Diode Circuits
Practical Aspects of pn Junction
anode
Reversed bias
+
-
+
-
Forward bias
cathode
The left hand diagram shows reverse bias, with positive on the
cathode and negative on the anode (via the lamp). No current flows.
The other diagram shows forward bias, with positive on the anode
and negative on the cathode. A current flows.
Polarization of the pn Junction
(1)
(3)
(2)
Forward bias
examples
(4)
Polarization of the pn Junction
(1)
(2)
Reversed bias
examples
(3)
(4)
Diode Ohms Check:
Checks preformed on Si diode, by reversing the leads on the
Digital Voltage Mutimeter (DMM).
-
DMM
+
1.
DMM = 0 
P
N
N
P
2.
DMM =  
Diode Voltages
To forward bias a
diode, the anode must
be more positive than
the cathode or LESS
NEGATIVE.
To reverse bias a
diode, the anode must
be less positive than
the cathode or MORE
NEGATIVE.
A conducting diode has about 0.6 volts across if silicon, 0.3 volts if germanium.
A Diode Puzzle
Which lamps are alight? Some may not be full brightness.
+
-
+
-
A Diode Puzzle
Which lamps are alight? Some may not be full brightness.
+
-
+
-
Exercise - a Diode Puzzle
Which lamps are alight? Some may not be full brightness.
+
-
+
-
Exercise - a Diode Puzzle
Which lamps are alight? Some may not be full brightness.
+
-
+
-
Diode Characteristic
circuit
Rp
Ev
A reading
R
D
V reading
A diode is a nonlinear device and typical linear circuit
analysis methods do not apply!
Diode Characteristic for Small-Signal Diodes
  vd  
  1
i D  I s exp 
  nVT  
kT
VT 
n ~ 1-2
q
VT ~ 26 mV
Vknee
less than 1mA at 300K
When the temperature is increasing the knee voltage Vknee
decreases by about 2mV/K
Analysis of Diode Circuits
Nodal analysis
Mesh analysis
Kirchhoff’s voltage law
Thevenin & Norton theorems
Vss  Ri D  v D
Vth/RTh
Slope=-1/RTh
Example 10.1
Vth
Analysis of Diode Circuits
+
i
+
o
Thevenin
equivalent
Vo
-
KVL
KCL
vD
iD
-
Vo  vD
io  iD
Their characteristics
intersect
Analysis of Diode Circuits
Nodal analysis
Mesh analysis
Kirchhoff’s voltage law
Thevenin & Norton theorems
Vss  Ri D  v D
Vth/RTh
Slope=-1/RTh
Example 10.1
Vth
Load-Line Analysis
Problem
If the circuit shown below has Vss=2V and R=1k and a diode with ch-tic
shown, find the diode voltage and current at the operating point
Vss  Ri D  v D
Repeat for:
Vss=10V and R=10k
VDQ=0.68V and iDQ=0.93mA
Zener Diode - Voltage Regulator (reverse biased)
A Zener diode is a type of diode that permits current not only in the forward
direction like a normal diode, but also in the reverse direction if the voltage is
larger than the breakdown voltage known as "Zener knee voltage" or "Zener
voltage".
Zener Diode - Voltage Regulator (reverse biased)
Problem
Find the output voltage for Vss=15V
and Vss=20V if R=1k and a Zener
diode has the ch-tic shown below.
Load Line analysis
Kirchhoff’s voltage law
Vss+ RiD+vD=0
Slope of the load is -1/R
Reverse bias region
Load Line Analysis of Complex Circuits
Thevenin Equivalent
Problem
Consider the Zener diode regulator shown in figure (a). Find the load voltage
vL and the source current iS if Vss=24V, R=1.2k and RL=6k.
Problem
Consider the Zener diode regulator shown in figure (a). Find the load voltage
vL and the source current iS if Vss=24V, R=1.2k and RL=6k.
Exercise – find Thevenin equivalent
Problem
Consider the Zener diode regulator shown in figure (a). Find the load voltage
vL and the source current iS if Vss=24V, R=1.2k and RL=6k.
Thevenin equivalent
VT=Vss*(RL/(R+RL))=20V
RT=(RRL)/(R+RL)=1k
Load line equation
VT + RTiD + VD = 0
VL=-VD=10V
iD=-10mA
Finally iS=(VSS-VL)/R=11.67 mA (from circuit “a”)
Exercise 10.4 & 10.5
Ideal diode Model
Useful for circuits with more than one diode
(1) Assume a state for each diode,
either “on” or “off” -2n combinations
(2) Assume a short circuit for diode
“on” and an open circuit for diode
“off”
(3) Check to see if the result is
consistent with the assumed state
for each diode (current must flow in
the forward direction for diode “on”
and the voltage across the diodes
assumed to be “off” must be positive
at the cathode – reverse bias)
(4) If the results are consistent with
the assumed states, the analysis is
finished. Otherwise return to step
(1) and choose a different
combination of diode states.
Problem
Analyze the circuit shown below using the ideal diode model. Start by
assuming the D1 is off and D2 is on.
7V
-3V
Not consistent with
the assumption that
D2 if off
Exercise 10.6 & 10.7 & 10.8
Problem
Analyze the circuit shown below using the ideal diode model. Start by
assuming the D1 is off and D2 is on.
7V
-3V
Not consistent with
the assumption that
D1 is off
Problem
Analyze the circuit shown below using the ideal diode model. Start by
assuming the D1 is off and D2 is on.
7V
-3V
This is OK
Piecewise Linear Diode Models
More accurate that the ideal diode model and do not relies on nonlinear
equation or graphical techniques.
(1) Diode V-I ch-tic approximated by
straight line segments
(2) We model each section of the diode
I-V ch-tic with R in series with a fixed
voltage source
v = Rai + Va
Problem
Find circuit models for the Zener-diode volt-ampere ch-tic shown in figure
below using the piecewise-linear diode model.
Draw a line
Look for intercept (0.6V)
& the reciprocal of the
slope (1/R)
(1.6V-0.6V)/100mA=10
Open circuit approximation
Repeat for the reverse bias
Exercise 10.7