Diode Applications and Types

Load Line Analysis• The load line plots all
possible combinations ofdiode current (ID) andvoltage (VD) for a givencircuit. The maximum ID
equals E/R, and themaximum VD equals E.
• The point where the loadline and the characteristic
curve intersect is the Qpoint, which identifies ID
and VD for a particular
diode in a given circuit.

Generic Values
Constants• Silicon Diode: VD = 0.7 V• Germanium Diode: VD = 0.3 V• GaAs Diode: VD= 1.2V.

Series Diode Configurations
Forward BiasAnalysis (for silicon)• VD = 0.7 V (or VD = Eif E < 0.7 V)• VR = E – VD
• ID = IR = IT = VR / R

Reverse Biased

Examples: Diode Circuits• Determine Vo and ID for the series circuit for
red= 1.8V

Example:• Determine Vo,VD2 and ID for the series circuit:Example:• Determine I1,I2 and ID2 for the following circuit:Parallel Configurations

Parallel configuration (Cont.`)V  0.7 V
DV  V  V  0.7 V
D1 D2 OV  9.3 VR
E  V 10 V  .7 VD
I  28 mARR.33kΩ
ID1

ID2



28 mA
2

 14 mA

Diodes as Gates• Diodes can be used as gates• When placed in forward and reverse biased
mode• Commonly used gates :– OR gate
– AND gate

OR GateAND GateHalf-Wave Rectification•

The diode only conducts when it is forward biased, therefore only
half of the AC cycle passes through the diode to the output.

The DC output voltage is 0.318Vm, where Vm = the peak AC voltage.

PIV (PRV)•Because the diode is only forward biased for one-half of the AC cycle, it isalso reverse biased for one-half cycle.

•It is important that the reverse breakdown voltage rating of the diode behigh enough to withstand the peak, reverse-biasing AC voltage.
PIV (or PRV) > Vm•••

PIV = Peak inverse voltage
PRV = Peak reverse voltage
Vm = Peak AC voltage

Full-Wave Rectification•

The rectification process can be
improved by using a full-wave
rectifier circuit.

•Full-wave rectification produces agreater DC output:••

Half-wave: Vdc = 0.318Vm
Full-wave: Vdc = 0.636Vm

Full-Wave Rectification (cont.)

Bridge Rectifier••

Four diodes are connected in a
bridge configuration
VDC = 0.636Vm

Center-Tapped Transformer
RectifierCenter-Tapped Transformer RectifierRequires• Two diodes• Center-tapped transformer
VDC = 0.636Vm

Summary of Rectifier Circuits
RectifierIdeal VDC

Realistic VDCHalf Wave Rectifier

VDC = 0.318Vm

VDC = 0.318Vm – 0.7

Bridge Rectifier

VDC = 0.636Vm

VDC = 0.636Vm – 2(0.7 V)

Center-Tapped Transformer
Rectifier

VDC = 0.636Vm

VDC = 0.636Vm – 0.7 V

Vm = peak of the AC voltage.
In the center tapped transformer rectifier circuit, the peak AC voltage
is the transformer secondary voltage to the tap.

Zener DiodeZener Equivalent circuit.
• Zener as regulator•Regulator is combination of elements to ensure output voltage is fairly constant.

OUTPUT CHARECTERISTICS OF
ZENER DIODEZENERBREAKDOWNFORWARD
BIAS

I
REVERSE
BIAS

V

ZENER DIODE WORK AS A VOLTAGE REGULATORIN REVERSE BIASZENER DIODE CHARACTERISTICS
 In the reverse bias region, a reverse leakage current flows
until the breakdown voltage is reached.
 At this point, the reverse current, called zener current IZ ,increases sharply.

VBRIZZENER DIODE CHARACTERISTICS(cont.)
 In the forward region the zener diode acts like a regular silicon
diode, with a 0.7 volt drop when it conducts.

IFFORWARD BIAS
REGION

0.7VVF The width of depletion region is very less at highimpurity concentration resulting in high electric field
intensity at depletion region sufficient to break thecovalent bonds and free the electrons.
 A large number of covalent bonds break resulting information of a large number of electron –hole pairs
and sudden increase in the reverse current.
 This is known as zener effect.

Zener Diodes
The Zener is a diode operated
in reverse bias at the Zener
Voltage (Vz).
• When Vi  VZ
– The Zener is ON– Voltage across the Zener is VZ– Zener current: IZ = IR – IRL– The Zener Power: PZ = VZIZ
• When Vi < VZ
– The Zener is OFF– The Zener acts as an open circuit

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Analysis Procedure: Zener Circuits• Fixed Vi& R fixed• Fixed Vi& R variable• Variable Vi& R fixedFixed Vi & R fixed• The simplest of Zener diode networks appears in
the following figure• The analysis is broken down to 2 steps• An application of the voltage divider rule will result in• If V ≥VZ, the Zener diode is “ON” and the equivalentmodel of ON will be substituted.
• If VZ ≥ V, the diode is “off” and the open-circuit
equivalence will be substituted.
`• The Zener diode current must be determined by
an application of Kirchhoff’s current law. That is,ExampleFixed Vi and Variable RL

Example

Example (cont.)Variable Vi & Fixed RL

Example

Diode Clippers•Clippers can be of any input waveform•Sinusoidal•Square•Saw tooth•Types•Series•ParallelSeries Clippers
The diode in a series clipper “clips”
any voltage that does not forward
bias it:• A reverse-biasing polarity• A forward-biasing polarity less than
0.7 V (for a silicon diode)42

Biased Clippers
Adding a DC source inseries with the clipping
diode changes theeffective forward bias ofthe diode.

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Parallel Clippers

The diode in a parallel clipper
circuit “clips” any voltage thatforward bias it.
DC biasing can be added inseries with the diode to change
the clipping level.

44

Example

Example- solution

Summary of Clipper Circuits

more…

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Summary of Clipper Circuits

48Clampers

A diode and capacitor can be
combined to “clamp” an AC
signal to a specific DC level.
Can you people differentiate
between clipper and clamper
circuits?

•clipper removes the unwanted
portion of the input signal
whereas• the clamper moves the input
signal upwards or downwards.

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Do you people knowwhat is Time constant ?

Why does capacitor charge
instantaneously? Comparewith 0-T/2 transition

Now write
KVL

Can you determine the time
constant in this case? Is itlarge as compared to T/2 – Ttransition?

Assumption:
Capacitor holds onto all its
chargeWrite KVLClampers types• Negative clamper circuits• Positive clamper circuits• Biased clamper circuits

Negative Clamper Circuits• Output is shifted towards negative sidePositive Clamper Circuits• Output is shifted towards positive sideBiased Clamper Circuits
The input signal can be any typeof waveform such as sine, square,and triangle waves.
The DC source lets you adjust
the DC clamping level.

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Example

Example (cont)Example (cont) if diode is Si• On state• Off stateSummary of Clamper Circuits60• The basic purpose of a power supply is to provide one or more
fixed voltages to the working circuit, with sufficient currenthandling capacity to maintain the operating conditions of thecircuit. These can be of two types:• UNREGULATED: The output of such supply changes due to :a) change in the loadb) change in the source voltage• REGULATED: The output of such supply does not change due to:a) change in the loadb) change in the source voltage
REGULATED POWER SUPPLY IS REUIRED FOR CRITICAL ELECTRONICAPPLICATIONS.

 A voltage regulator circuit automatically maintains the output
voltage of a power supply constant, regardless ofi) a change in the loadii) a change in the source voltage(V)STransformerand rectifierFilterRegulator+VDC-LOAD

 The zener diode is typically connected reverse
biased, in parallel with the load.
 Resistor Rs limits current to zener.
IRSIL+IZ+vDC (IN) DZ
VLRL-Voltage-Multiplier Circuits
Voltage multiplier circuits use a combination of diodes andcapacitors to step up the output voltage of rectifier circuits.
•••

Voltage Doubler
Voltage Tripler
Voltage Quadrupler

64Voltage Doubler

This half-wave voltage doubler’s output can be calculated by:
Vout = VC2 = 2Vm
where Vm = peak secondary voltage of the transformer

65Voltage Doubler•

Positive Half-Cycleo D1 conductso D2 is switched off
o Capacitor C1 charges to Vm•

Negative Half-Cycleo D1 is switched off
o D2 conductso Capacitor C2 charges to Vm

Vout = VC2 = 2Vm

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Voltage Tripler and Quadrupler

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Practical Applications•

Rectifier Circuits– Conversions of AC to DC for DC operated circuits– Battery Charging Circuits•

Simple Diode Circuits– Protective Circuits against– Overcurrent– Polarity Reversal– Currents caused by an inductive kick in a relay circuit•

Zener Circuits– Overvoltage Protection– Setting Reference Voltages

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