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Measuring Drain of a FET...

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Promocom

New Member
I built a preamp that uses a n-channel J-fet (J201) and the designer stressed that to perform correctly the drain on the device must be around 6 volts. He says that there is a big difference between individual devices and they should be hand picked and tested to find a good one.

Ive tried 6 different ones and they all measure 8.5v to 8.8v (9v suppy) and before I buy a bunch more I wan't to make sure I'm testing it right.

I'm testing it in circuit with no signal, measuring between drain and ground. I've also tried measuring between source and drain with similar results.

It seems funny that 6 in a row would measure so close to each other and theres supposed to be a magic one coming along that measures 2 or more volts less than the others... :)

Am I testing them wrong... hopefully. I tried e-mailing the designer but no results.

Thanks
 

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Roff

Well-Known Member
Where did you get these J201s? I ran some simulations of your circuit, and it appears that all your transistors have very low (~-0.3v) Vto, the gate turn-off voltage. All are near the lower end of the spec for Vto(-0.3v to -1.5v). Some may actually be outside this range. I suspect you may have gotten culls, or even rejects.
If your input is AC coupled (through a capacitor), you can try the following:
Instead of returning the 3 Meg resistor to ground, connect it to the wiper of a 100k potentiometer. Connect one end of the pot to GND, and connect the other end to a 470k resistor which connects to +9V. Vary the pot to get the bias point you want. See the modified schematic below.

What is the amplitude of your input signal?
 

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mozikluv

New Member
measuring fet drain

:D hi promocon,

based on your schem, Drain bias is 6K8 w/c will give you 1.32ma and a voltage of 6.8v and the voltage that will appear across the 2K2 R will be 2.2v for the Source.

you will notice that no voltage appears at the gate, however a large amount of current will flow from D to S. the device will not turn off til the gate goes negative. the bias is set by the zero (0v) gate voltage current thru the D to the S.

are you expecting to get a reading between the D and S of about 6V? you wont, based on the biases set for the D and S. it is the voltage that appear across the D resistor that you are looking for. check again.

but if you really want to have ar eading of exactly 6v, follow the suggestion of Ron H (hi ron, gud day to you). :D

if you are
 

Promocom

New Member
Hey thanks alot guys... for the time and the great suggestions. :)

I'm using it as an onboard guitar preamp so it basically runs directly from the magnetic pickup through a volume pot, no capacitor coupling. I think the signal is about 1v.

I got the Fairchild j201's from Newark, and I've got some smd samples coming directly from Fairchild I'm gonna try.

Mosikluv I did check voltage across r3 but nothing around 6v. I don't have the circuit breadboarded now and don't remember r2's drop.

I'll try your circuit changes Ron and see what I get, but are the changes you made assuming I get good semi's or is it to compensate for the bad ones I might have?

Here's where I got the circuit if you want.

https://www.till.com/articles/GuitarPreamp/

Thanks Again!
Bill
 

Sebi

Active Member
The magnetic guitar pick-up not give higher voltage as 10mV...
That is why You need a preamp.Most of cases with 1V can You drive a PA directly....
 

Roff

Well-Known Member
Re: measuring fet drain

mozikluv said:
:D hi promocon,

based on your schem, Drain bias is 6K8 w/c will give you 1.32ma and a voltage of 6.8v and the voltage that will appear across the 2K2 R will be 2.2v for the Source.

Mozikluv, I don't think you understand JFETs. This is totally wrong. The nominal Id drain current (and source current) in this circuit is 140ua, which makes the voltage on the drain Vd=9-Id*6.8k=8.05v. The source voltage is Vs=Id*2.2k=308mv. I verified these values with a Spice simulation and with the equations for JFET drain current.
Note that, in order to get 6v on the drain, Vto (gate turn-off voltage) must be -1.5v. The range of Vto as specified in the J201 datasheet is -0.3v to -1.5v, so it would be very unusual to find a part that will work in this circuit as Promocon wishes.
 
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