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Old 27th May 2004, 01:20 AM   #1
JoeBob is offline JoeBob  Canada
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Default Common Cathode 6922 output impedance?

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?
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Old 27th May 2004, 04:28 AM   #2
SY is offline SY  United States
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The source resistance will look like rp in parallel with the plate load resistor, assuming your bias scheme places the cathode at ground.
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Old 27th May 2004, 05:02 AM   #3
grataku is offline grataku  United States
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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.
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Old 27th May 2004, 07:37 AM   #4
JoeBob is offline JoeBob  Canada
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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.
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Old 27th May 2004, 11:43 AM   #5
SY is offline SY  United States
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Grataku, if I understand correctly, the Cgs of the source follower is indeed not a significant factor since the source signal "follows" the gate. But gate-drain capacitance (which does need to be charged and discharged) can be pretty significant.
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Old 27th May 2004, 01:38 PM   #6
JoeBob is offline JoeBob  Canada
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Well, thanks for the help guys. I checked, and I guess I will be needing the cathode follower, idealy the output impedance should be under 1k, no way I can get the 6922 alone to that ah well... Thanks again though.
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Old 27th May 2004, 03:43 PM   #7
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Two paralleled sections 6922 might just do the job. Although choosing the lesser evil will be tough...
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Old 27th May 2004, 10:17 PM   #8
grataku is offline grataku  United States
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Quote:
Originally posted by SY
Grataku, if I understand correctly, the Cgs of the source follower is indeed not a significant factor since the source signal "follows" the gate. But gate-drain capacitance (which does need to be charged and discharged) can be pretty significant.

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_Folder...osfetfolly.htm

IME if you run a mosfet hot enough it turns into a tube (soundwise)
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Old 27th May 2004, 10:43 PM   #9
SY is offline SY  United States
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Quote:
And I thought I was being real original but someone had already built something similar apparently with hi-power mosfets as well.
Well, someone else on this forum has said over and over that MOSFETs are dandy as source followers to drive output tubes. And he actually uses them to drive the screen grids of high perveance power tubes with grounded control grids. Who could that be, who could that be...?
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Old 27th May 2004, 11:42 PM   #10
JoeBob is offline JoeBob  Canada
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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?).
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