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Diodes
ESC102 : Introduction to Electronics
A.R. HarishDept. of EE, IIT Kanpur
Diode : I-V Characteristics
exp( ) 1
: Reverse Saturation Current
/ 26 at T 300K
dD S
T
S
T
vi I
nV
I
V kT q mV
Vd
n is called ideality factor and is equal to 1 for ideal diodes
I
VVγ
Slope = 1/rf
V
open circuit
rf
V > V
V < V
Piece-Wise Linear Model
Vγ is called cut-in or turn-on voltage and depends on nature of diode and range of current considered
10f f
Ir V V I r
V V
For most of our analysis, we will take Vγ = 0.7V and rf ~10Ω
Even Simpler Diode Models
open circuit
V > V
V < V
V
V
open circuit
rf
V > V
V < V
Constant voltage drop model
V V
I
V
I
V
Even Simpler Diode Models
Ideal diode model
V
I
V
Diode Models
open circuit
V > V
V < V
V
open circuit
V > 0
V < 0
V
open circuit
rf
V > V
V < V
+ vd -
I
exp( ) 1dD S
T
vi I
V
Simplicity
Accuracy
D
1K
10V
Analysis using ideal diode model
open circuit
V > 0
V < 0
1K
10V D
mAk
I 101
10
Analysis with a constant voltage diode model
D
1K
10V open circuit
0.7V
V > 0.7
V < 0.7
1K
10V 0.7V
mAk
I 3.91
7.010
V
open circuit
rf
V > V
V < V
Analysis with a constant voltage plus resistor diode model
D
1K
10V
mAI 208.9101000
7.010
D 5K
10V
5K2mA
Example
Find the current through the diode using ideal diode model
Is the diode forward biased? – Not Sure!!
Assume that it is forward biased
Carry out analysis and then check if current through the diode is in appropriate direction.
If not, diode is reverse biased and we carry out the analysis again!!
Example
D 5K
10V
5K2mA
102 0
5 DmA iK
10V
5K2mAiD 4Di mA
Current is positive, so our assumption is correct
Assume forward bias
D 5K
10V
2mA 4K
D 5K
10V
2mA 4K
Example Find the current through the 5K resistor using ideal diode model
Assume forward bias
D 5K
10V
2mA 4K
102 0
4 DmA iK
10V
2mA 4K iD
4.5Di mA
This is not possible.
Therefore, our assumption is incorrect
D 5K
10V
2mA 4K
5K
10V
2mA 4K
Assume reverse bias
VV
Vk
V
k
VmA
10
0
05
10
42
2
1
11
V1 V2
