# Ugent, Please help with circuit

Discussion in 'Homework Help' started by tinhnho, Feb 9, 2005.

1. ### tinhnho Thread Starter Active Member

Aug 31, 2004
34
0
Hi everyone

Yesterday my instructore gave out the hw which we never seen before. We just done with Lapplace transform stuffs. And i have not started anything like this hw, I dont even know where to start. Would someone please help me.It's due today. Thanks guys.

p/s: i know this is long ,but please take a look, thanks again

2. ### Brandon Senior Member

Dec 14, 2004
306
0
Not to bad. Ill show you #1 and #2 for Figure #1.

#1 Want to find the lowest usable value for Vbe for Figure 1 is the question.

gm=5mS. R=1k, Vcc=5v.

We know that ic=gm*Vbe from the relationships given for this problem. We also know that we never want 0 for ic (stated as well in the text).

Since it didn't say much else then that, we need to make an assumption. Let ic=1uA. Very small value. Solve for Vbe in 1uA=5mS*Vbe

#2 What is the max useable value of Vbe.

This is a little easier as we dont need to make an assumption. We knoiw Vcc=5v and R=1K. If we assume Vce=0v then we have a 5V drop across 1K and 5mA of current flowing as ic.

From ic=gm*Vbe we substitute and solve. 5mA=5mS*Vbe. We get 1 volt is the maximum value for Vs.

The rest of them involve about this much work as well. It might just be the wording that is getting you. When in doubt in a problem, make a assumption, write it down so the professor understands your working based upon that assumption and go from there, If you really wanted to get technical with #1 it would be justt Vbe>0, but had to make an assumption to get a numerical answer.

3. ### mozikluv AAC Fanatic!

Jan 22, 2004
1,437
1
hi,

oh boy i think i need a very big monitor just to read your uploaded item

my suggestion is brush up on the superposition theorem, what you have there is just determining the proper biasing of a bipolar or fet.