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- EIE209 Basic Electronics
Diode circuits
Contents
• Diode models
• Typical diode circuits & applications
• Load line concepts for nonlinear load analysis
Prof. C.K. Tse: Diode circuits
1
- What is a diode?
Simplest view (no physics):
— a unidirectional device
that allows current to flow in
one direction but not the
other.
Ideally, we regard a diode as
short circuit when voltage
applied to it in the forward
manner is positive.
Prof. C.K. Tse: Diode circuits
2
- Ideal characteristic
Bias conditions
Forward bias : vd > 0 — current can flow and id > 0.
Reverse bias : vd < 0 — current cannot flow and id = 0.
id id
Ideal diode with finite forward drop
(more realistic)
vd vd
0.7V
Prof. C.K. Tse: Diode circuits
3
- Real characteristic
Take a closer look at the characteristic around the
turning point.
The i-v characteristic is an exponential function.
id
From physics, we have
id = I ss (e q vd /kT
- 1)
Also, the diode can only stand the –VBD
negative voltage up to a certain vd
threshold VBD , beyond which breakdown 0.7V
the diode conducts reverse
current (breakdown).
Prof. C.K. Tse: Diode circuits
4
- Which model to use?
The choice depends on the external voltage magnitudes.
Ideal model: Ideal model: Ideal model:
id = 100/100 = 1 A id = 10/100 = 100 mA id = 2/100 = 20 mA
With 0.7V drop: With 0.7V drop: With 0.7V drop:
id = (100–0.7)/100 id = (10–0.7)/100 id = (2–0.7)/100
= 0.997 A = 93 mA = 13 mA
Prof. C.K. Tse: Diode circuits
5
- Example: rectifier circuit
The ideal model is valid if the external voltages are well above 0.7V.
What is the magnitude of vr ?
Crude — 50 V
Better — 50 – 1.4 = 48.6 V
Prof. C.K. Tse: Diode circuits
6
- Application examples
clamper dc restorer
half-wave doubler full-wave doubler
Prof. C.K. Tse: Diode circuits
7
- A nonlinear circuit problem
Suppose we wish to find vd , given that the external voltage Vs is not large
enough to validate the use of the ideal diode model.
??
Prof. C.K. Tse: Diode circuits
8
- Step 1 : locating the operating points
Recall: The characteristic curve/line for a device actually defines where the point (v, i) can lie.
We know
1 the operating point (vd, 2 the operating point (vR, iR)
id) is somewhere on the is somewhere on the
diode characteristic curve resistor characteristic curve
Prof. C.K. Tse: Diode circuits
9
- Step 2 : KVL & KCL constraints
We also know
from KCL : id = iR AND from KVL : vd + vR = Vs
= Vs
O+
=
Prof. C.K. Tse: Diode circuits
10
- Step 3 : enforcing KVL & KCL
Method: flip the resistor curve horizontally; and
push the two curves together horizontally until the y-axes are Vs apart.
push push
add to Vs
Prof. C.K. Tse: Diode circuits
11
- Solution : load line
The flipped resistor line is called the LOAD LINE.
Prof. C.K. Tse: Diode circuits
12
- General problem
How to find vd and id ?
+ vR – iR
id
R id
+ +
Vs _
:) vd
–
nonlinear device
vd
Prof. C.K. Tse: Diode circuits
13
- Basic load line construction
+ vR – iR
i
R id
slope = –1/R
+ +
Vs _
:) vd id device
– characteristic
nonlinear device
v
vd
Vs
Prof. C.K. Tse: Diode circuits
14
- Alternative construction
+ vR – iR
i
R id id when
device is
+ + short-circuit
Vs _
:) vd device
– characteristic
nonlinear device load line
v
vd when
device is open-
circuit
Prof. C.K. Tse: Diode circuits
15
- Tutorial problem
Find the operating point.
Prof. C.K. Tse: Diode circuits
16
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