# Jfet amplifier

Discussion in 'Homework Help' started by metal_militia7, Oct 16, 2010.

1. ### metal_militia7 Thread Starter New Member

Jun 13, 2010
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0
I have to build an common source amplifier with JFET whose gain is 3
R drain = 4500
Vdd = 15 V

RG = 100k

I have to use 2n5457
I used shockley's equation to find IDSS and VGS off . I wonder how do i get the load line to get Q point.

I need to find RS to get ID and plot the load line.
I am stuck at this point.
Besides, I do not want to use bypass capacitor at RS, so what is the gain of the circuit formula?
IS it AV= gm(RD||RL)/(1+ gm(RS))
Any help would be highly appreciated.
thanks

2. ### Jony130 AAC Fanatic!

Feb 17, 2009
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This is a homework or you try to build real jfet amp?

3. ### metal_militia7 Thread Starter New Member

Jun 13, 2010
13
0
it is sort of lab exercise, i have to figure out everything about this circuit and write my final report.
The point of my confusion is finding load line, and i measured my IDSS and VGSoff as 3.5mA and -1.85 V respectively.
I koow i need to figure out RS to get load line, but how ?? still doing my investigation...please just hints would be great.
thanks

4. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,936
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It's always go to star designs with Vd= 0.5*Vdd = 7.5V
From then

Id = 7.5V / Rd = 1.6mA

gm0 = 2*Idss/|Ugsoff| = 7mA/ 1.85 V = 3.783mS

So gm at Id = 1.6mA is equal:

gm = gm0*√(Id/Idss)
= 3.783mS * 0.676 = 2.55mS

So know we can find RS.

Av= Rd||RL / (1/gm + Rs) = 2.25K / (392Ω + Rs) = 3V/V

Rs = (Rd||RL / Av ) - 1/gm = ( 2.25K / 3 ) - 392Ω = 358Ω = 360Ω

And know we can find Vgs

`
$Vgs = Vgs_off * [ 1 - \sqrt{\frac{Id}{Idss}} ] = -0.6V$

And If you use self bias method then for Rs = 360 you get Id = 1.63mA

And you have to remember that the amplifier in which Rd=RL is not a good amplifier.
We should always design the amplifier that Rd<<RL

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Last edited: Oct 17, 2010
5. ### metal_militia7 Thread Starter New Member

Jun 13, 2010
13
0
Thank you that really helps.
you said that RD<< RL in JFET amplifier
so how do i choose RD, should i pick any value and that is less less than RL or there is some rule??
BUt In BJT's ...for a good design RL= RC is true right?
I used the same assumption in JFET that is why said i had 4.5k as RD.
Thank you

6. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,936
1,088
Well, all voltage amplifier should have Rout<<RL (ideal voltage amplifier would have Rout=0).
Why ??
This imagine tell all the story

If you chose Rd=RL the voltage gain drops badly, and max output voltage swing also drops.

So general rule is Rout = 0.1 R _load or better.
But sometimes respect this rule is impossible.