The simplistic Salas low voltage shunt regulator

[massimo]

Many people use the current low voltage version for DAC with 5 V p.s. 250 mA shouldn't be a big problem: set CCS for 300 mA or more.
Disco's has been designed for <4V to overcome the minimum mosfet Vgs.

[merlin el mago]

Originally Posted by Salas

Use RG58 on all your shunts for sense wiring. Use as thick main ''force'' wiring as you like. Remember that the sense nodes are voltage nodes, don't worry about gauge. Their mission is to skip the voltage drop on the current carrying force wires and ''see'' the remote nodes directly. I believe that you will see less stuff on your scope in difficult termination situations if you shield your sense wiring. If you need extra thin and flexible enough, see about Belden (RG style) 179DT also, which I like for signal too BTW.

The Belden RG 179DT is a very good cable coaxial with 75 ohms impedance for digital SPDIF connexions too

The clear brown that I used in mine Simplistic Phono RIAA is a Huber-Shuner RG179 all teflon dielectric for the conductor & the shield, is a multistranded copper silver plated & also is a 75 ohms impendace, is extra thin 2mm but isn't flexible due to the teflon.

Can I use it for remote sensing? os is better to buy a RG58?

[Salas]

Use it, no problem. The Rs connections are voltage sense, no current. Gauge is indifferent.

[dodo]

Originally Posted by Salas

Both the noted TO-247 and TO-220 Mosfets can work on all schematics. Both types tested. 2N5457, BF245A for Q6 exchangeable. 2SK170 Q6 for the lower voltage range symmetric also. Up to 10uF MKT or MKP for C2. 47uF if with chemical cap. Output RC Zobel values are standard. Film cap there.

Some qestion to the low voltage 1.2 shunt

1. Q9 jfet should be from GR group ? ( it's not so easy to find 2n5459 with Idss>10mA)
2. What was the reason to use short gate Jfet as Q5 ??

[Salas]

1. Works with BL around 8mA. The 220R brings it far down. Its the low pinch off and noise that is of interest, current we tweak. If with GR, the 220R value should be revised lower. 5-6mA or more 2N5459 is enough in this occasion for cascode's top.
2. Many times has been written in the thread that only very low pinch off Jfets can work under just Vbe (0.6-0.7V) provided for Q5's position.

[dodo]

Originally Posted by Salas

1. Works with BL around 8mA. The 220R brings it far down. Its the low pinch off and noise that is of interest, current we tweak. If with GR, the 220R value should be revised lower. 5-6mA or more 2N5459 is enough in this occasion for cascode's top.
2. Many times has been written in the thread that only very low pinch off Jfets can work under just Vbe (0.6-0.7V) provided for Q5's position.

So in case some Rs on the bottom cascode jfet is used , the rule of thumb is
Idss of the top casocode jfet > Id of the bottom jfet , where Id is settled by the value of the Rs ??

I'm little confused , because I was thinking the right rule is Idss of the top jfet always > Idss of the bottom cascode jfet.
I have to read once again Borbely articals and Siliconix AN103

[Salas]

It was confusing me too, because they don't discuss what happens if we degenerate the Idss. But I saw it works like this for real.

[AndrewT]

if a pair of cascoded jFETs are to be used.
Walt Jung says the voltage across the lower jFET should be at least twice the pinch off voltage.
This rule demands a low pinch off voltage for the lower jFET and also requires high transconductance.
A similar rule applies to the cascoding jFET. It too needs to see at least twice it's pinch off voltage to effectively cascode the inner/lower FET.
This cascoding FET must have a Vgs that is >2*Vpinch and this demands a highish Idss and a low transconductance.

I have found that
using BF244 or BF245 for the cascode and LSK170 for the lower jFET that
select a K170 with an Idss about 1mA to 1.4mA more than the required Id current and then the upper jFET needs approximately double this for it's Idss.
I don't know if this rule applies to other types.
eg. Id of the lower jFET is 3mA, then use a 4 to 4.4mA Idss 170 and add a 9mA 244 around it as the cascode.

[Salas]

The question is if we degenerate the inner jfet's idss to our needs, i.e lower than upper's, is it still the same condition? In practice it works as I saw. I don't know if perfect though. It comes very handy for using parts from our drawer. We may have the needed pinch offs but the lower jfet can be happier than OK for idss VS its upper in hand without using a resistor to drop.

[klewis]

I've made this layout for a + voltage v1.2 and was hoping that folks might take a look and give me feedback. I made it so that I could use a zener or resistor to set the voltage and with two pin out versions of Q2. All thoughts would be appreciated.

Thanks

Ken