Tài liệu EIE209 Basic Electronics Diode Circuits pdf

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

Prof. C.K. Tse: Diode circuits

0.7V

vd

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

Also, the diode can only stand the
negative voltage up to a certain
threshold VBD , beyond which
the diode conducts reverse
current (breakdown).

- 1)
–VBD
breakdown

Prof. C.K. Tse: Diode circuits

vd
0.7V

4


Which model to use?
The choice depends on the external voltage magnitudes.

Ideal model:

id = 100/100 = 1 A

With 0.7V drop:

id = (100–0.7)/100
= 0.997 A

Ideal model:

id = 10/100 = 100 mA
With 0.7V drop:

id = (10–0.7)/100
= 93 mA

Prof. C.K. Tse: Diode circuits

Ideal model:

id = 2/100 = 20 mA
With 0.7V drop:

id = (2–0.7)/100
= 13 mA

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
Prof. C.K. Tse: Diode circuits

full-wave doubler
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,
id) is somewhere on the
diode characteristic curve

2

the operating point (vR, iR)
is somewhere on the
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
+ vR –

How to find vd and id ?

iR

id

R

:)

Vs

+
_

id
+
vd
–

nonlinear device

vd

Prof. C.K. Tse: Diode circuits

13


Basic load line construction
+ vR –

iR

i

R

:)

Vs

+
_

id
+
vd
–

slope = –1/R

id

device
characteristic

nonlinear device

vd

Prof. C.K. Tse: Diode circuits

v

Vs

14


Alternative construction
+ vR –

iR

i

R

:)

Vs

+
_

id
+
vd
–

id when
device is
short-circuit

nonlinear device

device
characteristic
load line

v
vd when
device is opencircuit

Prof. C.K. Tse: Diode circuits

15

Tutorial problem
Find the operating point.

Prof. C.K. Tse: Diode circuits

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