Pertemuan6 Diode Aplc

7/29/2019 Pertemuan6 Diode Aplc

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Clippers & Clampers

Zener diode application


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Clippers

Clippers or diode limiting is a diode network that have the ability to

clip off a portion on the i/p signal without distorting the remaining part

of the alternating waveform.

Clippers are used to eliminate amplitude noise or to fabricate new

waveforms from an existing signal.

2 general of clippers:

a) Series clippers

b) Parallel clippers

Series Clippers

The series configuration is defined as one where the diode is in series

with the load.

A half-wave rectifier is the simplest form of diode clipper-one resistorand diode.

+

-

ViR

L Vo

-

+

2


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The diode clips any voltage that does not put it in forward

bias. That would be a reverse biasing polarity and a voltage less

than 0.7V for a silicon diode.

Clipper Diode Circuit

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VmVi

t0 T/2 T

+

-Vi

V

RL Vo

-

+

The half-wave rectifier with addition of dc supply is shown infollowing figure. The cct known as biased series clipper. The dcsupply have pronounced effect on the o/p of a clipper.

Our initial discussion will be limited to ideal diode.

Biased series clipper

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+ve region turn the diode ON.

-ve region turn the diode OFF.

Vi > V to turn ON the diode

In general diode is open cct (OFF state) and short cct (ON state)

For Vi > V the Vo = ViV

For Vi = V the Vo= 0 V

The complete cct shown above

Vm

Vi

t0 T/2 T

+

-

Vi=Vm

V

RL Vo

-

+

+ -

T/20

Vo

tT

Vm-V

Vi=V (diodes change state)


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Determine the o/p waveform for the network below:

Solution:

Example: Variations of the Clipper Circuit

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More Example:

Repeat previous example for the square-wave i/p.

+

-

Vi

V=5 V

RL Vo

-

+

+-Vi

t0 T/2 T

-10

20

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Parallel Clippers The diode connection is in parallel configuration

with the o/p. Diode is ideal

+

RL

V

-

+

-

Vi

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By taking the output across the diode, the output is now thevoltage when the diode is not conducting.

A DC source can also be added to change the diodes required

forward bias voltage.

Changing Output Perspective

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Example :

Determine the Vo and sketch the o/p waveform for

the below network

+

V=4 V

RL

Vo

-

+

-

Vi

10

t0 T/2 T

Vi

16

-16


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V=VoON4

Vi=VoOFF5

Vi=VoOFF6

Vi=VoOFF7

Vi=VoOFF16

V=VoON3

V=VoON2

V=VoON1

VoDiode stateVi

+

V=4 V

RL

Vo

-

+

-

Vi

+

V=4 V

RL

Vo

-

+

-

Vi

t0 T/2

Vo

16

4

Solution:

+ ve region

t0 T/2 T

Vi

16

-16

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Solution (continued):

- ve region

+V=4 V

VRL=0A

Vo

-

+-Vi id=0A

V=VoON-4

V=VoON-5

V=VoON-6

V=VoON-7

V=VoON-16

V=VoON-3

V=VoON-2

V=VoON-1

VoDiode stateVi

t0 T/2

Vo

16

4

T

t0 T/2 T

Vi

16

-16

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Example :

Repeat the previous example using a silicon diode with VD=0.7 V

+

V=4 V

RL

Vo

-

+

-Vi

VD=0.7 V

id=0A

V3.3

7.04

VVVi

0VVVi

D

D

Solution:

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Solution (continued):

For i/p voltages greater than 3.3 V the diode open cct and Vo=Vi.

For i/p voltages less than 3.3 V the diode short cct and the network result

as/;

+V=4 V

RL

Vo

-

+

-

Vi

VD=0.7 V

id=0AV3.3

7.04Vo

t0 T/2

Vo16

3.3

T

The resulting o/p waveform

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15

Clipper Circuits Summary


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Clipper Circuits Summary

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Clampers

The clamping network is to clamp a signal to a different dc level. Also

known as dc restorers. The clamping cct is often used in TV receivers as adc restorer.

The network consists of:

a) Capacitor

b) Diode

c) Resistive elementd) Independent dc supply (option)

The magnitude of R and C must be chosen such that the time constant

= RC is large enough to ensure that the voltage across the capacitor does

not discharge significantly during the interval the diode is nonconducting.

Our analysis basis that all capacitor is fully charge and discharge in 5

time constant.

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18

+

R Vo

-

+

-

Vi

Vi

t0 T/2 T

V

-V


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19

Operation of clamper+ ve region

+

RVo

-

+

-

Vi

C 0 - T/2: Diode is ON state (short-cct

equivalent)

Assume RC time is small and capacitorcharge to V volts very quickly

Vo=0 V (ideal diode)

- ve region T/2 T: Diode is OFF state (open-cctequivalent)

Both for the stored voltage across

capacitor and applied signal current

through cathode to anode KVL: - V- V- Vo = 0 and

Vo = -2V

+

R Vo

-

+

-

V

C

Vo

-

+V

Vi

t0 T/2 T

V

-V


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20

Tips : Clamping networkTotal swing o/p signal = the total swing i /p signal


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21

Example: Determine Vo for the network below:

+

R=100 kohmVo

-

+

-

Vi

C= 1 uF

V= 5 V

Vi

-20

10

T

0 t1 t2 t3 t4 t

f=1000 Hz

S l ti


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22

Solution:

Step 1:Consider the part of i/p signal that will forward bias the

diode. From network (t1 -t2:-ve region)+

R=100 kohm Vo

-

+-

20 V

Vc

5 V

Step 2:During ON state assume capacitor will charge to a voltage level

determined by the network. Find the store voltage capacitor & obtained

Vo

KVL: -20 +Vc5 = 0

Vc = 25v Vo = 5

Vi

-20

10

T

0 t1 t2 t3 t4t

f=1000 Hz

S l ti ( td)


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23

Solution (cntd):

Step 3:During OFF state assume capacitor will hold on its established

voltage level. From network (t2 -t3:+ve region)

+

R=100 kohm Vo

-

+-

10 V

C

Vc

5 V

KVL

Step 4:Obtained Vo

ms500ms)5(1005istimedischargetotalThe

ms100k)(1u)(100RC:by

determinedisgdischarginofconstantTime

V35Vo0Vo2510

0VoVc10:KVL

Vi

-20

10

T

0 t1 t2 t3 t4t

f=1000 Hz

Solution (cntd):


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Step 5:Checking!!! total swing o/p signal = total swing i/p signal

From network (t2 -t3: +ve region)Vo

35

0 t1 t2 t3 t4t

5

Solution (cntd):

Vi

-20

10

T

0 t1 t2 t3 t4t

f=1000 Hz

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Example:Repeat the previous

example using agermanium diode !!!

25

Summary of Clamper Circuits


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Summary of Clamper Circuits

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Zener Diodes

The zener diode is a special type of diodes that is designed to

work in the reverse breakdown region. But it also can operate in the forward bias region.

Zener diode is a main component to design voltage regulator

circuit for DC power supply.

Zener Diodes Characteristic

The I-V characteristics of a diode in Fig 3.19 shows that the

breakdown voltage of a diode is nearly constant over a wide-

range of reverse-bias currents.

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Operation region:

Forward bias-operate same as

normal diode

Reverse bias-small current flow

Breakdown-big current flow. This is

the region where the voltage zener is

constant

For normal diode breakdown voltage is

capable to destroy the diode but with

zener diode the current is limited by

connecting series resistor.

0

I

V

Forward

ReverseBreakdown

-Vz0.7

Fig. 3.19

28

Zener Diode


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The Zener is a diode operated in reverse bias at the Zener

Voltage (Vz).

Zener Diode

29


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Vi and R fixed Fixed DC voltage is applied in network

below, as is the load resistor.

The analysis can be determined with 2

steps.

30

Step 1:


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p

Determine the state of zener diode by removing it from

the network and calculating the voltage across the

resulting open cct.

L

L

L

RR

ViRVV

LR

R

RR

RVi

V:OFFdiodeZenerVzV

Vz-ViV:ONdiodeZenerVzV

Vz

Iz+

- Pzm

RL

Vi

Fig 3.20: Basic zener regulato

V+

-

RL

Vi

Fig 3.21: Determining the state othe zener diode

RV

L

+

-

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Step 2:


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Subtitute the appropriate equivalent cct and solve for the

desired unknowns.

The ON state will obtained the equivalent cct in

Fig below

LZR III:KCL LRZ III

Vi

R

VIdan

R

VIwhere

R

R

L

L

L

devicefor thespecifiedPPzwhich

VzIzPzzenerbydissipatedPower

ZM

VZ

+

-

RLVi

Fig 3.22

R

VL

+

-

IR

PZM

IZ

IL

32

E l


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33

Example:

a) Determine VL, VR, IZ and PZ in the network below.

b) Repeat part (a) with RL=3 k

VZ=10 V+

-

RL

Vi R=1 k ohmVL

+

-PZM=30 mW

IZ

1.2 kohm

VR+ -

Solution:


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So ut o :

Step 1: Remove zener diode & obtained the zener state

W0VzIzPz

0AIz

7.27V8.73-16V-ViV

8.73VVV

cct)(openstateOFFisdiodeVzV

V73.8k2.1k1

16k2.1

RR

ViRV

LR

L

L

L

V

+

-

RL

Vi=16 V

R=1 k ohm

VL

+

-

IZ

1.2 kohm

IR

0

I

V

Forward

ReverseBreakdown

-Vz0.7

Fig. 3.19

8.73

10

Resulting operating point

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Repeat part (a) with RL=3 k

35

Fixed Vi Variable R


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Fixed Vi, Variable RL For an offset Vz a specific range of resistor values need

to be choose to ensure zener diode is ON state.

Too small a load resistance will cause VL < Vz - diode is

OFF state.

Thus the minimum load resistance in previous example

need to be calculate. This can be expressed by the

equation below:

VzVi

RVzRminL

RL > RLmin zener diode is ON state diode canreplaced by Vz source equivalent

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With R changes the I also changes The table below


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With RL changes the IL also changes. The table belowdescribed relationship between RL ,IL,IR

minL

maxL

R

VzI

maxL

minL

R

VzI

zminLmaxR III

zmaxLminR III

Notes: Izmax = Izm

37

Example:


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p

a) For the network below, determine the range of RL and IL

that will result in VRL being maintained at 10 V.

b) Determine the maximum wattage rating of the diode.

RLVi=50 V

R=1 k ohm IZ

IR

+

-

Vz=10 VIzm = 32 mA

IL

38

Solution:(a)


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k1.25R250:rangeRThe

k1.25m8

10

I

Vz

R

mA8m32-m40I-II

mA40k1

40RVI

25040

k10

1050

10k1

Vz-Vi

RVzR

LL

LminLmax

RLmin

RR

Lmin

zmax

Solution: (b)

320m3210VzIP zmaxzmax

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Fixed RL Variable Vi


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Fixed RL, Variable Vi For a fixed values of RL in the network below, Vi must

be sufficiently LARGE to turn zener diode ON. The minimum turn-ON voltage Vi=Vimin is expressed

by :

Vz

Iz+

- Pzm

RL

Vi

R

L

L

L

LL

R

VzRRVimin

RR

ViRVzV

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The maximum value of Vi is limited by the maximumzener current, Izmax, thus

LIII zmaxRmax

RImaxVi maxR

IL is fixed at Vz/RL and Izmax is the maximum value sothe maximum value of Vi is expressed below:

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Example:


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Determine the range of values of Vi that will maintain the

zener diode of network below:

RL=1.2k ohmVi

R=220 ohm IZ

IR

+

-

Vz=20 V

Izm = 60 mA

IL

VL

+

-

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Solution:


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V36.87ViV23.67:ViofrangeThe

V87.3620220m67.76VzRIVimax

mA76.6716.67m60mIIzmaxI

mA16.671200

20

R

Vz

R

VI

V23.671200

202201200

R

VzRRVimin

maxR

LRMax

LL

LL

L

L

RL=1.2k ohmVi

R=220 ohm IZ

IR

+

-Vz=20 V

Izm = 60 mA

IL

VL

+

-

Equivalent cct in the ON state

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Practical Applications of Diode Circuits


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pp

Rectifier CircuitsConversions of AC to DC for DC operated circuits

Battery Charging Circuits

Simple Diode Circuits

Protective Circuits against OvercurrentPolarity Reversal Currents caused by an inductive kick

in a relay circuit

Zener Circuits

Overvoltage Protection

Setting Reference Voltages

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References:

1. Thomas L. Floyd, Electronic Devices, Eighth Edition, Prentice Hall, 2002.

2. Robert Boylestad, Electronic Devices and Circuit Theory, Seventh edition,Prentice Hall, 2002.

3. Puspa Inayat Khalid, Rubita Sudirman, Siti Hawa Ruslan,ModulPengajaran Elektronik 1, UTM, 2002.

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Pertemuan6 Diode Aplc

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