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Hi all,
I have a question about the Mu follower output of a cascade CCS.
The Mu output should be able to drive a low impedance. What if you were to take a low current driver tube, say a 12ax7, and load it with a cascade CCS. The 12ax7 should not be able to drive the miller capacitance of a big triode, like a 300b. But, can the Mu-output, on top of a 12ax7, drive 300b?
I've copied some notes from Gary Pimm below. (Thanks Gary!).
Thanks,
Dan
"Using a CCS as a combined CCS and mu follower
When an n-channel mosfet is used for a CCS we have an opportunity to get a second function almost for "free". The output signal can be taken from the source of the lower mosfet instead of the plate of the triode. Refer to the schematic above to see the circuit detail of where the signal is taken from.
When driving output triodes or interconnect cables that are high capacitance loads, this arrangement works very well. The driving triode is isolated from the load capacitance to maintain constant current operation at high frequencies and the next stage is driven from low source impedance."
--Gary Pimm
pimmlabs.com
Follow Ups:
" But, can the Mu-output, on top of a 12ax7, drive 300b?"
It depends on the CCS.
Gary's CCS have much lower output Z from the "mu" output than just a simple pair of dn2450s in cascode.
I believe it all has to do with the value of the current set resistor.
By biasing the gates of the MosFets upward, the current set resistor can be (needs to be) made a larger value for the same set current.
This applies more feedback for the MosFets so the output impedance of the mu output is lower.
I'm not sure a 12ax7 would have a low enough drive impedance using the mu output of a simple pair of MosFets in cascode and I know the 12ax7 wouldn't have enough drive current to drive the Miller capacitance of a 300b no matter what.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
OK, got it. I see there is no free lunch.
Thanks Tre,
Dan
The mu-output Z is based on delivered gm of the lower MOSFET. The drain-source voltage is based on the i*R voltage driven by the LND150 ccs. You can do this with the depletion mode FET's too in order to require a relatively large value of R-set...which de-sensitizes the circuit from minute changes in that resistor's value.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
" The Zout of the mu-output is a function of the gfs of the MOSFET you are using, the value of the MOSFET source resistor (Rset), and the internal plate resistance of the tube. For a "large" value source resistor the value approaches that of a simple source follower of 1/gfs and is less dependent on the tube anode resistance. For finite small Rset values, the anode current is a fraction of output current defined by 1/(gfs * Rset). The Zout thus also includes Rp / (gfs * Rset) in series.
The gfs increases, therefore Zout decreases, with increasing current. The plate resistance also decreases with increasing current, resulting also in decreased Zout at higher current. Unfortunately, higher current means a smaller value Rset, which tends to increase Zout counteracting some of the improvement due to higher gfs and lower Rp. "
"Anything you do to increase the resistance between bottom and top devices will decrease the output impedance at the "follower" output."
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Which of course is why the battery bias option, or the i*R generated voltage on the gate is so useful; it allows larger R_set values...:) Thanks for the linque.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
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