Jfet amplifier

Thread Starter

metal_militia7

Joined Jun 13, 2010
13
I have to build an common source amplifier with JFET whose gain is 3
R load = 4500
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
 

Thread Starter

metal_militia7

Joined Jun 13, 2010
13
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
 

Jony130

Joined Feb 17, 2009
5,145
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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Thread Starter

metal_militia7

Joined Jun 13, 2010
13
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
 

Jony130

Joined Feb 17, 2009
5,145
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.
 
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