Lecture Electrical Engineering: Lecture 10 - Dr. Nasim Zafar

In this chapter, you will learn about: Ideal & practical diodes; terminal characteristics of junction diodes; DC load line and quiescent conditions; piecewise linear model; small signal analysis of diodes; dynamic resistance, AC resistance; capacitance and switching response.

Fall Semester – 2012

COMSATS Institute of Information Technology

Virtual campusIslamabad

1.  The Ideal Diode Model

P­N Junction Diode Schematic Symbol:

I F

R +V

Reverse­Biased Diode Circuit:

R +V

5V 0.7V 4.3V 4.3V

4.3mA 1kΩ

6V 0.7V 5.3V

5.3V

530μA 10kΩ

R

V I

R

=

10V 0.7V 0.7V 1.5k Ω 1.8k Ω 2.61mA

2.61mA 3.03mA

2.61mA

S ideal

S D D prac

V I

Power Dissipation PD(max)

I0 and PD(max) Relationship:

(max) 0

D F

P I

V

=

where I0 = the limit on the average forward current

    PD(max) = the forward power dissipation rating of the diode

      VF = the diode forward voltage (0.7V for Si)

S D L

Example 5. 

A diode has a forward power dissipation rating of 500 

mW. What is the maximum allowable value of forward current for the device?

(max) 0

500mW

714.29mA 0.7V

D F

P I

V

0.2 0.4 0.6 0.8 20

40 60 80 100

20 40

60 80

1.0 2.0 3.0

R

V R

I

5.  Constant­Voltage Diode Model

        6.  Dynamic Resistance, AC Resistance

X T

X D

X X

I

I V

R I V

R I

V 1 for   

mA   2

0

V 3 for   

mA   2

2

X X

X X

V I

V I

Ø Small­signal analysis is performed at a DC bias point by perturbing the voltage by a small amount and observing the resulting linear current perturbation

V T s

V V D

D D

D

V

I e

V I dV

dI V

I

T D

D D

1 /

1

1

Ø Since there is a linear relationship between the small­signal current and small­signal voltage of a diode, the diode can be viewed as a linear resistor when only small changes in voltage are of interest

D

T d

I

V r

Small­Signal Resistance

(or Dynamic Resistance)

Small­signal analysis is performed around a bias point by 

1

D T

V V I

Small­Signal Analysis of Diodes:

Ø If  two  points  on  the  IV  curve  of  a  diode  are  close  enough,  the 

trajectory connecting  the first to the second  point  is like a line, with 

the  slope  being  the  proportionality  factor  between  change  in  voltage 

and change in current.   

T D

T

D T

s

VD

VD D

D D

D

V I

V

I V

I

dV

dI V

v If a sinusoidal voltage with small amplitude is applied, the 

resulting current is also a small sinusoid around a  value

Ø The operating point of a diode moves from one region to another the resistance of the diode will also change due to the nonlinear shape of the characteristic curve 

Ø The  type  of  applied  voltage  or  signal  will  define  the resistance level of interest 

Ø Three different types of applied voltage 

– DC or Static Resistance 

– AC or Dynamic Resistance 

– Average AC Resistance

) (

) 1 exp(

n

po n p

no

p s

s

L

n

eD L

p

eD J

kT

eVa J

J

a

i po

d

i no

N

n n

N

n p

state

Js : strong function of temperature

E

Typical Diodes

Diode Maximum Ratings.

Peak repetitve reverse voltage, VRRM Maximum allowable reverse voltage.

Nonrepetitive peak reverse voltage,

VRSM Maximum allowable value of a single event reverse voltage (VRSM >

RMS reverse voltage, VR(rms) VR(rms) = 0.707 VRRM

Average rectified forward current, I0 Maximum average diode current.

Nonrepetitive peak surge current,

IFSM Maximum allowable value of forward current surge (30A for 1N400X) Operating and storage junction

temperature, TJ or Tstg Temperature that diode can withstand.

Semiconductor Devices  Semiconductor Devices

Tunnel  diode  Photo­diode

Light Emitting diode & Laser Diode

BJT (Bipolar Junction Transistor)

Solar cell Photodetector

Ø Diode specifications.