1. Recall-Lecture 4 Current generated due to two main factors
Drift – movement of carriers due to the existence of electric field
Diffusion – movement of carriers due to gradient in concentrations

2. Recall-Lecture 4 Vbi Introduction of PN junction
Space charge region/depletion region
Built-in potential voltage Vbi
Reversed biased pn junction
no current flow
Forward biased pn junction
current flow due to diffusion of carriers.
Vbi

3. PN Junction Diode
The basic PN junction diode circuit symbol, and conventional current direction and voltage polarity.
The graphs shows the ideal I-V characteristics of a PN junction diode.
The diode current is an exponential function of diode voltage in the forward-bias region.
The current is very nearly zero in the reverse-bias region.

4. PN Junction Diode Temperature Effects
Both IS and VT are functions of temperature.
The diode characteristics vary with temperature.
For silicon diodes, the change is approximately 2 mV/oC.
Forward-biased PN junction characteristics versus temperature.
The required diode voltage, V to produce a given current decreases with an increase in temperature.

5. Analysis of PN Junction Diode in a Circuit

6. CIRCUIT REPRESENTATION OF DIODE
The I -V characteristics of the ideal diode. i vD
Reverse bias
Conducting state
V = 0V
Reverse biased
open circuit
Conducting state
short circuit

7. CIRCUIT REPRESENTATION OF DIODE – Piecewise Linear ModelvD
Reverse bias
Conducting state
Reverse biased
open circuit V
VD = V for diode to turn on.
Hence during conducting state: = V
Represented as a battery of voltage = V

8. CIRCUIT REPRESENTATION OF DIODE – Piecewise Linear ModelvD
Reverse bias
Conducting state
Reverse biased
open circuit V
VD ≥ V for diode to turn on.
Hence during conducting state: = V
Represented as a battery of voltage = V and forward resistance, rf in series rf VD

9. Diode Circuits: DC Analysis and Models
Example
Consider a circuit with a dc voltage VPS applied across a resistor and a diode.
Applying KVL, we can write,
or,
The diode voltage VD and current ID are related by the ideal diode equation: (IS is assumed to be known for a particular diode)
Equation contains only one unknown, VD:

10. Why do you need to use these models?

11. Diode Circuits: Direct Approach
Question
Determine the diode voltage and current for the circuit.
Consider IS = A.
and
ITERATION METHOD

12. Diode Circuits: Using Models
Example
Determine the diode voltage, current and the
power dissipated by the diode using
piecewise linear model.
Assume piecewise linear diode parameters of
V = 0.6 V and rf = 10 Ω.
Solution:
The diode current is determined by:
Power dissipated: VD x ID = x 2.19 = 1.36 mW

13. DIODE DC ANALYSIS
Find I and VO for the circuit shown below if the diode cut in voltage is V = 0.7V + VO - I
I = mA
Vo = -0.35V

14. Find I and VO for the circuit shown below if the diode cut in voltage is V = 0.7V+ VO -
I = 0.372mA
Vo = 0.14V

15. Example 2 Determine ID if V = 0.7V
R = 4k
b) If VPS = 8V, what must be the value of R to get ID equal to part (a)

16. DC Load Line A linear line equation ID versus VD
Obtain the equation using KVL

17. V 2 Use KVL: 2ID + VD – 5 = 0 ID = -VD + 5 = - VD + 2.5
The value of ID at VD = V V 2
Use KVL:
2ID + VD – 5 = 0
ID = -VD = - VD + 2.5

18. DIODE AC EQUIVALENT

19. Sinusoidal Analysis The total input voltage vI = dc VPS + ac vi
iD = IDQ + id
vD = VDQ + vd
IDQ and VDQ are the DC diode current
and voltage respectively.

20. Diode Circuits: AC Equivalent Circuit
Current-voltage Relation
The relation between the diode current and voltage can be written as:
VDQ = dc quiescent voltage
vd = ac component
The -1 term in the equation is neglected.
The equation can be written as:
If vd << VT, the equation can be expanded into linear series as:
The DC diode current Is:

21. iD = ID [ 1 + vd/VT] iD = ID + ID vd / VT = ID + id where id = ID vd / VT using Ohm’s law: I = V/R hence, id = vd / rd compare with id = ID vd / VT which reveals that rd = VT / ID CONCLUSION: During AC analysis the diode is equivalent to a resistor, rd

22. IDQ
VDQ = V rd id
DC equivalent
AC equivalent

23. Example 1 VDQ = V IDQ Analyze the circuit (by determining VO & vo ).
Assume circuit and diode parameters of
VPS = 5 V, R = 5 kΩ, Vγ = 0.6 V & vi = 0.1 sin ωt
VDQ = V
IDQ

24. rd id

25. CALCULATE DC CURRENT, ID CALCULATE AC CURRENT, id
DC ANALYSIS
AC ANALYSIS
DIODE = MODEL 1 ,2 OR 3
CALCULATE rd
DIODE = RESISTOR, rd
CALCULATE DC CURRENT, ID
CALCULATE AC CURRENT, id

26. EXAMPLE 2
Assume the circuit and diode parameters for the circuit below are
VPS = 10V, R = 20k, V = 0.7V, and vi = 0.2 sin t. Determine the current, IDQ and the time varying current, id