Chapter 2 - Diode Circuits [Clippers]

7/26/2019 Chapter 2 - Diode Circuits [Clippers]

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CHAPTER 2DIODE

CIRCUITS

BY AZRUL GHAZALI

CONTENT

1. RECTIFIERS

Half-wave Rectifier

Full-wave Rectifier

Bridge Rectifier

Rectifiers with Capacitor Filter

2. VOLTAGE REGULATORS3. CLIPPERS & CLAMPERS

Series-based Diode Clippers

Parallel-based Diode Clippers

Clamping Circuits


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REVISITING ZENER DIODE Zener diode consists of a silicon PN junction. When forward biased, it behaves like a

normal diode. When reverse biased, as soon as its reverse voltage exceeds the

breakdown voltage (VZ), current starts to flow through the diode. In this region, thevoltage across diode remains nearly constant while the current varies (small internalresistance).

Important!!!!Zener diode operates in reversebiased region.

ZENER DIODE MODEL & DIODE APPLICATIONS

In most circuit analysis, thesmall internal resistance, rZcan be neglected, ie, assumerZ= 0 .

Zener diode applications:

(1) Voltage regulation

(2) Voltage overload

protection


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VOLTAGE REGULATOR

(max)(min)

(min)(min)(max)(max)

(max)1.09.0 SZS

ZSLZSL

ZVVV

VVIVVII

LZ

ZSS

II

VVR

VOLTAGE REGULATORFIGURE OF MERITS Source Regulation

Measure of change in output voltage with a change in sourcevoltage

Load regulation

Measure of change in output voltage with a change in loadcurrent

%100xVs

VregulationSource O

%100)(

)()(x

v

vvregulationLoad

loadfullO

loadfullOloadnoO

As source regulation and load regulation factors approach zero,the circuit reaches ideality.


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Example 4

The voltage regulator is to power up a car radio at VL= 9V from an

automobile battery whose voltage may vary from 11V to 13.6V. Thecurrent in the radio will vary between 0 (off) to 100mA (full volume). A 9VZener diode is used, and assumeIZ(min)= 0.1IZ(max). Find the series resistance,Riand minimum power of the diode,PZ.

Example 5

From example 4, assume non-ideal Zener diode with Zener resistance, rZ = 2. Determine the source regulation and load regulation of the circuit.


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CHOOSING THE CORRECT DIODE AND RESISTOR

FOR A KNOWN LOADBEING PRACTICAL

Let say, as an example, the input is unregulated 12V supply with 0.5V variation, and youneed a stable regulated output of 8V to power a 100mA device (load).

STEP 1: Choose the right Zener diode.

There are many Zener diodes in the market with differentstandard Zener Voltages (2.4V, 3.3V, 6.8V, 7.5V, 8.2V,9.1V, 10V, etc) and with different diode power rating(500mW, 1.3W, etc). In this example, 8.2V Zener diode ischosen since it is close enough to the target voltage of8V.

STEP 2: Estimate maximum current in the circuit, IS

We know that IS= IL+ IZ. The load current is100mA. Youcan assume IZto be 10% of IL. This will give a maximumcurrent of 110mA for IS.

CHOOSING THE CORRECT DIODE AND RESISTORFOR A KNOWN LOADBEING PRACTICAL

STEP 3: Select the Power Rating of Zener diode

We can use the maximum current in the circuit to calculate the power lost in the diode.P = 110mA x 8.2V = 0.9W. Therefore, 1.3W power rated Zener diode should be sufficient tobe used in the circuit.

STEP 2: Select Resistor RS

The maximum voltage drop across the resistor, VRs= 12.58 = 4.5V, and therefore,according to Ohms law, RS= VRs/IS= 4.5V/110mA = 40.9. So, a 43resistor is selected.

STEP 2: Select the Power Rating of Resistor RS

The power rating in the resistor, PRs= VRsx IS= 4.5V x 110mA = 0.495W. Choosing 0.5W 43resistor is too risky, especially if supply voltage fluctuates even higher. Therefore, to be safe,a 1W 43resistor is selected for RS.


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CLIPPER & CLAMPING CIRCUITS

?

Circuit clippers or limiter circuits eliminates portion of a signal thatare above or below a specified level.

Positive Clipping

(Single Limiter)

Negative Clipping

(Single Limiter)

Parallel Clipping(Double Limiter)

Biased Clipping

CLIPPER & CLAMPING CIRCUITS

?

Clamping circuits shifts the entire signal voltage by a dc level, ie,the signal is clamped to a different dc level.

Positive Clamping

Negative Clamping


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SERIES AND SHUNT CLIPPER CIRCUITS

Clippers may be classified into two types, based on the

positioning of the diode.

Series Clippers, where diode is in series with RL

Shunt (Parallel) Clippers, where diode is shunted acrossRL

BASIC SHUNT CLIPPER CIRCUIT

Diode D1 is OFF when VS< VB+ V. No current flow throughdiode. Hence, Vout= VS.

Diode DI is ON when VS> VB+ V.Current flow through diode.Voltage across diode is V. Hence, Vout= V+ VB


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CLIPPER CIRCUITPARALLEL CLIPPING

During positive cycle of input, D1 is ON while D2 is OFF. Output

voltage is limited by V+ VB1

During negative cycle of input, D2 is ON while D1 is OFF. Output

voltage is limited by V+ VB2in the negative (reverse bias)direction.

MODIFIED CLIPPER CIRCUIT - BIASED CLIPPING

Diode is OFF when VBVS< V, orsimply VS> VB- V. No current flowthrough diode. Hence, Vout= VS

Diode is ON when VBVS> V, or

simply VS< VB+ V. Current flowsthrough diode. Voltage acrossdiode is V. Hence, Vout= V- VB


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VOLTAGE TRANSFER FUNCTION

CLIPPER CIRCUIT USING ZENER DIODE

Single Zener Diode Clipping

During +ve cycle of input, Zenerdiode is in reverse bias. When VS> VZ, output across diode is fixedor limited at VZ. In other words,Vout= VZwhen VS> VZin the +vecycle.

Duringve cyle of input, Zenerdiode is in forward bias. Diodewill turned on when VS> V.Output across diode is then fixedor limited at V. In other words,Vout= V when VS> Vin thevecycle.

Can you explain theoperation of double Zenerdiode clipper circuit?


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Example 6

Find the steady-state output of the clipper circuit below.

CLAMPING CIRCUIT (DC LEVEL RESTORER)

Capacitor in clamping circuit provides the DC offset from the storedcharge.

The capacitor forms the circuit time constant with the load resistor,which determines the range of frequencies which clamper will beeffective. In general, RC time constant must be made 10 timeslarger than the period of the input signal, ie, Tin> 10RC.

Assume capacitor isinitially uncharged.

Important!!!Output swing of output (VO,pp) mustequal to input waveform (VI,pp) .

Vpp

Vpp


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OPERATION OF CLAMPING CIRCUITIDEALDIODE

During +ve cycle of input, diode is ON. Assuming RC is very small,capacitor charges to peak voltage of input voltage very quickly.Voltage across the capacitor, vC= Vm.

Duringve cycle of input, diode is OFF. From KVL, output voltage,vout= vinvC, where vCis the dc offset which equals to Vm.

Vm

+ -

Positive Cycle of Input

KVL: -vin+ vc+vout= 0 where vc= Vm

OPERATION OF CLAMPING CIRCUIT WITH BIASSUPPLY VOLTAGE

The operation is similar to the unbiased clamping circuit.

During +ve cycle of input, diode is ON. Assuming RC is very small,capacitor charges to peak voltage of input voltage very quickly.From KVL, voltage across the capacitor, vC= Vm- VBias.

Duringve cycle of input, diode is OFF. From KVL, output voltage,vout= vinvC, where vCis the dc offset which equals to Vm.

Vm- Vbias+ -

Positive Cycle of InputKVL: -vin+ vc+ vout= 0, where vc= Vm- Vbias


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Example 7

Find the steady-state output of the diode clamper circuit shownbelow.

Example 8

Find the steady-state output of the diode clamper circuit shownbelow.

Chapter 2 - Diode Circuits [Clippers]

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