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Old 25th November 2011, 05:19 PM   #11
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Really, is it now ?

http://www.onsemi.com/pub_link/Colla...TF202THC-D.PDF
(mind the boundary dashes)

There's not an abundant choice of low capacitance JFETs, if not SMD (otherwise Sanyo might be an option). And if small size wouldn't be an issue, you'd get away with a K30, same same as K118, but regular/taller TO-92 package.
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Old 25th November 2011, 06:00 PM   #12
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Quote:
Originally Posted by jacco vermeulen View Post
Really, is it now ?

http://www.onsemi.com/pub_link/Colla...TF202THC-D.PDF
(mind the boundary dashes)

There's not an abundant choice of low capacitance JFETs, if not SMD (otherwise Sanyo might be an option). And if small size wouldn't be an issue, you'd get away with a K30, same same as K118, but regular/taller TO-92 package.
So are you saying the 2SK53 has an inbuilt G-S resistance as shown for the TF202THC ?

Small signal JFET's read as a "diode" between G and S and G and D with a forward volt drop of around 1.1 volts.
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Old 25th November 2011, 06:28 PM   #13
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More examples ?
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File Type: pdf 2SK2219.pdf (103.9 KB, 46 views)
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Old 29th November 2011, 01:26 PM   #14
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So if I understand correctly, in the enclosed schematic, the bias is made with the voltage created by the diode to the gate ? If we could find the damn 2sk53 pinout and specs. it could be more easy.

Thanks
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Old 29th November 2011, 01:42 PM   #15
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A diode won't bias the FET. As mentioned the diode is an intrinsic property of a junction FET, if that is what they are trying to show. It doesn't bias it though.

You need a gate to source resistor for biasing, maybe of several or many megohms to prevent loading of the electret source.

What sort of FET's do electet mics use ? You can not leave the gate just floating so if the electret source has no DC path you have to bias the FET somehow.
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Old 29th November 2011, 03:36 PM   #16
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The resistors inside the CP-Y are two 30K that comes from the power supply ground and goes to both outputs of the cartridge.

There's no resistors coming from the gate

The two 30K are connected to the ground, not to V+ so that's why I assumed that it was a source follower and not a gain stage.

As you specify that I need a resistor to bias the gate with a source follower, I really don't know what to do now.

Here is what I have but don't know now how it was connected to the FET.
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Old 29th November 2011, 04:03 PM   #17
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Originally Posted by Mooly View Post
A diode won't bias the FET. As mentioned the diode is an intrinsic property of a junction FET, if that is what they are trying to show. It doesn't bias it though.

You need a gate to source resistor for biasing, maybe of several or many megohms to prevent loading of the electret source.

What sort of FET's do electet mics use ? You can not leave the gate just floating so if the electret source has no DC path you have to bias the FET somehow.
No, the gate voltage equilibrates with the back to back diodes biased by the gate leakage current. Try it some time on the bench, 2SK170 and back to back diodes. For the n'th time there are no resistors in an electret capsule.

Find a 2SK123 data sheet and you will see the operating current in a capsule is outside the Idss range, which allows for the gate floating to a slightly positive voltage (due to drain to gate leakage).

If you don't use a FET designed for capsules, just cut off the drain lead of another FET and connect the gate to source as shown as a diode.
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Old 29th November 2011, 04:17 PM   #18
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Originally Posted by legarem View Post
The resistors inside the CP-Y are two 30K that comes from the power supply ground and goes to both outputs of the cartridge.

There's no resistors coming from the gate

The two 30K are connected to the ground, not to V+ so that's why I assumed that it was a source follower and not a gain stage.

As you specify that I need a resistor to bias the gate with a source follower, I really don't know what to do now.

Here is what I have but don't know now how it was connected to the FET.
Looks like a source follower so the diode must go to the source not ground.
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Old 29th November 2011, 07:00 PM   #19
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Originally Posted by scott wurcer View Post
No, the gate voltage equilibrates with the back to back diodes biased by the gate leakage current. Try it some time on the bench, 2SK170 and back to back diodes. For the n'th time there are no resistors in an electret capsule.

Find a 2SK123 data sheet and you will see the operating current in a capsule is outside the Idss range, which allows for the gate floating to a slightly positive voltage (due to drain to gate leakage).

If you don't use a FET designed for capsules, just cut off the drain lead of another FET and connect the gate to source as shown as a diode.
Many many thanks for your insight Scott.

Sorry if these are daft questions just trying to expand my knowledge now.
You say "the gate voltage equilibrates with the back to back diodes biased by the gate leakage current. Diodes ? plural, two of. Do you mean the intrinsic "diodes" found in a normal junction FET and if so, that the leakage is both sufficient and presumably equally matched between the two ? Sufficient on their own to be used for biasing. (I am thinking that as the FET is symetrical there must be two diodes.)

Is there any reason why just one diode is shown in the picture and also in the device in Jaccos link.


I appreciate the electret in isolation has no DC path of its own... I have just never heard of FET's being left essentially with the gate OC to self bias if that understanding is correct.

So legarem is going to need a fairly specific FET for this to work.
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Old 29th November 2011, 07:10 PM   #20
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Originally Posted by Mooly View Post

So legarem is going to need a fairly specific FET for this to work.
This is what I strongly suspect
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