm driving it, the f3 is somewhere in the AM radio band.
Joebob,
I looked at the model for mosfets, it seems to me that the in the follower both Cgs and Cgd are in series so I am not seeing them low pass filter but rather as a source of local feedback.
In the reference I have given the author makes a point about the mosfet being a better follower than the tube (vide infra). I wouldn't get into a glass vs. sand dispute if it was a BJT but when if we talk mosfet the sound is soo sweet and the output impedance soo low...
SY
sorry for taking the thread off topic but what I was planning to do with my not-so-original idea was a driver stage for some big triode. I have a pair of 300BXLS and I would like to drive them a little bit differently. Have you ever done such thing?
I looked at the model for mosfets, it seems to me that the in the follower both Cgs and Cgd are in series so I am not seeing them low pass filter but rather as a source of local feedback.
In the reference I have given the author makes a point about the mosfet being a better follower than the tube (vide infra). I wouldn't get into a glass vs. sand dispute if it was a BJT but when if we talk mosfet the sound is soo sweet and the output impedance soo low...
SY
sorry for taking the thread off topic but what I was planning to do with my not-so-original idea was a driver stage for some big triode. I have a pair of 300BXLS and I would like to drive them a little bit differently. Have you ever done such thing?
THE CRITICS EYE....
Hi,
If you look at low Zout as THE GOAL per se then I think you're missing the bigger picture.
In the context of tubes you'll usually find yourself driving high enough impedances with mostly stray capacitance and Miller effects the worry about in the Zout context and than some...
What's often more important is the voltage swing and current capability of the driver stage so you don't run into slew limiting.
I'm well aware high voltage FETs do exist and both tubes and FETs can live symbiotically...just trying to point out that bigger DHTs such as the 300B like to be kicked by the driver stage.
After all that's the one you'll be listeneng to at the end of the day.
Cheers,
Hi,
If you look at low Zout as THE GOAL per se then I think you're missing the bigger picture.
In the context of tubes you'll usually find yourself driving high enough impedances with mostly stray capacitance and Miller effects the worry about in the Zout context and than some...
What's often more important is the voltage swing and current capability of the driver stage so you don't run into slew limiting.
I'm well aware high voltage FETs do exist and both tubes and FETs can live symbiotically...just trying to point out that bigger DHTs such as the 300B like to be kicked by the driver stage.
After all that's the one you'll be listeneng to at the end of the day.
Cheers,
With 300Bs? No. But they're just triodes, albeit expensive ones. Frank's on point- you just need to swing enough volts to drive 'em and enough current to overcome the input capacitance. That's it- you don't need a stage that will swing a kilovolt at half an amp! It's an easier load than the sweep tubes I'm driving, and I'm not doing anything exotic other than the MOSFET source followers.
The price of the tubes is a very important point, since I don't know what tomorrow holds and I may not be able to swing another 300 bucks for a new pair of KR300B I would like to build a driver stage with enough nuts to drive bigger and hopefully cheaper tubes.
SY if you're at liberty, what are you driving exactly with your mosfet follower?
Anyway, fun discussion. The tube forum is quickly becoming my favorite
SY if you're at liberty, what are you driving exactly with your mosfet follower?
Anyway, fun discussion. The tube forum is quickly becoming my favorite
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