pspice simulation of Jfet

[electronic_satya]

Hi all,

I am trying to simulate a JFET in orcad Pspice and i am facing the following problem

The pspice calculation of the drain current is different from my hand calculation. Could somebody please explain me where am i making mistake?

The JFET iam using is J2N3819 and the model parameters are

.model J2N3819 NJF(Beta=2.0m Betatce=-.5 Rd=1 Rs=1 Lambda=2.25m Vto=-3
+ Vtotc=-2.5m Is=33.57f Isr=322.4f N=1 Nr=2 Xti=3 Alpha=311.7u
+ Vk=243.6 Cgd=1.6p M=.3622 Pb=1 Fc=.5 Cgs=2.414p Kf=9.882E-18
+ Af=1)
* National pid=50 case=TO92
* 88-08-01 rmn BVmin=25

My calculation

@Vgs = 0V

ID = IDSS = Beta * Vto ^2
= 2 * 9 = 18mA

Pspice calculated value
2.446mA

Thanks in advance,
Regards,
Satya

 

[godfreyl]

The resistor is limiting the current. With 4K, you can't get more than 10V / 4K = 2.5mA.

 

[electronic_satya]

Hi,
Thank you very much for the reply. I have one more small clarification here the JFET in the circuit is actually capable of giving IDSS (=18mA here) but not able to give so much current because of external circuit. Does it mean that the JFET performance characteristics are actually dependent on the external circuit?

Thanks in advance,
regards,
satya

 

[godfreyl]

Does it mean that the JFET performance characteristics are actually dependent on the external circuit?

Sort of. If you look in the datasheet, you see that IDSS is the drain current when Vgs = 0 and Vds = 15V. So it will be different with different drain voltages.

From the curves below, you can see the current does not change much when VDs is more than about 4V, so IDSS is almost constant, except at low voltages.

There's something else very important to know about JFETs; If you look in the datasheet, it says IDSS is "2mA minimum, 10mA typical, 20mA maximum". That means that if you buy 10 of them, they will probably all be different.