2[1].DIODE APPLICATIONS_final.ppt

7/27/2019 2[1].DIODE APPLICATIONS_final.ppt

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DIODEAPPLICATIONS

Robert L. Boylestad


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RESISTANCE OF CRYSTAL

DIODE As the operating point of the diode moves from one region to

another the resistance of the diode will also change due to thenonlinear shape of the characteristic curve. The type of appliedvoltage of signal will define the resistance level of interest

Forward Resistance.

a. DC Forward Resistance/DC or Static Resistance

b. AC Forward Resistance/AC or Dynamic Resistance.

Reverse Resistance

Average AC Resistance


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DC Forward Resistance/DC or

Static Resistance The application of a dc voltage to a circuit containing a semiconductor

diode will result in an operating point on the characteristic curve that will notchange with time.

The resistance of the diode at the operating point can be found simply byfinding the corresponding levels of VD and ID

The dc resistance levels at the knee and below will be greater than theresistance levels obtained for the vertical rise section of the characteristics.

The resistance level at the reverse bias region will naturally be quite high.

The lower the current through the diode the higher the dc resistance level.


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Fig: dc resistance (static resistance)

of a diode at a particular operating

point


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AC Forward Resistance /Dynamic

Resistance.

The dc resistance is independent of the shape of the characteristic inthe region surrounding the point of interest.

If the sinusoidal input is applied, the operating point will move up anddown region of the characteristics with specific change of current andvoltage.

With no applied varying signal the Q (quiescent means stillunvarying) point will be fixed and decided by applied dc levels.

A straight line drawn tangent to the curve through the Q-point will

define a particular change in voltage and current that can be used todetermine the ac or dynamic resistance.

The lower the Q point of operation the higher the ac resistance.


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Fig: ac resistance (dynamic

resistance) of a diode at a particular

operating point


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LOAD-LINE ANALYSIS

The applied load will normally have an importantimpact on the point of region of operation of a

device.

A load line is used in graphic analysis ofcircuits,

The operating point is where the parameters of

the nonlinear device and the parameters of the

linear circuit match, according to how they are

connected while still adhering to their internalsystems

The point of operation is usually called the

quiescent point or Q-point
http://en.wikipedia.org/wiki/Circuithttp://en.wikipedia.org/wiki/Circuit


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Fig: Load Line Analysis


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LOAD-LINE ANALYSIS In the example on the right, the nonlinear

diode is placed in series with a linear circuitconsisting of a resistorand a voltage source.

The load line represents the relationship

between current and voltage in the linear part

of the circuit while the exponential represents

the relationship between current and voltage

in the nonlinear device. Since the current

going through three elements in seriesshould be the same, the operating point of

the circuit will be at the intersection of the

exponential with the load line
http://en.wikipedia.org/wiki/Resistorhttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Currenthttp://en.wikipedia.org/wiki/Currenthttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Resistor


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Fig: symbol of diode


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RECTIFIERRobert L. Boylestad


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RECTIFIER The diode rectifier converts the input

sinusoidal voltage, Vs to a uni-polar

output Vo. Types of rectifier circuits:

HALF WAVE RECTIFIER

FULL WAVE RECTIFIER

Rectifier is simply a device that rectifies or

removes or shifts part of an AC signal


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Why do we need it?

Biasing is always provided by dc signal

(i.e. biasing is independent of time). For

biasing of a circuit we need dc supply. Inthat case rectifier converts AC signal into

DC for biasing.


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HALF WAVE RECTIFIER

For Ideal diode

For (+ve) half cycle:

diode is on.

V0 =Vi (t)

For (-ve) half cycle:

diode is offV0=0


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HALF WAVE RECTIFIER


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HALF WAVE RECTIFIER

For Practical diode:

Vr=0.7 volt (for Si)/ 0.3 volt (Ge)

For (+ve ) half cycle

diode turns on when Vi=Vr/Vd

and upto this point V0=0. For (-ve ) half cycle

diode is off i.e. V0 =0.


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HALF-WAVE RECTIFICATION

During the positive half cycle of the

input voltage the polarity of thevoltage across the secondaryforward biases the diode. As aresult a current IL flows through theload resistor, RL. The forward

biased diode offers a very lowresistance and hence the voltagedrop across it is very small. Thusthe voltage appearing across theload is practically the same as theinput voltage at every instant


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HALF-WAVE RECTIFICATION

During the negative half cycle of the inputvoltage the polarity of the secondaryvoltage gets reversed. As a result, the

diode is reverse biased. Practically nocurrent flows through the circuit andalmost no voltage is developed across theresistor. All input voltage appears across

the diode itself. The process of removing one half the input

signal to establish a dc level is called half

wave rectification


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Disadvantages

of half wave rectifierThe ac supply delivers power

only half the time and theoutput is low.


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EFFICIENCY OF HALF WAVE

RECTIFICATION


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Efficiency, h is the ratio of

the dc output power to acinput power

Thus


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FULL-WAVE RECTIFICATION

A Full Wave Rectifier is a circuit, whichconverts an ac voltage into a pulsating dc

voltage using both half cycles of the

applied ac voltage

Full wave rectifier:

a. Centre-tapped Full-wave Rectifier b. Bridge Rectifier


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CENTRE TAP FULL WAVE

RECTIFIER


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RECTIFIERDuring the positive half cycle of

the input voltage, diode D1

becomes forward biased and D2becomes reverse biased. HenceD1 conducts and D2 remains OFF.

The load current flows through D1and the voltage drop across RLwill be equal to the input voltage.


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RECTIFIER During the negative half cycle of the input

voltage, diode D1 becomes reverse biased

and D2 becomes forward biased. HenceD1 remains OFF and D2 conducts.

The load current flows through D2 and thevoltage drop across RL will be equal to the

input voltage


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FULL-WAVE BRIDGE RECTIFIER A bridge circuit is a type ofelectrical circuit in

which the current in a conductorsplits into twoparallel paths and then recombines into a singleconductor, thereby enclosing a loop

Advantages:

a. The need for centre-tapped transformer is eliminated

b. The output is twice that of the centre tap circuit forthe same secondary voltage.

c. PIV is one-half that of the centre tapped circuit.

Disadvantages:

a. It requires four diodes

b. Two diodes conducts during each half cycle-power/voltage drop is twice than centre tap. This

objectionable when secondary voltage is small.
http://en.wikipedia.org/wiki/Electrical_circuithttp://en.wikipedia.org/wiki/Current_(electricity)http://en.wikipedia.org/wiki/Conductor_(material)http://en.wikipedia.org/wiki/Conductor_(material)http://en.wikipedia.org/wiki/Current_(electricity)http://en.wikipedia.org/wiki/Electrical_circuit


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EFFICIENCY OF FULL WAVERECTIFIER


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Efficiency, h is the ratio ofthe dc output power to ac

input power


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RIPPLE FACTOR

The ratio of r.m.s. value of ac component

to the dc component in the rectifier output

is known as Ripple factor.

rms value of ac componentRipple Factor=

Value of dc component

ac

dc

I

I


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ZENER DIODE

When the reverse bias on a crystal diode is increased, a

critical voltage called breakdown voltage is reachedwhere the reverse current increases sharply to a highvalue is the knee of the reverse characteristics.

This breakdown voltage is called Zener voltage and thesharp increase in current is called the Zener current.

Zener voltage depends upon amount of doping. Forheavily doped diode the depletion layer shall be thin and

the Zener voltage will be low and for lightly doped diodethe voltage is high.

Hence, a properly doped crystal diode which has a sharpbreakdown voltage is known as a Zener diode.


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A zener diode is like an ordinary diode except it

is properly doped so as to have sharpbreakdown voltage.

A zener diode is always reverseconnected/reverse biased.

Has a sharp breakdown voltage called zenervoltage Vz

When forward biased, its characteristics its justlike an ordinary diode.

A zener diode is not immediately burnt in thebreakdown region as long as the current is lessthan the burnt out value.

Points to remember


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Equivalent circuit of a zener diode

a. On state

b. Off state

Zener diode as a voltage stabilizer

ZENER DIODE


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Fig: zener diode

2[1].DIODE APPLICATIONS_final.ppt

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