Building a Tiny FET Impedance Converter

[antonis]

Antonis was faster on the keyboard than I 

That's because I've cheated with not taking time to answer for the filter order...
(plus the fact that I've allways availiable the letter "π" on my keyboard)

[JHanko]

Bare with me here, I'm old school. I think I understand Lawrence's formula as 1 / (2 x Pi) x R x C , but I don't understand Antonis's formula. What is 1/2n?

[Groovenut]

Bare with me here, I'm old school. I think I understand Lawrence's formula as 1 / (2 x Pi) x R x C , but I don't understand Antonis's formula. What is 1/2n?

I think that "n" is supposed to be the symbol for Pi

[JHanko]

OK, that makes sense. Never heard of that before... "π" I would have recognized.
I'm going to put together a parts list and run it by you guys before I place the order.

[Gus]

Did you search?
MMBFJ201

[JHanko]

Did you search?
MMBFJ201

Search what? I don't understand what you're asking here.

[PRR]

The FET is critical.

Till picked a FET with Vp (Vgs(off)) generally larger than guitar peak voltage. This is "stacked" several times: bias, across the FET, and more across Drain resistor. He could do this under 9V supply. Pretty-much: random-grab J201 may not all work to complete satisfaction.

With 5V supply the design gets too tight to do well. Instead I pick a low-low Vp FET and force it up to about half supply, and omit a Drain output. This is very predictable and satisfactory IF the JFET Vp is small. Much-much less than 2.5V. (The 1.5V limit on J201 is too high for happiness.)

Yes, this does add parts. I offer it for consideration, not as a done deal design.

We must also pick a JFET which can pass the peak current, roughly double the idle current, which is about 0.5mA.

So Vgs(off) less than 1V and minimum Idss at-least 1mA.

I am not keeping up with currently available JFET selections. I hope others here will recognize some readily available part.

> about 10Hz

Ah! Then we could go several octaves higher. 0.005 or even 0.002uFd (5nFd or 2nFd). That may not save any size.

The much larger output cap is needed because I can't find the input impedance of your transmitter. Lower impedance needs bigger cap to pass the same bass. As it is intended as a "Mic" input, it could be as low as 1K Ohms. 5uFd into 1K gives about 33Hz. If you can confirm the box input is say 10K then 0.5uFd may be ample. Likewise if you can stand a very bass-shy sound on the airwaves (you may be able to boost bass back on the amp) then you could use a lower value. However in this zone around 1uFd we have a choice of electrolytic or film. And 0.5uFd film may be much larger than 5uFd electrolytic.

[JHanko]

The FET is critical.

Till picked a FET with Vp (Vgs(off)) generally larger than guitar peak voltage. This is "stacked" several times: bias, across the FET, and more across Drain resistor. He could do this under 9V supply. Pretty-much: random-grab J201 may not all work to complete satisfaction.

With 5V supply the design gets too tight to do well. Instead I pick a low-low Vp FET and force it up to about half supply, and omit a Drain output. This is very predictable and satisfactory IF the JFET Vp is small. Much-much less than 2.5V. (The 1.5V limit on J201 is too high for happiness.)

Yes, this does add parts. I offer it for consideration, not as a done deal design.

We must also pick a JFET which can pass the peak current, roughly double the idle current, which is about 0.5mA.

So Vgs(off) less than 1V and minimum Idss at-least 1mA.

I am not keeping up with currently available JFET selections. I hope others here will recognize some readily available part.

> about 10Hz

Ah! Then we could go several octaves higher. 0.005 or even 0.002uFd (5nFd or 2nFd). That may not save any size.

The much larger output cap is needed because I can't find the input impedance of your transmitter. Lower impedance needs bigger cap to pass the same bass. As it is intended as a "Mic" input, it could be as low as 1K Ohms. 5uFd into 1K gives about 33Hz. If you can confirm the box input is say 10K then 0.5uFd may be ample. Likewise if you can stand a very bass-shy sound on the airwaves (you may be able to boost bass back on the amp) then you could use a lower value. However in this zone around 1uFd we have a choice of electrolytic or film. And 0.5uFd film may be much larger than 5uFd electrolytic.

I'm beginning to think that I have bit off more than I can chew with this project. With the exception of the capacitor/frequency information at the end, I cannot understand any of this. Perhaps it is best that I just fork over the $100 and get the cable designed for the job. I really appreciate everyone taking the time to help bring this to life. I'm very sorry I couldn't grasp enough of it and for wasting the group's time.

[antonis]

I'm very sorry I couldn't grasp enough of it and for wasting the group's time.

Don't even mention it...
(although we are paid by fruitful answer we can manage our credits with the forum admins..)

[Gus]

Do a search for MMBFJ201 and then a search for J201
Compare the datasheet PDFs

[Groovenut]

Ok I just went through Mousers current SMD N channel Jfet offerings and the closest available to Paul's "impossible"  fet is the MMBFJ201, though it does exceed the V_GSOFF by a half volt on the high limit. So plug and play may not be possible. The other option is to buy 10 of them and hand sort for V_GSOFF until you find an acceptable part, then use that part for your project. It might be $5 well spent (plus the gained education in the process).

[JHanko]

Ok I just went through Mousers current SMD N channel Jfet offerings and the closest available to Paul's "impossible"  fet is the MMBFJ201, though it does exceed the V_GSOFF by a half volt on hte high limit. So plug and play may not be possible. The other option is to buy 10 of them and hand sort for V_GSOFF until you find an acceptable part, then use that part for your project. It might be $5 well spent (plus the gained education in the process).

How easy is it to check for V_GSOFF? I don't have a scope, but I have a few nice power supplies and a Fluke 87.

[JHanko]

Do a search for MMBFJ201 and then a search for J201
Compare the datasheet PDFs

Yes, I did. I only understood what I was told to look for. I posted my findings in reply #10.

[induction]

How easy is it to check for V_GSOFF? I don't have a scope, but I have a few nice power supplies and a Fluke 87.

Very easy. See here and here.

[JHanko]

This one looks pretty simple, but how would I connect to the JFET to test it? It's not like I can just slip it into a breadboard.

[Groovenut]

alligator clip wires and old component leads for pins. clip to the fet legs and then to the pins (old comp leads) in the breadboard

should do the trick

[PRR]

> How easy is it to check for VGSOFF?

Breadboard it. A happy Source voltage will be 2.8V to 3.5V.

[JHanko]

alligator clip wires and old component leads for pins. clip to the fet legs and then to the pins (old comp leads) in the breadboard

should do the trick

I'll give it a shot. Those legs are TINY!

[JHanko]

> How easy is it to check for VGSOFF?

Breadboard it. A happy Source voltage will be 2.8V to 3.5V.

I have no idea what this means or how to do it...

[Groovenut]

> How easy is it to check for VGSOFF?

Breadboard it. A happy Source voltage will be 2.8V to 3.5V.

I have no idea what this means or how to do it...

I'm pretty sure Paul means to breadboard the circuit with the values on his schematic and measure the source voltage referenced to ground. Try all the fets in the breadboard circuit and any fet that measures 2.8V to 3.5V will be happy in the circuit.