### Author Topic: JFET Substitution  (Read 3634 times)

#### tehmessiah75

##### JFET Substitution
« on: November 25, 2017, 10:32:42 PM »
Sorry if I've posted this in the wrong section.
I had a search and found some information but honestly have no idea and couldn't find the correct place to post.

I am building the Seymor Duncan Clean Boost circuit from the following link.
This circuit uses a 2N5457 JFET which is no longer readily available as a TO-92 package

• Can I substitute the 2N5458 in its place?
• What is the difference between the 2N5457 and the 2N5458?
• Is there a gain difference?
• If so how much more gain would there be?
I am not familiar with JFETs and how they work in a circuit and appolagise for for what might seem like stupid questions

#### Rob Strand

##### Re: JFET Substitution
« Reply #1 on: November 25, 2017, 11:28:57 PM »

Quote
Can I substitute the 2N5458 in its place?
Yes.

Quote
What is the difference between the 2N5457 and the 2N5458?
They have different specs.  The main thing is the pinchoff voltage of the 2N5458 is higher.

Quote
Is there a gain difference?

Quote
If so how much more gain would there be?

The tolerances of JFETs are quite high.  That means you could see large differences even with different 2N5457's.

Typically, by only adjusting the drain resistor of the boost stage to get the correct bias point, you might see 3dB lower gain on the 2N5458.

If however you modified the circuit to include a resistor and capacitor in the source, like this,

Then you can reduce the difference somewhat. My initial guess would be 270 ohms and 47uF.    But that is changing the circuit, and only focussing on the *typical* gain.

The internet:  answers without the need for understanding.

#### tehmessiah75

##### Re: JFET Substitution
« Reply #2 on: November 26, 2017, 06:06:31 AM »
Quote
My initial guess would be 270 ohms and 47uF.
• Is that guess based on a 12v circuit or 9v, because you have stated 12v in the schematic there?
• I cant read those component values on that image, can you please write them down?
Cheers
« Last Edit: November 26, 2017, 07:42:09 PM by tehmessiah75 »

#### Rob Strand

##### Re: JFET Substitution
« Reply #3 on: November 27, 2017, 03:51:31 PM »
Quote
Is that guess based on a 12v circuit or 9v, because you have stated 12v in the schematic there?
I cant read those component values on that image, can you please write them down?

Oh,  that schematic wasn't intended to be used as is.   I was just showing an example circuit how to connect the added resistor and capacitor.

The idea was:
- Use the SD schematic
- For the boost stage, on the SD schematic add the parallel resistor and capacitor (220R + 47u) between the source and ground.
- The schematic I gave is only showing an example of a circuit  with those components in place, they are R2 and C2.   (Because it was an example the values on that schematic are not 220R and 47u).
The internet:  answers without the need for understanding.

#### tehmessiah75

##### Re: JFET Substitution
« Reply #4 on: November 27, 2017, 11:03:28 PM »
Thanks Rob for the clarification.

Is there a tutorial somewhere that explains the details of how a JFET works in a circuit like this?
I would like to know more about them.

#### Rob Strand

##### Re: JFET Substitution
« Reply #5 on: November 28, 2017, 03:55:33 PM »
I'm sure not what is more suitable for you.  The problem with electronics in general is the "simple" articles don't leave you much and they give example circuits without explaining much of the underlying ideas behind them.  On the other side of the coin are more formal articles more suitable for engineers and those who like a mathematical approach.  For amplifiers a lot of ideas follow from Transistors (BJTs), so you might not see a lot written down specifically for JFETs.

http://www.kennethkuhn.com/students/ee351/jfet_basics.pdf

http://www.linearsystems.com/lsdata/others/FET_Principles_Circuits_Part_1_4_Ray_Martson_Nuts_Volts.pdf

http://www.electronics-tutorials.ws/transistor/tran_5.html

http://staff.utar.edu.my/limsk/Basic%20Electronics/Chapter%204%20JFET%20Theory%20and%20Applications.pdf

http://www.hobbyprojects.com/field_effect_transistor/junction_fet_as_an_amplifier.html

Use these as a start.  You might find you have to google specific aspects.
Keep in mind you want JFETs not MOSFETs, they are different devices but have some similarities.
« Last Edit: November 29, 2017, 04:29:39 PM by Rob Strand »
The internet:  answers without the need for understanding.

#### diydave

##### Re: JFET Substitution
« Reply #6 on: November 29, 2017, 02:54:38 AM »
Is there a tutorial somewhere that explains the details of how a JFET works in a circuit like this?
I would like to know more about them.

A spice-simulator (e.g. ltspice) can help as well. You can test any number of circuits and try to grasp the ins & outs of all the info you're reading.
With Ltspice, you can even use a piece of recording (wav-file) and simulate the outcome.
I lost track of the hours I've spend on simulating all kinds of things, but I sure learned a lot from it.

#### Elijah-Baley

##### Re: JFET Substitution
« Reply #7 on: November 29, 2017, 05:21:36 AM »
There's a way to know the right voltages value of the JFET in case of debug?
Thanks!
«There is something even higher than the justice which you have been filled with. There is a human impulse known as mercy, a human act known as forgiveness.»
Elijah Baley in Isaac Asimov's The Cave Of Steel