• Support PF! Buy your school textbooks, materials and every day products Here!

Thevenin equivalent question

  • #1
123
0

Homework Statement


Determine the Thevenin equivalent with respect to the terminals a,b

Homework Equations


Rth=Voc/Isc

The Attempt at a Solution


I'm doing this practice problem and I calculated the the short circuit current below:

500uA*100ohm/(100ohm+1310ohm)= 35.460 uA

since 80*ib=current in v2, 35.460 A * 80=2836.87 uA

So Isc=2836.87

Now for the open circuit voltage..
80ib* 50kohm= 40.0*10^5 ib
Now to solve for ib is where I'm lost.
In the solution they have 4*10^-5v2=-160ib, but I have no idea where it came from?
 

Attachments

Answers and Replies

  • #2
NascentOxygen
Staff Emeritus
Science Advisor
9,244
1,071
A fraction of v2 is fed back to influence the input. That fraction is given as 4x10-5v2, so substitute in this your value for v2 (which you have in terms of ib).
 
  • Like
Likes orangeincup
  • #3
123
0
Thank you. I have a question about calcuating Isc vs. Voc, why isn't Voc simply Isc/50kohms?

I know the circuit is suppose to be "open" but isn't it still the same thing?
 
  • #4
NascentOxygen
Staff Emeritus
Science Advisor
9,244
1,071
Thank you. I have a question about calcuating Isc vs. Voc, why isn't Voc simply Isc/50kohms?
It would be if the Thèvenin resistance were 50kΩ, but it isn't. That's what you are trying to find! The feedback changes the output impedance. And it changes the input impedance, and it changes the gain.
 
Last edited:
  • #5
123
0
So is the Vth or Voc I'm calculating simply the voltage across the 80ib dependent current source?

In the open circuit voltage I thought the 50kohm resistor would still be part of the circuit, and the voltage across that would equal the voltage across a-b?
 
  • #6
NascentOxygen
Staff Emeritus
Science Advisor
9,244
1,071
So is the Vth or Voc I'm calculating simply the voltage across the 80ib dependent current source?
Yes, but the 50k remains in place, it's part of the model. (Without some resistive load, the voltage across a current source would be infinite.)

In the open circuit voltage I thought the 50kohm resistor would still be part of the circuit, and the voltage across that would equal the voltage across a-b?
Yes, and yes.
 
Last edited:

Related Threads on Thevenin equivalent question

  • Last Post
Replies
12
Views
2K
  • Last Post
Replies
2
Views
1K
Replies
1
Views
3K
  • Last Post
Replies
20
Views
941
  • Last Post
Replies
4
Views
2K
  • Last Post
Replies
4
Views
6K
  • Last Post
Replies
13
Views
1K
  • Last Post
Replies
3
Views
733
  • Last Post
Replies
5
Views
946
Replies
12
Views
823
Top