J201 gain

[phil123456]

Hello,

I wanted to build a humbucker preamp, allowing me to learn JFET amplification in that context

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 (http://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

[Rob Strand]

Check the DC voltage on the drain.

You can decrease the 2k2 to reduce the voltage on the drain to around mid supply, maybe a bit above. Decreasing the 2k2 will also increase the signal gain.

I cant find any gain (hfe) like for bjt in the datasheet

JFETs don't work like BJT's.

For both BJT and JFET amplifiers the signal gain is

A   =  - R3 / (R2 + r)

where r depends on the bias point of the BJT or JFET.
r gets smaller when more current flows through the device.

For a BJT r is usually small.   So the signal gain become -R2/R2 and
doesn't depend much on r.

For a JFET it can be small, or in the same order as R2, or even larger than R2.
When r is large the signal gain will be low.

[temol]

Try increasing R3, for example to 27k. With 9V supply voltage and R3 6k8, drain voltage is almost 8V. With 27k drain voltage is around 4.5V
Check this plots from ltspice. Input signal and output from default R3 and R3 27k.



T.

[phil123456]

ok so this is my first ltspice sim

- indeed augmenting R3 4.7k->6.8k already gives a 4db gain in AC analysis

- transient analysis : my vout is distorted, maybe I am missing something

[phil123456]

For a JFET it can be small, or in the same order as R2, or even larger than R2.
When r is large the signal gain will be low.

how do I figure r ? would be fun to be able to do the calulations myself

[phil123456]

Try increasing R3, for example to 27k. With 9V supply voltage and R3 6k8, drain voltage is almost 8V.

how do you figure drain voltage ? how can you tell voltage is too high/low for it to work ?

regards

[temol]

Probe the signal before the 4u7 cap. You'll see how the sinewave get's clipped when drain voltage is set close to the supply voltage. If the drain voltage is set to 1/2 of the power supply there is "plenty" of room before clipping occurs.

T.

[anotherjim]

It doesn't have to be 1/2 supply volts at the drain. On 9v, anywhere from 3v to 6v should be workable. JFET's are far too variable in quality to expect constancy in designs with fixed resistor values.

If the drain voltage is something in a workable range, then a quick way to increase gain would be to fit a 22uF cap across the Source resistor. That will increase distortion.

[PRR]

>> A   =  - R3 / (R2 + r)
>> where r depends on the bias point of the BJT or JFET.
>> r gets smaller when more current flows through the device

> how do I figure r ?

Roughly from datasheet info:

Rob's "r" is 1/Gm, Transconductance. Page 3 shows Gm varying with current (as Rob said).

So first figure the current. We don't have a solid clue without more study. But this is said to work. To get good swing the drain should be "about halfway" on the supply. Or since there is stuff happening in the source also, maybe a bit higher. "5V on drain" is a good bet. Then the remaining 4V across 6.8k is 0.6mA, close-enough. Entering the graph at 0.6mA, read up to the curve, over to the Gm scale, we get 2 milli-Mho. 0.002 Mho. Invert, this is 500 Ohms.

In the plan proposed the load is 6.8k, parallel with 51k/47k, parallel with the external load. Pencil about 5.6k. In the bottom we have 2.2k + 500r, 2.7k. 5.6k/2.7k is about 2. (6dB, not the 3dB promised, not the small gain you observed).

Why the discrepancy? The JFET was tested at Vds=10V and we may be under 4V where the curves go soft (some clue in the next graph- we may have 100k "plate resistance" at the lower voltage). Also the graph is for parts with Vgs(off) of 2.5V and 5.5V, yours will be different. 

The *simple* way to get more gain is to bypass R2 with like 10uFd cap. Gain may go towards 10, which is too much for clean guitar under a 9V supply. The next step is a cap and resistor in series, to get a in-between gain.

[antonis]

And of course, don't underestimate the value of R1..

Total signal gain is - g_mx(R3//R4//R_load)x[R1/(R1+Rs)] / (1 + g_m x R2), where Rs is your guitar output impedance..

Or - g_mx(R3//R4//R_load)x[R1/(R1+Rs)], for grounded Sourse resistor..

[phil123456]

The next step is a cap and resistor in series, to get a in-between gain.

the first part makes sens and reminds me my electronic courses; trying to figure things from crappy graphs in a datasheet :-)

- but I really thought this was going to work as is , how to figure that cap/res ? why do people give such crap unfinished circuit ?
- is there an easier way to boost pickup signal ? with an op amp or something ? (yet in the 0v-9v roam)
- also why is there no decouplig capacitor on the input ? is the 3M res supposed to have a more stable gate signal ?

regards

[antonis]

<- but I really thought this was going to work as is , how to figure that cap/res ? why do people give such crap unfinished circuit ?>

Because they can.. 

Cap/res could be figured out by selecting appropriate resistor value for the gain of your taste..
(it doesn't upset bias due to DC blocking cap..)

Cap value could be figured from 0.159/(C x f), where f is lowest frequency of interest and the result should be an impedance much greater than 1/g_m - or you may alternatively take into account 1/g_m in series with cap impendance..

<- is there an easier way to boost pickup signal ? with an op amp or something ? (yet in the 0v-9v roam)>

Of course..
A non-iverting op-amp configuration, where R1 should be the bias to 4.5V resistor and Rf/Rg =1 could exibit +6dB gain..

I should also propose Jack's minibooster in case you wish to swim into "active load" troubled waters..
http://www.muzique.com/amz/mini.htm

<- also why is there no decouplig capacitor on the input ? is the 3M res supposed to have a more stable gate signal ?>

There should be one but, maybe, the designer considered guitar tone cap in series with guitar output..
(which doesn't stand for most cases..)

Just for the record: that kind of cap should be called "coupling - "decoupling" is called the cap proposed by Paul (Sourse resistor by-pass..)

The 3M3 resistor doesn't have to do anything with gate signal.. - it biases Gate to zero Volts DC for V_GS to be set at -IDxR_S(Self-bias technique..)
(it actually DOES have to do with signal attenuation, due to voltage dividing effect with previous stage input impedance but I presume it isn't your original query..)

[amptramp]

This is one of the reasons designers are shying away from JFET designs.  The Idss and pinchoff voltage vary so much that a design that works once like the Tillman preamp you are describing needs to have select-on-test source and drain resistors to keep the signal centred within the output bounds and providing the gain you want.  Typically the gain should be around three but it can vary all over the place with different resistor values.

[phil123456]

A non-iverting op-amp configuration, where R1 should be the bias to 4.5V resistor and Rf/Rg =1 could exibit +6dB gain..

yeah.. I guess I'll stay away from transistors then lol , I'll give it a try

thanks for all the infos, I'll definetely save this page

[antonis]

A non-iverting op-amp configuration, where R1 should be the bias to 4.5V resistor and Rf/Rg =1 could exibit +6dB gain..

yeah.. I guess I'll stay away from transistors then lol , I'll give it a try

So, you tell us you've invented the "transistorless" op-amp..??