ECE 220 PreLab

1
ECE 220 Pre-Lab

• Lecture 3

2
When and Where?

• Turn in to the Stockroom
• Due at 400 PM on Friday
• Please give yourself time to finish!!!
• Incomplete parts and a late turn-in hurt

3
Outline

• Purpose of the lab
• What to turn in
• Results you should expect
• Questions

4
Purpose

• To get more familiar with Thevenin and Norton
  equivalent circuits
• To learn more about Transistors through
  experimentation

5
Part 1 Like this
6
Turn in?

• Calculation of Rth
• It will be an algebraic equation with no known
  values

7
What should I expect?

• An equation that includes Vth/VL somewhere in it
• Try to manipulate the equation to get that ratio
• It will help the problem solve itself
• Nothing else I can say
• Anything more would be the answer

8
Well, one more thing

• Use voltage division
• As usual, it will make your life a little easier
• The rest is algebra

9
Part 2 Something like this
10
I couldnt resist!!!

• Just kidding. ?
• Part 2 is omitted.

11
Part 3 Something like this
12
Turn in?

• Print-out of simulated schematic with R3 5k
• Calculations verifying Spice results with
  equations in Figure 3
• Plot of load current with R3 swept from 0 to 10k
• Calculations for the estimates of Norton current
  and resistance
• Value of Norton resistance for an ideal current
  source
• Discussion of how close the circuit is to an
  ideal current source

13
What should I expect?

• Your calculations should agree with the
  equations.
• If not, something must be wrong with your circuit

14
What should I expect?
15
Part 4 Like this
16
Turn in?

• Your computation of Vthev and Rthev across the
  terminals a and b
• Calculations showing Vthev and Rthev after
  plugging in values from (b)
• Calculations for VL with the 4 different loads
• One simulated schematic using Figure 4 and one of
  the loads across a and b
• All data obtained from the 4 simulations
• Explanation of how data was obtained

17
What should I expect?

• For (a), you should have an equation for Vthev
  and Rthev with no known values (all variables)
• Also, note that they give a hint saying that
  Rthev should be equal to R4 when you finish
• For (b), simply plug the numbers they give into
  your equations, PSpice can help verify your
  results

18
What should I expect?

• For (c), simply attach your Thevenin circuit to
  the load circuit and make the calculations for VL
  using the 4 resistances
• Again, use Spice to verify your answers, because
  you have to do it anyway!!!

19
Back to Thevenin Resistance

• To find Rthev for part (a), there is indeed a
  trick
• Short out all independent voltage sources and
  open all independent current sources
• Notice what happens to the dependent current
  source when the voltage sources are shorted
• Finally, remember that Zero current is an open
  circuitNOT a short!!!!

20
Back to Thevenin Voltage

• To find Vthev for part (a), there is also a trick
• To do this you must notice that Vab, which is
  Vthev, is the total voltage across all the
  elements going from terminal a to b
• Notice that the voltage across the current source
  is not helpful, because you dont know it
• But, knowing the current from the source, you can
  calculate the total voltage in the other
  branch!!!!

21
One Last Look
22
Questions?

• The only stupid question is the one you didnt
  ask!!!!

23
The End

• Come to my office hours or send me an email for
  help.
• See you in 2 weeks!!!