Electrical Circuits and Electronics

Part 4
Diodes

Pearson Education, Inc.

 Part 4: Diodes
1. Understand diode operation and select diodes for various
applications.
2. Analyze nonlinear circuits using the graphical technique.

3. Analyze and design simple voltage-regulator circuits.

4. Solve circuits using the ideal-diode model and

piecewise-linear models.

5. Understand various rectifier and wave-shaping circuits.

6. Understand small-signal equivalent circuits.

 Basic diode Physics

• Diodes Consist of a junction between two types of semiconducting material

(usually, silicon with carefully selected impurities).
• On one side of the junction, the impurities create n-type material, in which
large numbers of electrons move freely.
• On the other side of the junction, the impurities create p-type material, in
which holes (positively charged particles) predominate.
• With no external applied voltage, an electric-field barrier appears naturally
at the pn junction.
•With external applied voltage, the diode conducts very little current for n-
side polarity and large current for the p-side polarity of applied voltage.
 Basic diode concepts

• Diodes readily conduct current from anode to cathode, but do not allow
current to flow in the opposite direction.
• Forward-bias region : current flows easily through the diode in the
direction of the arrowhead of the circuit symbol.
• Reverse-bias region : for moderate negative values of VD , the current iD is
very small in magnitude.
•Reverse-breakdown region : for sufficiently large reverse-bias voltage, and
large magnitude flow of current.
Non-linear I-V Characteristic

“On”

“Off”
 Zener Diodes
• Diodes that are intended to operate in the breakdown region are called
Zener diodes.
• Zener diodes are useful in applications for which a constant voltage in
breakdown is desirable.

Volt-Amper Characteristic
 Load-Line Analysis of Diode Circuits
• Many of the techniques that we have studied for linear circuits do not
apply for circuits involving diodes.
• Graphical methods provide one approach to analysis of nonlinear circuits.
• Connecting points A and B results in a plot called the load line.
• The operating point is the intersection of the load line and the diode
characteristic

VSS = RiD + v D

Load-line analysis of the circuit

 Zener-Diode Voltage-Regulator Circuits
• A voltage regulator circuit provides a nearly constant voltage to a load from a
variable source.
• The Zener diode has a breakdown voltage (region with negative values for VD)
and iD is equal to the desired output current.
• The resistor R limits the diode current to a safe value so that the Zener diode
does not overheat.

VSS + RiD + vD = 0
 Load-Line Analysis of Complex Circuits

• Analysis of a circuit containing a single nonlinear element can be

accomplished by load-line analysis of a simplified circuit.

•ofThe load line is constructed to find the operating point on the characteristic
the nonlinear device.

• Voltages and currents can be determined in the original circuit.

 Shockley Equation

• Relationship between current and voltage for a junction diode :

⎡ ⎛ vD ⎞ ⎤ kT
iD = I s ⎢exp⎜⎜ ⎟⎟ − 1⎥ VT =
⎣ ⎝ nVT ⎠ ⎦ q
• Is : Saturation current, n: emission coefficient , VT : Thermal voltage

k = 1.38 × 10–23 J/K is Boltzmann’s constant and q = 1.60 × 10–19 C is the magnitude
of the electrical charge of an electron. At a temperature of 300 K, we have :
VT ≅ 26 mV
⎛ vD ⎞
iD ≈ I s exp⎜⎜ ⎟⎟ for v D >> VT
⎝ nVT ⎠
iD ≈ −I s for vD << -VT until reaching reverse breakdown
William Shockley, (Nobel Laureate, 1956; from
Electrons and Holes in Semiconductors)
 Ideal-Diode Model
• Graphical load-line and Shockley equation are too cumbersome for more
complex circuits. Instead, we use simpler models to approximate diode
behavior.
• The ideal diode acts as a short circuit for forward currents and as an open
circuit with reverse voltage applied.
• If iD is positive, vD is zero, the diode is in the on state
• If vD is negative, iD is zero, the diode is is in the off state

Volt-Ampere characteristic for the ideal diode

 Analysis of Ideal-Diode Circuits
A step-by-step procedure for analyzing circuits that contain ideal diode is to :

1. Assume a state for each diode, either on (i.e., a short circuit) or off (i.e., an
open circuit). For n diodes there are 2n possible combinations of diode states.

2. Analyze the circuit to determine the current through the diodes assumed to be
on and the voltage across the diodes assumed to be 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 diodes assumed to be on.
Furthermore, the voltage across the diodes assumed to be off must be positive at
the cathode (i.e., 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.
Analysis of Ideal-Diode Circuits

Assume D1 off and D2 on:

vD1= 7V

D1 would
be on!
Analysis of Ideal-Diode Circuits

Assume D1 off and D2 off:

Both diodes should be on!

vD1= 10V vD2= 3V
Analysis of Ideal-Diode Circuits

Assume D1 on and D2 on:

 Analysis of Ideal-Diode Circuits

i1 i2

i3

10V − 3V
i1 = = 1.75 mA
4 KΩ
3V
i3 = = 0.5 mA
6 KΩ
i1 + i2 = i3 → i2 = i3 − i1 = 0.5 mA − 1.75 mA = −1.25 mA
Not consistent with direction of current through D2!
Analysis of Ideal-Diode Circuits

Assume D1 on and D2 off:

This works!
6V
vD2= -3V
iD1=1mA
 Piecewise-Linear Diode Models
• More accurate model than the ideal-diode assumption
• Approximate the volt-ampere characteristic by straight-
line segments
• Model each section of the diode characteristic with a
resistance in series with a constant-voltage source

v = Ra i + Va

Circuit and volt-ampere characteristic for piecewise-linear

Piecewise-Linear Diode Models
 Piecewise-Linear Diode Models
Example:

Solution: Since the 3Vsource has a polarity that results in forward bias of the diode we assume
that the operating point is on line segment A. Consequently the equivalent circuit for the diode
is the one for segment A. Solving we find that i = 80mA.
D
 Types of Diodes and Their Uses

PN Junction Diodes: Are used to allow current to flow in one direction while blocking
current flow in the opposite direction. The pn junction diode is
the typical diode that has been used in the previous circuits.

P N
A C

Schematic Symbol for a PN Representative Structure for a PN

Junction Diode Junction Diode

Zener Diodes: Are specifically designed to operate under reverse breakdown

conditions. These diodes have a very accurate and specific
reverse breakdown voltage.

A C

Schematic Symbol for a Zener

Diode
 Types of Diodes and Their Uses

Schottky Diodes:

These diodes are designed to have a very fast switching time

A C which makes them a great diode for digital circuit applications.
They are very common in computers because of their ability to
Schematic Symbol for a Schottky be switched on and off so quickly.
Diode

Shockley Diodes:

The Shockley diode is a four-layer diode while other diodes

are normally made with only two layers. These types of
A C
diodes are generally used in switching applications and to
control the average power delivered to a load.
Schematic Symbol for a four-layer
Shockley Diode
 Types of Diodes and Their Uses

Light-Emitting Diodes:

Light-emitting diodes are designed with a very large bandgap so

movement of carriers across their depletion region emits photons
of light energy. Lower bandgap LEDs (Light-Emitting Diodes)
emit infrared radiation, while LEDs with higher bandgap energy
emit visible light. Many traffic lights are now starting to use
LEDs because they are extremely bright and last longer than
regular bulbs for a relatively low cost.

The arrows in the LED representation

A C indicate emitted light.

Schematic Symbol for a Light-

Emitting Diode
 Types of Diodes and Their Uses

Photodiodes:
While LEDs emit light, Photodiodes are sensitive to received
light. They are constructed so their pn junction can be exposed to
A C the outside through a clear window or lens.
In Photoconductive mode, the saturation current increases in
proportion to the intensity of the received light.

λ In Photovoltaic mode, when the pn junction is exposed to a

A C
certain wavelength of light, the diode generates voltage and can
be used as an energy source. This type of diode is used in the
production of solar power. (Definitions from Kristin Ackerson, Virginia Tech EE)
Schematic Symbols for Photodiodes
 Practical circuit : Rectifier Circuits

• Rectifiers : convert AC power into DC power

• These circuits allow power to flow only from the source to the load,
unidirectional converters.

• Basis for electronic power supplies and battery-charging circuits: such as

computer circuits or television circuits.

• Signal processing, such as demodulation of a radio signal (Demodulation is the

process of retrieving the message, such as a voice or video signal)

•There are many possible ways to construct rectifier circuits using diodes. The two
basic types of rectifier circuits are:

• The Half Wave Rectifier

• The Full Wave Rectifier
The D.C Power Supply
 Half-Wave Rectifier -1-
• Half-Wave Rectifier Circuits is composed of a sinusoidal source, a
diode and a resistive load RL
• Only the positive half-cycles of the source voltage appear across
the load (reverse biased diode)
• The diode conducts only when the source voltage Vs is positive :
• For positive half-cycle of input, source forces positive
current through diode, diode is on, Vo= Vs.
• During negative half cycle, negative current can’t exist in
ideal diode, diode is off, current in resistor is zero and Vo=0.
 Half Wave Rectifier -2-
output voltage
input voltage
R VO

0 VO 0

• Diode is on for negative input voltages VO

• When diode is on the output voltage is zero

R
• Diode is off for positive input voltages
• When diode is off the output voltage is the same as input voltage VO
 Half Wave Rectifier -3-

output voltage
R VO
input voltage

VO 0

• Diode is on for positive voltages VO

• When diode is on the output voltage is zero

R
• Diode is off for negative voltages
• When diode is off the output voltage is the same as input voltage VO
 Full-Wave Rectifier Circuits -1-

• The full wave rectifier consists of two diodes and a resister

• During the positive half cycle of the full wave rectifier. Note that diode A
is forward biased and diode B is reverse biased. Note the direction of the
current through the load.
• During the negative half cycle, the polarity reverses. Diode B is forward
biased and diode A is reverse biased. Note that the direction of current
through the load has not changed even though the secondary voltage has
changed polarity. Thus another positive half cycle is produced across the
load.
Full-Wave Rectifier Circuits 2-
Full-Wave Rectifier Circu
Full-Wave Rectifier Circuits -3-

Graëtz bridge
 Full-Wave Rectifier Circ
Full-Wave Rectifier Circuits -4-
 Wave-Shaping Circuits :Clipper circuits
• Clippers, limiters or clipping circuits make use of non-linear properties of
diode, that is the diode conducts the current in forward direction and does not
conduct in reverse direction.
• These circuits are primarily wave shaping circuits.
• They clip or remove certain portion of ac voltage applied to the input of
circuit.
 Diode Positive Clipper with positive bias voltage -1-

R
VB
VB VO 0
VB
0

Positive peak is clipped to V

B

• Diode conducts for input voltages above VB VO

• When diode conducts the output voltage = VB VB

• Diode opens for input voltages below VB VO

• When diode opens the output voltage is the same as input voltage VB

( No current, no drop at “R” )

 Diode Negative Clipper with negative bias voltage -2-

0 VO 0
VB -Vb
-VB

Negative peak is clipped to -V

B

• Diode conducts for input voltages below (-VB)

• When diode conducts the output voltage = (-VB)

• Diode opens for input voltages above (-VB)

• When diode opens the output voltage is the same as input voltage

( No current, no drop at “R” )

 Diode Double Clipper with bias voltage -3-

VB
VB VO
VB 0
0 -VB
-VB
-VB

Positive peak is clipped to V

B
Negative peak is clipped to –V
B
R R

• Diode1 (Left) conducts for input voltages above VB

VO VO
• When diode conducts the output voltage = V B
VB VB

• Diode2 (Right) conducts for input voltages below -VB

R
• When diode conducts the output voltage = -VB
Between V and –V no diode conducts & no current, no drop at “R” VO
B B
Output voltage = Input voltage VB -VB
 6V
part of the input waveform above 6 V or less than 9 V. (We are assuming ideal
diodes.) When the input voltage is between 9 and +6 V, both diodes are off and
Clipper circuits using Zener diodes
no current ows. Then, there is no drop across R and the output voltage vo is equal
to the input voltage vin . On the other hand, when vin is larger than 6 V, diode A is
(a) Circuit diagram

on and the output voltage is 6 V, because the diode connects the 6-V battery to the

15 V Portion of vin clipped

R off by diode A
+
2k
+ A 9V
+ 6V
vin(t) 15 sin(vt) + vo(t)
vo(t)
6V B
t

(a) Circuit diagram

9V
Diode
vo (V)
Section 10.7 Wave-Shaping C
rtion of vin clipped
off by diode A Diode (b) Waveforms
A on
6
Figure 10.29 Clipper circuit.
2k 2k
+ 8.4 V 1 8.4 V
+
1
vin(t) vin(t)
v (V)
t 5.4 V 9 6 5.4 Vin

Both diodes off

(a) Circuit of Figure 10.29 with batteries replaced (b) Simpler circuit
by Zener diodes and allowance made for a
0.6-V forward diodeDiode
drop B on
9
Figure 10.30 Circuits with nearly the same performance as the circuit of
 Example :Clipper circuits -1-

•0.6Sketch the transfer characteristics to scale for the circuits below. Allow a
V forward drop for the diodes. And skectch the output waveform to
scale if Vin(t) = 15 sin(ⱳt).

9.4 V
 Example :Clipper circuits -2-
For this circuit all of diodes are off if -1.8<v0<10. With the diodes off, no current
flows and V0=Vin When Vin exeeds 10V, D1 turns on and D2 is in reverse
breakdown. Then V0=9.4+0.6=10V. When Vin becomes less than -1.8V diodes D3,
D4, and D5 turn on and V0= -3 x 0.6 = -1.8V.

V0(V)

10 V 10 V

-1.8
Vin(V)
-1.8 -1.8
10 V
 Wave-Shaping Circuits
Wave-Shaping :Clamp
Circuits circuits
: Clamp Circuits
•Clamping circuits are used to hold either positive or
negative extremity to a reference voltage level.

• In a clamp circuit, a variable DC voltage is added to

the input waveform so that one of the peaks of the
output is clamped to a specified value.

•Inserting a reference voltage between the cathode of

the diode and common, the positive extremity is held
at – 5 V level (positive peaks are clamped to -5V).

• The capacitance is a large value, and we can consider

the voltage across the capacitor to be constant.

• The output voltage of the circuit is given by :

V0(t)= Vin(t)-Vc
Reversing the direction of the diode causes the negative InClamp
this example, after
Circuits
peak to be clamped instead of the positive peak. charging Vc=10V
 Wave-Shaping Circuits :Clamp circuits
Positive peak clamps to zero

+C -
VP

0
0 + VP
VP R
-VP - 0

-
-VP
+
VO
C -2VP
R C
+ - -2VP
- VP
VP R
+

During the first positive half cycle the diode acts like a short circuit. The capacitor
charges to peak value of input voltage Vp. During this interval the output Vo which
is taken across the short circuit will be zero. During the negative half cycle, the
diode is open. C is very large, so Vc will stay almost constant for the next cycles.
 Wave-Shaping Circuits :Clamp circuits
Positive peak clamps to +VB

C
VP
+ - VO=-(VP-VB)+VP
=VB
+ VP-VB
0 R
VP
-
VB
-VP
0

2VP
C
+ - VO
+ VP-VB
+
C
- VO=-(VP-VB)-VP
VP R =-2VP+VB
- VP-VB
- VB
VP R
+

During the positive half cycle of the input signal the diode is forward biased
and acts like a short circuit. The capacitor charges to Vp-VB .
 Linear Small-Signal Equivalent Circuits
• The operating point or Q point of the diode is the quiescent or no-signal
condition. The Q point is obtained graphically.
• Find a linear small-signal equivalent circuit for the nonlinear device to use in
the AC analysis.
• The DC supply voltage results in operation at the quiescent point (Q point).
• The small-signal equivalent circuit for a diode is a resistance (dynamic
resistance).

−1
⎛ di D ⎞ ⎡⎛ di ⎞ ⎤
ΔiD ≅ ⎜⎜ ⎟⎟ Δv D rd ≅ ⎢⎜⎜ D ⎟⎟ ⎥
⎝ dv D ⎠Q ⎢⎣⎝ dv D ⎠Q ⎥⎦

vd
id =
Diode characteristic illustrating the Q point. rd
 Linear Small-Signal Equivalent Circuits : Quiescent point

• For signals that cause small

changes from the Q point, the diode
acts simply as a linear resistance.

• As the Q point moves higher, a

fixed-amplitude AC voltage
produces an AC current of larger
amplitude.

• As the Q-point current IDQ

increases, the resistance becoms
smaller.
 Notation for Currents and Voltages

§ vD and iD represent the total instantaneous diode voltage and current. At

times, we may wish to emphasize the time-varying nature of these quantities,
and then we use vD(t) and iD(t)
§ VDQ and IDQ represent the dc diode current and voltage at the quiescent
point.
§ vd and id represent the (small) ac signals. If we wish to emphasize their time
varying nature, we use vd(t) and id(t).

Illustration of diode currents