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In Reply to: RE: CCS Mu output question posted by Tre' on March 05, 2017 at 16:12:50
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.
Follow Ups:
" 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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