Rowan Hall 238A

beard@rowan.edu

http://rowan.jkbeard.com

April 24, 2007

Electronics I for M.E.ECE 09-311 2Load Line Exercises

James K. Beard, Ph.D.

Electronics I for M.E.April 24, 2007 Slide 2

The Load Line

Definition: v-i curve of Thévenin equivalent of a linear circuit

Purpose: To provide a graphical interface with a nonlinear circuit element such as a diode or transistor

Usage: Draw v-i curve of the nonlinear element, Draw load line over it, and Intersection of the lines is the operating point.

Examples follow for BJT, MOSFET, and diode

Electronics I for M.E.April 24, 2007 Slide 3

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

0 1 2 3 4 5 6

Electronics I for M.E.April 24, 2007 Slide 4

0

5

10

15

20

25

0 5 10 15 20

Collector Voltage, Volts

Co

llec

tor

Cu

rren

t, m

A

0.0 uA10.0 uA20.0 uA30.0 uA40.0 uA50.0 uA60.0 uA

Electronics I for M.E.April 24, 2007 Slide 5

0

5

10

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0 5 10 15 20

Drain Voltage, Volts

Dra

in C

urre

nt,

mA

Bound 2.0 V 3.0 V 4.0 V 5.0 V 6.0 V 7.0 V 8.0 V

Electronics I for M.E.April 24, 2007 Slide 6

Input-Output Voltage Curve

Called the voltage transfer characteristic curve A plot of the output voltage versus the input

voltage The voltage transfer characteristic from load line:

Draw the family of transistor v-i curves Draw the collector load line Find the proper transistor v-i curve from an input

voltage, Read the output voltage from the load line intersection Construct a table of input voltages versus output

voltages and plot the table

Electronics I for M.E.April 24, 2007 Slide 7

The Voltage TransferCharacteristic From Load Line Draw the family of transistor v-i curves Draw the collector load line For each of a set of input voltages

Find the proper transistor v-i curve for this voltage, Read the output voltage from the load line intersection

with the v-i plot, and Add the result to a table.

Plot the table

Electronics I for M.E.April 24, 2007 Slide 8

Ci

CEv

300Bi A

200Bi A

400Bi A

500Bi A

100Bi A

0Bi A

Saturation line

Cutoff line

Load line

Electronics I for M.E.April 24, 2007 Slide 9

Exercise 1: The Diode v-i Curve

Draw a load line for a 6 V source and a 75 Ohm resistor over the diode v-i curve

Read the voltage and current at the intersection

Check the results by using a constant voltage drop model for the diode, Vf = 0.7 Volts

Electronics I for M.E.April 24, 2007 Slide 10

Exercise 2: The BJT v-i Curves

Draw a load line for a 15 Volt source and a 750 Ohm collector load resistor

For a base series resistance of 1 kOhms and Vf equal to 0.7 Volts: Find the base current for input voltages of 1.7 Volts,

2.7 Volts, etc. and Find the output voltages from the intersections of the

proper v-i curves with the load line and put them in a table

Plot the table

Electronics I for M.E.April 24, 2007 Slide 11

Exercise 3: The MOSFET v-i Curves Draw a load line for a 20 Volt source and a 400

Ohm drain load resistor For a threshold voltage VTR of 2 Volts:

Find the drain current for input voltages of 2 Volts, 3 Volts, etc. and

Find the output voltages from the intersections of the proper v-i curves with the load line and put them in a table

Plot the table