If you do a few calculations based on John's previous post you will see the answer.
He said he is dropping 15 volts across a 200 ohm resistor, so the front end and the folded cascode together are drawing 75 mA. If you run the front end with "blue" Toshiba JFETs, they will draw about 5 mA. If you run them with "violet" JFETs, they will draw about 10 mA. So that means he is running at least 65 mA in the output stage. With 15 volts across the output devices, you need a part rated at 30 volts that can dissipate 1 watt and run at 65 mA. There are no complementary audio JFETs that can even come close to meeting these requirements. However, this is easy for the TO-220 sized Hitachi lateral MOSFETs.
He said he is dropping 15 volts across a 200 ohm resistor, so the front end and the folded cascode together are drawing 75 mA. If you run the front end with "blue" Toshiba JFETs, they will draw about 5 mA. If you run them with "violet" JFETs, they will draw about 10 mA. So that means he is running at least 65 mA in the output stage. With 15 volts across the output devices, you need a part rated at 30 volts that can dissipate 1 watt and run at 65 mA. There are no complementary audio JFETs that can even come close to meeting these requirements. However, this is easy for the TO-220 sized Hitachi lateral MOSFETs.
MOSFET Source Follower
I wish to refer to the schematic showed in Post #759.
I have been trying to come up with a MOSFET version for some time, but have not been able to come up with something more intellegent (elegant) than this (see PDF attachment).
The problem with this is of course the lack of adjustment for bias. One can perhaps replace the Zeners with resistors or trimmers, but I am still not entirely happy.
Any hints / suggestions ?
Patrick
I wish to refer to the schematic showed in Post #759.
I have been trying to come up with a MOSFET version for some time, but have not been able to come up with something more intellegent (elegant) than this (see PDF attachment).
The problem with this is of course the lack of adjustment for bias. One can perhaps replace the Zeners with resistors or trimmers, but I am still not entirely happy.
Any hints / suggestions ?
Patrick
John,
Your last post concerning the voltage-drop accross the 200 Ohm resistors, my close observation of the BT Line stage board and additional simulations, gives me reason to conclude that there are, apart from the 2 gain-set resistors, 4 bias source-resistors mounted underneath the PCB to prevent the JFETS from exceeding their maximum dissipation level.
Is this conclusion correct?
Thanks again.
Your last post concerning the voltage-drop accross the 200 Ohm resistors, my close observation of the BT Line stage board and additional simulations, gives me reason to conclude that there are, apart from the 2 gain-set resistors, 4 bias source-resistors mounted underneath the PCB to prevent the JFETS from exceeding their maximum dissipation level.
Is this conclusion correct?
Thanks again.
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