help with jfets

[drk]

Hello, im trying to build a circuit with some jfets and then a lm386 as the poweramp. well, i went and built it but theres something wrong with the 2nd jfet(the 2N5459 in the schematic). heres a partial schematic, if i bypass the 2nd part(putting some wire in parallel after the 1st transistor) it works, but if i dont bypass it doesnt work with the 2nd transistor, no sound.

im just starting to work around with this so i dont know what could be wrong... im thinking probably the resistor to V+ is to high or something... but i kinda copied the part from other circuit so dont know..


PS: some components are a bit dificult to read:
C1 - 10uF
Q1 - 2N5458
Q2 - 2N5459
C3 - 3.3nF

[R.G.]

The second JFET as no DC bias applied to the gate. It's floating.

[drk]

do you mean something like this?

if so, what would the values be in between?
thanks for replying

[rikkards]

If I would guess something to bias it in the middle (i.e 4.5v)?

do you mean something like this?

if so, what would the values be in between?
thanks for replying

[morcey2]

do you mean something like this?
(snip)
if so, what would the values be in between?
thanks for replying

Instead of adding a bias network like that, just add a resistor to ground between C? and Q2, just like on Q1.  Anything 200k or higher should be fine.  Make R6 a trim pot and bias for about 4.5V on the drain of Q2.

Matt

[demonstar]

The first transistor is using a self-bias system. Try using a similar idea for the second one. JFETs are renowned for being a pain to bias.

[drk]

thanks all, gonna try your ideas 

[Brian Marshall]

first things first-

the 10M input resistor is ridiculously high.  I wouldn't go over 1M, especially on a gain circuit.  You could completely eliminate the capacitor and one of the resistors on the input.  A giant electrolytic cap with a 10M on either side...  not a great idea since electrolytics are fairly leaky.

As already mentioned Q2 has no voltage reference.  Just use a resistor to ground, and perhaps try to calculate a roll off frequency you like withe the capacitor preceding it.

If you want repeatable results you will probably need to replace either the drain or source resistors with trim pots....  Biasing at the Gate is probably a better practice, but requires more parts, and the advantages aren't always that noticable.

[drk]

hey, i've tryed some of your ideas and its working now  just need to match both transistors
now its something like this:


one last question, i dont really know how to choose the capacitors value, can anyone say a link or something were it explains??
thanks all

[R.G.]

F = 1/(2*pi*R*C)

or its equivalent statements:

R = 1/(2*pi*F*C)   and C = 1/(2*pi*R*F)

You pick the caapcitor to cause the frequency boost/rolloff you want.