Question about biasing jfet stages with different voltages

[Atodovax]

Hi everyone. I have a quick question. Im working on a three stage jfet overdrive like the eighteen and im using 18volts. Im wondering how could i leave 18volts in tthe first stage and power the second two stages with 9 volts. Will a simple VCC- 10k - 9v- 10k - ground- voltage divider work to power the trimmers of both 2dn and third stages? Im trying to get symetric clipping on the next stages but to retain the first one at 18v. Thanks .
PS: Would it be the same if i leave everything at 18v and just bias the second and third jfet to 4.5 volt or around 1/2 of 9v?

[FiveseveN]

Since there's petty current involved a resistive divider could work. But what makes you think any of the stages will clip symmetrically? Use a couple of diodes if that's what you want.

[Atodovax]

Since there's petty current involved a resistive divider could work. But what makes you think any of the stages will clip symmetrically? Use a couple of diodes if that's what you want.

Well, i have symmetric clipping right now running at 18 volts but im afraid that if i lower the drain voltages taken directly from VCC (18v), i might have some asymmetric clipping on the lower cycles. Would it be the same to acommodate the drain voltages to 2/3of VCC from 18v or taking the drains of the junction of the voltage divider froming 9 volts and then acommodatin them to 2/3 of 9v? I mean only with the last two stages

[antonis]

Would it be the same if i leave everything at 18v and just bias the second and third jfet to 4.5 volt or around 1/2 of 9v?

No..

You'll get asymmetrical clipping..

Better use a 9V1 Zener diode  (with CLR of course) to feed 2^nd & 3_rd stage..

[Atodovax]

Would it be the same if i leave everything at 18v and just bias the second and third jfet to 4.5 volt or around 1/2 of 9v?

No..

You'll get asymmetrical clipping..

Better use a 9V1 Zener diode  (with CLR of course) to feed 2^nd & 3_rd stage..

Thanks Im trying with resistors but after i make the voltage divider, the voltage drops when i conect that junction to the drains of the transistors. How could i implement the diode + CLR to that schematic? Thanks in advance Antonis !

[antonis]

Im trying with resistors but after i make the voltage divider, the voltage drops when i conect that junction to the drains of the transistors.

Of course it does, as far as, voltage divider current isn't much larger than the sum of drains current..

How could i implement the diode + CLR to that schematic?

Right after 1^st stage Drain..
I presume you do know how to calculate CLR value..

https://www.electronics-tutorials.ws/diode/diode_7.html

[Atodovax]

Im trying with resistors but after i make the voltage divider, the voltage drops when i conect that junction to the drains of the transistors.

Of course it does, as far as, voltage divider current isn't much larger than the sum of drains current..

How could i implement the diode + CLR to that schematic?

Right after 1^st stage Drain..
I presume you do know how to calculate CLR value..

Unfortunatly i cannot draw over that schematic with my phone but i assume i could put a 10k resistor after de first stage with a 9v zenner to ground and a 100uf cap to ground and take the 9 volts from there? Is the inductor necesary? I dont have any on hand right now. Do you think the 10k 100uf RC filter would not be enought for the circuit?

[antonis]

No.. You've got it all wrong..

Give me some time to draw you a schematic..

[Atodovax]

No.. You've got it all wrong..

Give me some time to draw you a schematic..

Thanks

[antonis]

470 Ohm series resistor is estimated for 20mA total current draw (Zener + Q2/Q3 stages..)

Of course, a better solution should be CLR/Zener/Cap joint  placed at the Base of a series pass n-p-n BJT but try the above configuration at first..

[Atodovax]

470 Ohm series resistor is estimated for 20mA total current draw (Zener + Q2/Q3 stages..)

Of course, a better solution should be CLR/Zener/Cap joint  placed at the Base of a series pass n-p-n BJT but try the above configuration at first..

Thanks! Thats the way i was thinking but with a 10k resistor. THanks so much Im going to try that and report back. I just tried with a 78l09 regulator in there instead of the diode and resistor but a lot of oscilation was introduced.. I dont know why

[Atodovax]

470 Ohm series resistor is estimated for 20mA total current draw (Zener + Q2/Q3 stages..)

Of course, a better solution should be CLR/Zener/Cap joint  placed at the Base of a series pass n-p-n BJT but try the above configuration at first..

Well this worked like a charm Thanks Antonis!

[amptramp]

470 Ohm series resistor is estimated for 20mA total current draw (Zener + Q2/Q3 stages..)

Of course, a better solution should be CLR/Zener/Cap joint  placed at the Base of a series pass n-p-n BJT but try the above configuration at first..

Thanks! Thats the way i was thinking but with a 10k resistor. THanks so much Im going to try that and report back. I just tried with a 78l09 regulator in there instead of the diode and resistor but a lot of oscilation was introduced.. I dont know why

A 78L09 requires a 0.33 µF capacitor to ground at its input and a 0.01 µF capacitor to ground at the output and these should not be electrolytic.  All regulators have stability isues that require input and output capacitors located physically near the regulator package.