humbucker preamp - j201 gain is 1:1, why ?

[rompelstilchen]

Hello,

I wanted to build a humbucker preamp, allowing me to learn JFET amplification in the context of electric guitars

someone pointed once this circuit to me

I was happy that it worked on firt try ,

but the gain is 1:1

I cant find any gain (hfe) like for bjt in the datasheet (https://www.experimentalistsanonymous.com/diy/Datasheets/J201.pdf)

is there something I a missing (becos of the parts I use are a bit off maybe ? on sunday, no store is open, I used spare parts)

sheers

 

[FvM]

The gain will be constituted by actual JFET gm, source and drain resistors and possible load impedance. JFET have a relative large type variation, indicated by the Idss and Vgs(off) range in the datasheet. Depending on the actual gm, the voltage gain of this circuit isn't necessarily larger than 1.

As a first point, you want to check the actual operation point by measuring the J201 source and drain voltages. Knowing this values, you can calculate actual gm and gain.

Simulating the circuit in Ltspice is a simple way to understand it's operation better.

 

[Easy peasy]

the fet looks like it will be biased off - make sure it is not dead ...

 

[rompelstilchen]

I got these values, how can I calc gm and gain ?

what is the difference between gm and gain ?



- - - Updated - - -

I got these values, how can I calc gm and gain ?

what is the difference between gm and gain ?

 

[FvM]

Both values can be measured in the AC simulation.

 

[Audioguru]

The original schematic uses a 6.8k drain resistor and says you do not need much gain for an electric guitar and the gain is only +3dB which is 1.414 times. But you reduced the drain resistor to 4.7k so the gain is reduced to about 1 time. Jfets are not all the same, some will have a little more gain than others, even with the same part number and manufacturer.
Since R2 has a fairly high value to DC bias the Jfet then the voltage gain is low. Bypass R2 with a 2.2uF capacitor and you will have so much gain that it will blast lots of fuzz and overdrive distortions.

A bipolar transistor's hFE is its DC current gain, not its AC or DC voltage gain. A Jfet does not use hFE, instead it uses gm in a different calculation.
Your 1M input resistor value will reduce the high frequency response of the guitar pickup.

Why is the polarity of your 4.7uF output capacitor backwards??

 

[geekckts]

Currently, your JFET is biased at 0.22mA(from operating point in pic1);
From the datasheet(pg3), the gfs(alternatively gm) is ~ 1.1E-3 mhos.
So the gain of the stage is given by;
Av≅ gfs/(1+gfs*R2)*R3 which is equal to 1.5 V/V, ignoring output conductance;
But, from your measurement, looks like the gain is more closer to 1 V/V; Either the gfs is lower than the datasheet or I am missing some attenuation factor(for ex: series resistance).

So to crank up the gain you probably need higher bias current => higher gfs(gm);
i.e. Suggest to vary the Resistor R2(source resistance), ideally put a 1K in series with a potentiometer and vary it;

HIH. thanks
https://neurophysics.ucsd.edu/courses/physics_120/N-channel JFET Active Circuits.pdf