fet Matching

[Clint Eastwood]

So if you measure the same Idss, you can bet they have very similar Vgsoff, indeed they will have the same curve.

That is not actually true. Well maybe it is OK to assume that as long as you qualify that statement by adding 'if they are the same type of JFET from the same manufacturer and the same production run'.

The relationship between Vgs(off) and Idss varies considerably depending on the geometry of the JFET and the doping of the semiconductor material.

The transfer characteristic for normal operation is defined by Vgs(off) and Idss which determine the two end points of the forward transfer conductance curve used for typical operation.

This curve can be derived from these two end-point parameters using the following equation:   Id = Idss(1-Vgs/Vgs(off))^2

Sorry for not being clear, yes I meant for a certain jfet type. BF245 jfets will allways have very similarly shaped curves, if not  they would not be BF245's. 

[Clint Eastwood]

This is the seller:

https://www.ebay.es/itm/304157747953?var=603269161743

The fet matching circuit works with a genuine j201, so I absolute convinced that this bf245a are fake.

I hope I am wrong, but..  looking at the picture, I think the Fairchild 'F' looks suspicious. The horizontal part of the F is too long. Incredible it would be, you have one life on this earth and then you decide to fake jfets...

[FSFX]

This is the seller:

https://www.ebay.es/itm/304157747953?var=603269161743

The fet matching circuit works with a genuine j201, so I absolute convinced that this bf245a are fake.

I hope I am wrong, but..  looking at the picture, I think the Fairchild 'F' looks suspicious. The horizontal part of the F is too long. Incredible it would be, you have one life on this earth and then you decide to fake jfets...

The problem is that so many of these devices are no longer manufactured but there is still a demand for them by people like guitar pedal builders.
So that is why there is an incentive for fakers to sell all of this remarked rubbish. If people didn't want to buy things like TO-92 J201s and these other JFETs then there would be less fakes around.

It's time for pedal builders to start using modern alternatives to all of these obsolete devices.

If it doesn't look like this then it is not genuine Fairchild.

[spoontex]

This is the seller:

https://www.ebay.es/itm/304157747953?var=603269161743

The fet matching circuit works with a genuine j201, so I absolute convinced that this bf245a are fake.

I hope I am wrong, but..  looking at the picture, I think the Fairchild 'F' looks suspicious. The horizontal part of the F is too long. Incredible it would be, you have one life on this earth and then you decide to fake jfets...

The problem is that so many of these devices are no longer manufactured but there is still a demand for them by people like guitar pedal builders.
So that is why there is an incentive for fakers to sell all of this remarked rubbish. If people didn't want to buy things like TO-92 J201s and these other JFETs then there would be less fakes around.

It's time for pedal builders to start using modern alternatives to all of these obsolete devices.

If it doesn't look like this then it is not genuine Fairchild.

Very useful information!

Thanks.