Basic Electronics
R.A.D.D. Dharmasiri
Department of Physics
University of Sri Jayewardenepura
Diode
 PN junction, fitted in an enclosure with conducting
terminals forms a diode
 P side : Anode
 N side : Cathode
P type
N type
ANODE
CATHODE
Types of Diodes
Shottky Diode
General Purpose
Diode
Light Emitting Diode
(LED)
Ideal Diode
• The ideal diode may be considered the most fundamental
nonlinear circuit element which is a two-terminal device.
• The terminal characteristic of the ideal diode can be
interpreted as follows:
• Negative applied voltage – no current flows and the diode
behaves as an open circuit (reverse biased)
• Positive applied current – zero voltage drop appears across
the diode and the ideal diode behaves as a short circuit
(forward biased)
Figure 1: The ideal diode: (a) diode circuit symbol; (b) i–v
characteristic; (c) equivalent circuit in the reverse direction; (d)
equivalent circuit in the forward direction.
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The external circuit must
be designed to limit the
forward current through
a conducting diode, and
the reverse voltage
across a cutoff diode, to
predetermined values.
Figure 2 The two modes of
operation of ideal diodes and the
use of an external circuit to limit
the forward current (a) and the
reverse voltage (b).
• The I-V characteristic of the ideal diode is
highly nonlinear.
• Although it consists of two straight-line
segments, they are at 90° to one another.
• A nonlinear curve that consists of two straightline segments is said to be piece-wise linear.
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• If a device having a piecewise-linear characteristic is
used in a particular application in such a way that the
signal across its terminals swings along only one of the
linear segments, then the device can be considered a
linear circuit element as far as that particular circuit
application is considered.
• On the other hand, if signals swing past one or more of
the break points in the characteristic, linear analysis is
no longer possible.
A simple application
The Rectifier
Figure 3 (a) Rectifier circuit. (b) Input waveform. (c) Equivalent circuit when
vI  0. (d) Equivalent circuit when vI ≤ 0. (e) Output waveform.
Example 1:
•For the circuit in figure 3, sketch the
transfer characteristic v0 versus vp.
•sketch the waveform of vD.
•If vI have a peak value of 10 V and R = 1
kΩ, find the peak value of iD and the dc
component of vD.
Example 2:
Assuming the diodes to be ideal, find the values of I and
V in the circuits of figure 4.
Figure 4
Example 3:
Assuming the diodes to be ideal, find the values of I and V in the
circuits of figure 5.
Figure 5
Example 4:
Figure 6 shows a circuit for an ac voltmeter. It utilizes a movingcoil meter that gives a full-scale reading when the average current
flowing through it is 1 mA. The moving-coil meter has a 50 Ω
resistance. Assuming the diode to be ideal, find the value of R that
results in the meter indicating a full scale reading when the input
sine wave voltage v1 is 20 VPP.
Figure 6
Terminal characteristics of Junction Diodes
•
This section will discuss the characteristics of real diodes –
specifically, semiconductor junction diodes made of silicon.
•
Figure 7 shows the i–v characteristic of a silicon junction
diode.
•
The same characteristic is shown in figure 8 with some
scales expanded and others compressed to reveal details.
•
The characteristic curve consists of three distinct regions:
1. The forward bias region
2. The reverse bias region
3. The breakdown region
Figure 7: The i–v characteristic of a silicon junction
diode.
Figure 8: The diode i–v relationship with some scales expanded
and others compressed in order to reveal details.
Forward Bias Region
• When terminal voltage v is positive diode is in
forward bias region
• Current is negligibly small for v smaller than
about 0.5V
• ‘Cut-in voltage’ or ‘On-voltage’ or ‘Diode
forward voltage drop (Vd)’
• Region is forward but small bias and only a
small forward current is conducted
Forward Bias Region
• As potential difference is increased above cutin voltage diode current becomes appreciable
• Diode presents a very low resistance
• For a fully conducting diode the voltage drop
lies in a narrow range: approximately 0.6 - 0.8
V
• A conducting diode is assumed to have a 0.7V
drop across it
Reverse Bias Region
• Reverse bias region of diode is entered when
diode voltage v is made negative
• Real diodes have a small reverse current
• Reverse current is temperature dependent: at
sufficiently high temperatures reverse current
can be large
• Reverse current increases in magnitude
slightly as reverse voltage increases
Breakdown Region
• Breakdown region is when magnitude of
reverse voltage exceeds a threshold value
• Breakdown voltage : voltage at knee of i-v
curve - VZK
• In breakdown region reverse current increases
rapidly with a very small increase in reverse
voltage drop
• Diode breakdown is normally not destructive
if power dissipated is limited
Forward Characteristics &
Temperature
• Forward i-v characteristic varies with temperature
• At a given constant
diode current voltage
i
T2
T1
drop across diode
decreases by
-2 mV/C
approximately 2mV for
every 1C increase in
temperature
T2 > T1
v
Modelling Diode Forward Characteristic
•The Exponential Model
•The Piecewise-Linear Model
•The Constant-Voltage-Drop Model
•The Ideal -Diode Model
•The Small-Signal Model
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Modelling
Diode
Forward
Characteristic
• A simple diode circuit can be used to model
the operation of the diode.
R
ID
+
VDD
VD
Figure 9
Exponential Model
•
•
•
•
Most accurate description of diode operation
Severely non-linear and is more difficult to use
Need to use forward characteristics of diode
Graphical analysis using a load line
ID =
VDD – VD
R
ID  IS e
VD
nVT
• The current Is is usually called the saturation
current
• intersection between exponential model and load
line gives the operating point of circuit
Exponential Model
i
VDD/R
Diode characteristic
Load line
Operating point
ID
Slope = -1/R
0
VD
VDD
v
The Piecewise-linear Model
• The analysis can be greatly simplified if we can find linear
relationships to describe the diode terminal characteristics.
• An attempt in this direction is illustrated in figure 9, where the
exponential curve is approximated by two straight lines, line A
with zero slope and line B with a slope of 1/rD.
• It can be seen that for the particular case shown in figure 13,
over the current range of 0.1 mA to 10 mA the voltages
predicted by the straight-lines model shown differ from those
predicted by the exponential model by less than 50 mV.
Figure 10 Approximating the diode forward characteristic with
two straight lines: the piecewise-linear model.
Obviously the choice of these two straight lines is not unique; one
can obtain a closer approximation by restricting the current range
over which the approximation is required.
The straight-lines (or piecewise-linear) model of figure 9 can be
described by
iD  0,
vD  VD 0
iD  (vD  VD 0 ) / rD ,
vD  VD 0
Where VD0 is the intercept of line B on the voltage axis and rD is the
inverse of the slope of line B. For the particular example shown, VD0
=0.65 V and rD = 20 Ω.
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The piecewise-linear model described in the above equation can
be represented by the equivalent circuit shown in figure 11.
Note that an ideal diode is included in this model to constrain iD to
flow in the forward direction only. This model is also known as the
battery-plus-resistance model.
Figure 11: Piecewise-linear model of the diode forward characteristic and its
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equivalent circuit representation.
Example : calculate the current ID and VD of simple diode circuit shown in figure
9 by utilizing the piecewise-linear model whose parameters are given in figure
10 (VD0 =0.65 V and rD = 20 Ω).where VDD = 5 V and R = 1kΩ
Solution:
Replacing the diode in the circuit of figure
9 with the equivalent circuit model of
figure 11 results in the circuit in figure 12,
from which we can write for the current ID,
VDD  VD 0
ID 
= 4.26 mA
R  rD
VD  VD 0  I D rD = 0.735 V
Figure 12: The circuit of Fig. 9 with
the diode replaced with its
piecewise-linear model of figure 11.
The Constant-Voltage-Drop Model
• An even simpler model of the diode forward
characteristics can be obtained if we use a
vertical straight line to approximate the fastrising part of the exponential curve as shown in
figure 13.
• The resulting model simply says that a forwardconducting diode exhibits a constant voltage
drop VD.
Figure 13Development of the constant-voltage-drop model of the diode
forward characteristics. A vertical straight line (B) is used to approximate the
fast-rising exponential. Observe that this simple model predicts VD to within
0.1 V over the current range of 0.1 mA to 10 mA.
The constant-voltage-drop model can be represented by the equivalent circuit
shown in figure 14.
The constant-voltage-drop model is the one most frequently employed in the
initial phases of analysis and design. This is especially true if at these stages one
does not have detailed information about the diode characteristics, which is often
case.
Figure 14 The constant-voltage-drop model of the diode forward characteris
and its equivalent-circuit representation.
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Ideal Diode Model
• In applications that involve voltages much
greater than the diode voltage drop diode
voltage drop can be neglected
I
V
Ideal Diode Model
• VDD = 5V and R = 1kΩ in circuit
• The ideal diode model leads to:
VD = 0V
ID =
= 5mA
• For very quick analysis this model gives a gross
estimate
• Ideal diode model is most useful in determining which
diodes are on and which are off in a multi-diode circuit
Modeling the Diode Forward Characteristic
Modeling the Diode Forward Characteristic
Modeling the Diode Reverse characteristics
• The very steep i-v curve that the diode exhibits in
the breakdown region and the almost-constant
voltage drop that this indicates suggests that
diodes operating in the breakdown region can be
used in the design of voltage regulators.
• This in fact turns out to be an important
application of diodes operating in the reverse
breakdown region, and special diodes are
manufactured to operate specifically in the
breakdown region.
• Such diodes are called breakdown diodes or, more
commonly, zener diodes.
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Figure 15: The diode i–v characteristic with the breakdown
region shown in some detail.
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Specifying and Modeling the Zener Diode
• Figure 15 shows details of the diode i-v characteristics in the
breakdown region.
• We observe that for currents greater than the knee current IZK, i-v
characteristic is almost a straight line.
• The manufacturer usually specifies the voltage across the zener
diode VZ at a specified test current, IZT.
• As the current through the zener deviates from IZT, the voltage
across it will change, though only slightly.
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Specifying and Modeling the Zener Diode
Figure 15 shows that corresponding to current change ΔI the
zener voltage changes by ΔV, which is related to ΔI by
ΔV = r2ΔI
Where r2 is the inverse of the slope of the almost-linear i-v curve
at point Q.
Resistance r2 is the incremental resistance of the zener diode at
operating point Q.
It is also know as the dynamic resistance of the zener, and its
value is specified on the device data sheet.
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Specifying and Modeling the Zener Diode
The almost-linear i-v characteristic of the
zener diode suggests that the device can
be modeled as indicated in figure 16.
The equivalent circuit model of figure 22
can be analytically described by
VZ = VZ0 + rzIZ
and it applies for IZ > IZK and, obviously, VZ
> VZ0.
Figure 16: Model for the
zener diode.
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Zener Equivalent Circuits
Zener Diode : Voltage Regulator (Shunt Regulator)
• A zener diode can be used as a voltage regulator to
provide a constant voltage from a source whose
voltage may vary over a sufficient range
Zener Diode : Voltage Regulator (Shunt Regulator)
• Zener diode of zener voltage VZ is connected in
reverse across load RL
• Constant output across RL is needed
• Series resistance R absorbs output voltage
fluctuations to maintain a constant voltage across
load
• Zener diode maintains a constant voltage of VZ across
load as long as the input voltage does not fall below
VZ
Zener Diode : Voltage Regulator (Shunt Regulator)
• If input voltage increases, since zener is in
breakdown region output remains constant at VZ
• Excess voltage is dropped across series resistance R,
• Increase in total current I
• Zener will conduct increase in current and load
current remains constant
Zener Diode : Voltage Regulator (Shunt Regulator)
• If input voltage is constant and load resistance
decreases -> increase in load current
• Extra current cannot come from the source since the
drop in R will not change as zener is within regulating
range
• Additional load current will come from decreasing
current through the zener diode, IZ
Example
The 6.8 V zener diode in the circuit of figure 16 is specified to have VZ =
6.8 V at IZ = 5 mA, rz= 20 Ω, and IZk=0.2 mA. The supply voltage V+ is
nominally 10 V but can vary by ±1 V.
1.
2.
3.
4.
5.
6.
Find VO with no load and with V+ at its nominal value.
Find the change in VO resulting from the ±1 V change in V+. Note that
(ΔVO/ ΔV+), usually expressed in mV/V, is know as line regulation.
Find the change in VO resulting from connecting a load resistance RL that
draws a current IL= 1 mA, and hence find the load regulation (ΔVO/ Δ IL)
in mV/mA.
Find the change in VO when RL= 2 kΩ.
Find the change in VO when RL= 0.5 kΩ.
What is the minimum value of RL for which the diode still operates in the
breakdown region?
Figure 16: (a) Circuit for Example 13. (b) The circuit with the zener diode
replaced with its equivalent circuit model.
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Diode Applications
1. Rectifier Circuits (Half wave Rectifier and Full wave
Rectifier circuits)
2. Clippers
3. Clampers
4. Voltage Multipliers circuits (voltage Doublers,
Voltage Triplers and Quadruples)
Clippers
• There are a variety of diode networks called clippers that have
the ability to “clip” off a portion of the input signal without
distorting the remaining part of the alternating waveform.
• The half-wave rectifier is an example of the simplest form of
diode clipper—one resistor and diode.
• There are two general categories of clippers: series and
parallel
Series Clippers
Parallel Clippers
Clampers
• The clamping network is one that will “clamp” a signal to a
different dc level.
• The network must have a capacitor, a diode, and a resistive
element.
• it can also employ an independent dc supply to introduce an
additional shift.
• The magnitude of R and C must be chosen such that the time
constant τ = RC is large enough to ensure that the voltage across
the capacitor does not discharge significantly during the interval
the diode is non conducting.
Clamper
During the interval 0 → T/2 the network will appear as shown in
Figure below.
During the interval T/2 → T the network will appear as shown in
Figure below.
Applying Kirchhoff’s voltage law around the input loop will
result
Voltage Multipliers Circuits
• Voltage multiplier is a specialized circuit producing an output
which is theoretically an integer times the AC peak input, for
example, 2, 3, or 4 times the AC peak input.
Half-wave voltage Doublers
• During the positive voltage half-cycle across the transformer,
secondary diode D1 conducts (and diode D2 iscut off), charging
capacitor C1 up to the peak rectified voltage (Vm).
Half-wave voltage Doublers
• During the negative half-cycle of the secondary voltage, diode D1
is cut off and diode D2 conducts charging capacitor C2. Since
diode D2 acts as a short during the negative half-cycle we can
sum the voltages around the outside loop
• On the next positive half-cycle, diode D2 is nonconducting and
capacitor C2 will discharge through the load.
Half-wave voltage Doublers
• The voltage across capacitor C2 drops during the positive halfcycle (at the input) and the capacitor is recharged up to 2Vm
during the negative half-cycle.
• The output waveform across capacitor C2 is that of a half-wave
signal filtered by a capacitor filter.
Half-wave voltage Doublers
full-wave voltage Doublers