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Key Lecture Concepts for CoE225/EE 271
(Mostly Digital Electronics)
1
LESSON 2:
Analysis of Circuits with TwoTerminal Nonlinear Devices Using
LOAD CURVES and Mathematical Model (TRIAL AND ERROR)
Lesson Overview:
This lesson has three learning objectives.
The first is to
master a
trial and error method to find the Q point of a diode in series with a resistor using
the diode equations. Trial and error is an important technique to use to solve any
nonlinear problem which requires the solution of a transcendental equation. By this
method the diode current and voltage can be found with high precision by using just
two assumptions or “educated guesses” of either the value of the current or voltage.
It is an alternative to grinding out a mathematical solution or relying on computer
analysis; it enables the engineer to quickly visualize the system that is being designed
or analyzed.
Exercising
the trial and error method will give you a feeling for the typical
small changes in diode voltage values required for large changes in diode current.
The second learning objective is to use graphical methods to find the Qpt. of nonlinear
devices in series or in parallel. Graphical approaches are particularly useful when i/v
characteristics can not be described mathematically; they are also important for
designing and inventing new circuits and devices and for understanding the factors
involved in highspeed switching.
The graphical analyses taught in this lesson will be used
throughout the course. It is well worth the effort to struggle a little to learn how to use
graphical analysis versus relying only on mathematical equations.
The third learning objective is to analyze circuits with more than one nonlinear device,
e.g. multidiode circuits. For multidiode circuits it is necessary to make an assumption
for each diode on whether its Qpoint is in the 1
st
or 3
rd
quadrant.
Then the circuit is
analyzed based on these assumptions and the results checked by comparing the Qpoint
location of each diode versus the original assumption. If the assumption is wrong, the
analysis must be repeated using a new set of assumptions for the states of each of the diodes.
The analysis is not as difficult as it may seem, because a “feeling” for the best initial
assumptions and how to modify them to use a correct set of assumptions does develop after
solving problems. A “short or open” approximation to the state of the diode (the previously
described ideal diode characteristic) can be used to simplify the analysis.
A) Analysis by Iteration (Trial and Error Method)
The iteration method will be taught by solving the problem of finding the voltage and current
of a forward biased diode in series with a 10K resistor driven by a total applied voltage of ten
volts. The applied voltage could be from a 10 volt supply line and ground or alternatively
from plus and minus five volt lines.
The circuits are shown in the margin.
The parameters for
the diode will be taken to be
I
S
= 10
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This note was uploaded on 02/18/2010 for the course ECET 271 taught by Professor Hu during the Spring '05 term at NJIT.
 Spring '05
 Hu

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