JFET CSA phase-shift

[CadelFistro]

Hello guys! I am new, and would very much like to ask you guys a little problem that i have trouble finding information on.

The problem regards a simple JFET Common-Source-Amp that i am writing a report on, and i am wondering:

Within midpoint frequencies such an amplifier has 180degrees phase-shift between the gate and the drain. How do i prove this mathematically?

Best Regards
CadelFistro

 

[Harald Kapp]

You use the equation for Ids=f(Vgs). Ids flows through the load resistor Rl from drain to V+. This creates a voltage drop across Rl. The voltage from drain to source, Vds, is the difference Vds=V+ - V(Rl). Now check the sign of Vds vs. Vgs.

Of course you have to determine the operating point of your circuit first. Since IDS=f(Vgs, VP) (VP being the pinch off voltage of the transistor), you typically set Vgs somewhere between 0V and Vp.

You may want to read up this article.