Mini-A to Mini-J

[grataku]

A simple enough permutation of the Mini-A it would seem, not very original either.
The PS voltage for the mini-A seems to be perfect for a clean swap.

I have the mini-A circuit breadboarded, I have the j-fets and a higher value resistor to get the feedback down from the original Mini-A which uses higher gain mosfet for the differential.

Anyone with me?

 

[winslow]

I'm curious.

 

[rtirion]

Jee

Wouldn't the drive of the upstream MOSFET require a healthy bias
through the tiny JFET?

I think you would use a 2SJ109 JFET. According to the toshiba
datasheet these have a max Pd of 200mW. Sadly no Tja is
specified, so let's asume 150 to 200K/W.

At Ids of 8mA and 15V across a JFET half means 0,12W.
That's 0,24W for the 2SJ109 pair, and +36 to +50 degrees celsius
above ambient for this very small unit.

Too rich for me, so I think you will need to cascode.

btw I am building one of these jfet-cascode A.... right now.
Will keep you posted.

Regards

 

[grataku]

Re: Jee

Wouldn't the drive of the upstream MOSFET require a healthy bias
through the tiny JFET?

What is the 'upstream' mosfet? I am confused.
if you mean the ccs that's going to change to something more appropriate

 

[rtirion]

I am sorry. I should have specified Q7, original schematic by
Grey. You can find the schematic here.

At higher frequencys (>1k) Q7 needs more and more current.

That's why you need the relative high bias for the differential pair.

Hope this helps