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Old 5th October 2014, 01:05 AM   #1
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Default Floated cascode with jFETs for high voltage?

Hi,

I was doing some experiments in Spice and came up with something I'd like to share.

a.png

A long time ago, I found this use of cascoding jFETs across their turn-on voltage and thought it was quite interesting. Although the 2sk170 (J1) has a breakdown of 40V, the 2SK246 (J3) has a breakdown of 50V and gives a bit more protection.

It doesn't buy me much protection, but what If I replace J3 with a depletion-mode n-channel MOSFET such as a Supertex DN2540? Turn-on voltage is about 2.4V with a breakdown of a whopping 400V!

Ok, now we're talking.

Oh, wait... there's no such thing as a P-channel depletion-mode MOSFET! Drat!

Here's what I came up with to emulate one.

b.png

The Q5 CCS draws current through the LED and Q2 to keep them on. The voltage point at the gate of the TP2540 p-channel enhancement MOSFET (M2) is determined by the base of Q1 + 2 diode drops and the forward drop of the LED. The source for M2 gives a steady 2 volts or so for J2. I'm kinda happy with this. Don't forget to notice the miller comp barely exists to stabilize it and that's the value for +6dB of gain(!) Not shown are the 120V rails

Now to build it.
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Last edited by davygrvy; 5th October 2014 at 01:12 AM.
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Old 5th October 2014, 02:17 AM   #2
mlloyd1 is offline mlloyd1  United States
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thanks for sharing.
looks interesting.
please keeping posting after you build and test!

mlloyd1
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Old 7th October 2014, 08:03 AM   #3
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Anyone ever seen a diff input like this before?
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