Hi,
I´ve seen a few different cascoded Jfet BOZ-type circuits on this forum and my question is: Could such a circuit be optimized to amplify a line level signal up to something like 30Vrms?
The purpose is to build an SS gainstage capable of driving a power triode in a hybrid arrangement.
The triode in question (808) requires class A2 operation to produce any useful power and the resulting grid current demands would be taken care of by a Mosfet follower driver stage.
Here´s a link to a schematic in another thread, ignore the resistor values and focus on the topology:
http://www.diyaudio.com/forums/attachment.php?postid=1302735&stamp=1189942198
My idea is to use a J310 Jfet operating at ~12mA and 5V d-s with an IRF620 mosfet, 4k7 drain load resistor, 68R source resistor and 150V (or so) rail voltage.
The whole thing would be cap coupled to the Mosfet driver stage.
Unless I´ve missed something fundamental this circuit should be able to produce something like a +-50V peak output signal which would cover my needs. The gain should also be high enough to allow a few dBs of global NFB from the output to the Jfets source pin.
Thoughts and comments are most welcome!
Best regards,
Daniel
I´ve seen a few different cascoded Jfet BOZ-type circuits on this forum and my question is: Could such a circuit be optimized to amplify a line level signal up to something like 30Vrms?
The purpose is to build an SS gainstage capable of driving a power triode in a hybrid arrangement.
The triode in question (808) requires class A2 operation to produce any useful power and the resulting grid current demands would be taken care of by a Mosfet follower driver stage.
Here´s a link to a schematic in another thread, ignore the resistor values and focus on the topology:
http://www.diyaudio.com/forums/attachment.php?postid=1302735&stamp=1189942198
My idea is to use a J310 Jfet operating at ~12mA and 5V d-s with an IRF620 mosfet, 4k7 drain load resistor, 68R source resistor and 150V (or so) rail voltage.
The whole thing would be cap coupled to the Mosfet driver stage.
Unless I´ve missed something fundamental this circuit should be able to produce something like a +-50V peak output signal which would cover my needs. The gain should also be high enough to allow a few dBs of global NFB from the output to the Jfets source pin.
Thoughts and comments are most welcome!
Best regards,
Daniel
Yes, there will be a follower between the Jfet circuit and the output stage. 2SK2700 seems like a good choice there due to it´s low reverse transfer capacitance.
If necessary I could always stack up two or more jfets in // to get a "beefier" input stage, but I think 12mA and 4,7k drain load *should* be enough to drive the Mosfet follower.
There´s no point in aiming for a sky high bandwidth as the amps response will be limited by the output transformer anyway.
If necessary I could always stack up two or more jfets in // to get a "beefier" input stage, but I think 12mA and 4,7k drain load *should* be enough to drive the Mosfet follower.
There´s no point in aiming for a sky high bandwidth as the amps response will be limited by the output transformer anyway.
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