# Help with JFETs and BJTs

#### Saffire

Joined Mar 29, 2005
1
Hi, new here,

Have some homework due soon, but I haven't been paying much attention in class lately, so I have no clue on how to analyze JFETs and BJTs. I sorta understand how they work, but the book isn't very explicit on how to analyze and solve problems with them.

If you guys could help me solve these 2 problems (I think they should be pretty easy) and walk me through, that would be great.

1) In circuit Figure 1, the JFETs are not matched. Device Q1 has parameters Idss1 = 2 mA and Vp1 = -1V, and device Q2 has parameters Idss2 = 5mA and Vp2 = -4V

a) Find the current id
B) Find the voltage Vgs1

2) An npn BJT with Bf = 120 and Vsat = -.1 V is connected to the circuit of Figure 2 with i1 = 15 uA, R1 = 2.7 kOhms. What is the smallest value of v1 that will ensure that Q1 operates in the constant-current region?

Attached is a picture file with both figures on it. Sorry for the crappy paint skills. If you guys could explain how to analyze these as you go (not worried about the answers as much), that would be wonderful as I'm clueless on where to start.

Thanks!

*edit* If there is an online resource in which I can see how similar problems are solved, or a program I can use to see how it is solved, that would be great as well.

#### mozikluv

Joined Jan 22, 2004
1,435
hi

your fig.1 jfets are configured as a cascode to provide high input impedance and at the same time provide stable current source and the output is connected to a bjt configured is a common emitter and it should be biased in a forward active region.

am quite surprised that your book does not expicitly explain the formulas used in biasing at forward active region using current source and voltage saturation. it's pretty hard writing the formulas since the forum no longer have the engineering symbols.