Chapter five diode circuit applications · The three basic types of rectifier circuits are: 1. The Half Wave Rectifier 2. The Full Wave Rectifier 3. The Bridge Rectifier Half-wave

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Chapter five diode circuit applications

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Power supply: a group of circuits that convert the standard ac voltage (120 V, 60 Hz)

provided by the wall outlet to constant dc voltage

Transformer : a device that step up or step down the ac voltage provided by the wall

outlet to a desired amplitude through the action of a magnetic field

Rectifier: a diode circuits that converts the ac input voltage to a pulsating dc voltage

The pulsating dc voltage is only suitable to be used as a battery charger, but not good

enough to be used as a dc power supply in a radio, stereo system, computer and so

on.

There are two basic types of rectifier circuits:

Half-wave rectifier

Full-wave rectifier - Center-tapped & Bridge full-wave rectifier

In summary, a full-wave rectified signal has less ripple than a half-wave rectified

signal and is thus better to apply to a filter.

Filter: a circuit used to reduce the fluctuation in the rectified output voltage or ripple.

This provides a steadier dc voltage.

Regulator: a circuit used to produces a constant dc output voltage by reducing the

ripple to negligible amount. One part of power supply.

Regulator - Zener diode regulator

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Diode Rectifier Circuits

One of the important applications of a semiconductor diode is in rectification of AC

signals to DC. Diodes are very commonly used for obtaining DC voltage supplies from

the readily available AC voltage. There are many possible ways to construct rectifier

circuits using diodes. The three basic types of rectifier circuits are:

1. The Half Wave Rectifier

2. The Full Wave Rectifier

3. The Bridge Rectifier

Half-wave Rectifier

The easiest rectifier to understand is the half wave rectifier. A simple half-wave rectifier

using an ideal diode and a load is shown in Figure

Circuit operation

Let‘s look at the operation of this single diode rectifier when connected across an

alternating voltage source vs. Since the diode only conducts when the anode is positive

with respect to the cathode, current will flow only during the positive half cycle of the

input voltage.

The supply voltage is given by:

is the angular frequency in rad/s.

We are interested in obtaining DC voltage across the ―load resistance‖ RL.During the

positive half cycle of the source, the ideal diode is forward biased and operates as a

closed switch. The source voltage is directly connected across the load. During the

Simple half-wave rectifier circuit

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negative half cycle, the diode is reverse biased and acts as an open switch. The source

voltage is disconnected from the load. As no current flows through the load, the load

voltage vo is zero. Both the load voltage and current are of one polarity and hence said

to be rectified. The waveforms for source voltage vS and output voltage vo are shown in

figure

We notice that the output voltage varies between the peak voltage Vm and zero in each

cycle. This variation is called ―ripple‖, and the corresponding voltage is called the

peak-to-peak ripple voltage, Vp-p.

Average load voltage and current

If a DC voltmeter is connected to measure the output voltage of the half-wave rectifier

(i.e., across the load resistance), the reading obtained would be the average load voltage

Vave, also called the DC output voltage. The meter averages out the pulses and displays

this average.

Source and output voltages

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The output voltage waveform and average voltage are shown in figure

The output vo may be viewed as a DC voltage plus a ripple voltage. As we can see, the

output has a large amount of ripple.

Average Load Current

Just as we can convert a peak voltage to average voltage, we can also convert a peak

current to an average current. The value of the average load current is the value that

would be measured by a DC ammeter.

where IL is the average current passing through the load resistance.

Peak Inverse Voltage

The maximum amount of reverse bias that a diode will be exposed to is called the peak

inverse voltage or PIV. For the half wave rectifier, the value of PIV is:

Output voltage and average voltage for half-wave rectifier

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The reasoning for the above equation is that when the diode is reverse biased, there is

no voltage across the load. Therefore, all of the secondary voltage (Vm) appears across

the diode. The PIV is important because it determines the minimum allowable value of

reverse voltage for any diode used in the circuit.

Example A load resistance is connected across a half wave rectifier. The input

supply voltage is 230V (rms) at 50 Hz. Determine the DC output (average) voltage,

peak-to-peak ripple in the output voltage (Vp-p), and the output ripple frequency (fr).

solution

Full-wave Rectifier

The full-wave rectifier can be classified into two distinct types.

(i) Centre-tapped transformer full-wave rectifier:-

The full wave rectifier consists of two diodes and a resister as shown in Figure The

transformer has a centre-tapped secondary winding. This secondary winding has a lead

attached to the centre of the winding. The voltage from the centre tap to either end

terminal on this winding is equal to one half of the total voltage measured end-to-end.

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Circuit Operation

Figure shows the operation during the positive half cycle of the full wave rectifier. Note

that diode D1 is forward biased and diode D2 is reverse biased. Note the direction of the

current through the load.

During the negative half cycle, (figure) the polarity reverses. Diode D2 is forward

biased and diode D1 is reverse biased. Note that the direction of current through the load

has not changed even though the secondary voltage has changed polarity. Thus another

positive half cycle is produced across the load.

figure 5 Full-wave rectifier- Circuit operation during positive half cycle

Full-wave rectifier – circuit operation during negative half cycle

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Calculating Load Voltage and Currents

Using the ideal diode model, the peak load voltage for the full wave rectifier is mV .

The full wave rectifier produces twice as many output pulses as the half wave rectifier.

This is the same as saying that the full wave rectifier has twice the output frequency of a

half wave rectifier. For this reason, the average load voltage (i.e.DC output voltage) is

found as

Figure below illustrates the average dc voltage for a full wave rectifier.


When one of the diodes in a full-wave rectifier is reverse biased, the peak voltage

across that diode will be approximately equal to Vm. This point is illustrated in figure .

With the polarities shown, D1 is conducting and D2 is reverse biased. Thus the cathode

of D1 will be at Vm. Since this point is connected directly to the cathode of D2, its

cathode will also be Vm. With –Vm applied to the anode of D2, the total voltage across

the diode D2 is 2Vm. Therefore, the maximum reverse voltage across either diode will

be twice the peak load voltage.

Example

In the full-wave rectifier circuit of figure 5 , the transformer has a turns ratio of 1:2.

The transformer primary winding is connected across an AC source of 230V (rms), 50

Hz. The load resistor is . For this circuit, determine the DC output voltage, peak-

to-peak ripple in the output voltage, and output ripple frequency.

Solution

The rms value of secondary voltage = 460 V

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RMS value of v2 (and v3) = 230 V

Peak value of v2 (and v3): √

DC Output voltage (i.e. average load voltage):

The peak-to-peak ripple voltage can be calculated as:

Ripple frequency = 100 Hz, which is twice the AC supply frequency of 50 Hz.

ii) Bridge type full-wave rectifier:-

In many power supply circuits, the bridge rectifier (Figure ) is used. The bridge rectifier

produces almost double the output voltage as a full wave center-tapped transformer

rectifier using the same secondary voltage. The advantage of using this circuit is that no

center-tapped transformer is required.

Basic Circuit Operation

During the positive half cycle (Figure) , both D3 and D1 are forward biased. At the

same time, both D2 and D4 are reverse biased. Note the direction of current flow

through the load. During the negative half cycle (Figure ) D2 and D4 are forward biased

and D1 and D3 are reverse biased. Again note that current through the load is in the

same direction although the secondary winding polarity has reversed.

figure 6 Operation during positive half cycle

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In order to understand the Peak Inverse Voltage across each diode, look at figure 8. It is

a simplified version of figure 7 showing the circuit conditions during the positive half

cycle. The load and ground connections are removed because we are concerned with the

diode conditions only. In this circuit, diodes D1 and D3 are forward biased and act like

closed switches. They can be replaced with wires. Diodes D2 and D4 are reverse biased

and act like open switches.

The circuit of figure 8 is redrawn below. We can see that both diodes are reverse biased,

in parallel, and directly across the secondary winding. The peak inverse voltage is

therefore equal to Vm.

Therefore, Peak inverse voltage = Vm

Advantages of a bridge rectifier

(i) In the bridge circuit a transformer without a centre tap is used.

figure 7 Operation during negative half cycle

Figure 8: Equivalent bridge rectifier circuit during

positive half cycle

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(ii) The bridge circuit requires a smaller transformer as compared to a full-wave

rectifier giving the identical rectified dc output voltage.

(iii) For the same dc output voltage, the PIV rating of a diode in a bridge rectifier is half

of that for a full -wave circuit.

(iv) The bridge circuit is more appropriate for high-voltage applications, thus, making

the circuit compact.

Disadvantages of a bridge rectifier

(i) Two or more diodes are required in case of a bridge rectifier, as a full-wave rectifier

uses two diodes whereas a bridge rectifier uses four diodes.

(ii) The amount of power dissipated in a bridge circuit is higher as compared to a full-

wave rectifier. Hence, the bridge rectifier is not efficient as far as low voltages are

concerned.

Diode rectifier for power supply

The purpose of a power supply is to take electrical energy in one form and convert it

into another. There are many types of power supply. Most are designed to convert high

voltage AC mains electricity to a suitable low voltage supply for electronic circuits and

other devices such as computers, fax machines and telecommunication equipment.

A power supply can by broken down into a series of blocks, each of which performs a

particular function.

transformer first steps down high voltage AC to low voltage AC.

rectifier circuit is then used to convert AC to DC. This DC, however, contains ripples,

which can be smoothened by a filter circuit.

Power supplies can be ‗regulated‘ or ‗unregulated‘. A regulated power supply

maintains a constant DC output voltage through ‗feedback action‘. The output voltage

of an unregulated supply, on the other hand, will not remain constant. It will vary

depending on varying operating conditions, for example when the magnitude of input

AC voltage changes.

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Types of Diodes and Their Uses

1. PN Junction Diodes:

Are used to allow current to flow in one direction while blocking current flow in the

opposite direction. The pn junction diode is the typical diode that has been used in the

previous circuits.

2. Zener Diodes:

Are specifically designed to operate under reverse breakdown conditions. These diodes

have a very accurate and specific reverse breakdown voltage.

When a zener diode is forward biased it behaves like an ordinary silicon diode. When

the zener diode is reverse biased with a voltage less than VZ it blocks the current, like

an ordinary silicon diode.

Breakdown Mechanisms

Zener effect

Occurs in heavily doping semiconductor

Breakdown voltage is less than 5v.

Carriers generated by electric field---field ionization.

Temperature coefficient(TC) is negative.

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Avalanche effect.

Occurs in slightly doping semiconductor

Breakdown voltage is more than 7v.

Carriers generated by collision.

Temperature coefficient(TC) is positive.

Reverse Breakdown

When a large reverse bias voltage is applied, breakdown occurs and an enormous

current flows through the diode

Zener breakdown is a result of the large electric field inside the depletion region

that breaks electrons or holes off their covalent bonds.

Avalanche breakdown is a result of electrons or holes colliding with the fixed

ions inside the depletion region.

Zener breakdown is observed in highly doped p-n junctions and occurs for

voltages of about 5 V or less.

Avalanche breakdown is observed in less highly doped p-n junctions.

V-I Characteristics of Zener Diode

• Zener diodes are manufactured to have a very low reverse bias breakdown voltage

• Since the breakdown at the zener voltage is so sharp, these devices are often used in

voltage regulators to provide precise voltage references. The actual zener voltage is

device dependent.

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3. Schottky Diodes:

These diodes are designed to have a very fast switching time which makes them a great

diode for digital circuit applications. They are very common in computers because of

their ability to be switched on and off so quickly.

When a diode with a low forward voltage drop is required the Schottky diode may be

used. The Schottky diode turns on at about 0.2 Volts compared to 0.7 Volts for the Si

diode and it is characterized by a very fast switching times. The symbol, for the

Schottky diode is The fabrication process of the Schottky diode is different that of the

standard pn junction Si diode. The Schottky diode has a metallic layer in the place of

the p region.

Symbol of Schottky diode

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4. Shockley Diodes:

The Shockley diode is a four-layer diode while other diodes are normally made with

only two layers. These types of diodes are generally used to control the average power

delivered to a load.

Named after its inventor, a Shockley diode is a PNPN device having two terminals as

shown in Fig. (i). This device acts as a switch and consists of four alternate P-type and

N-type layers in a single crystal. The various layers are labelled as P1, N1, P2 and N2

for identification. Since a P-region adjacent to an N-region may be considered a

junction diode, the Shockley diode is equivalent to three junction diodes connected in

series as shown in Fig. (ii). The symbol of Shockley diode is shown in Fig. (iii).

Working

(i) When Shockley diode is forward biased (i.e., anode is positive w.r.t. cathode), diodes

D1and D3 would be forward-biased while diode D2 would be reverse-biased. Since

diode D2 offers very high resistance (being reverse biased) and the three diodes are in

series, the Shockley diode presents a very high resistance. As the *forward voltage

increases, the reverse bias across D2 is also increased. At some forward voltage (called

breakover voltage VBO), reverse breakdown of D2 occurs. Since this breakdown results

in reduced resistance, the Shockley diode presents a very low resistance. From now

onwards, the Shockley diode behaves as a conventional forward-biased diode; the

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forward current being determined by the applied voltage and external load resistance.

This behavior of Shockley diode is indicated on its V-I characteristic in Fig..

(ii) When Shockley diode is reverse biased (i.e., anode is negative w.r.t. cathode),

diodes D1 and D3 would be reverse-biased while diode D2 would be forward-biased. If

reverse voltage is increased sufficiently, the reverse voltage breakdown (point A in Fig.

of Shockley diode is reached. At this point, diodes D1 and D3 would go into reverse-

voltage breakdown, the reverse current flowing through them would rise rapidly and the

heat produced by this current flow could ruin the entire device. For this reason,

Shockley diode should never be operated with a reverse voltage sufficient to reach the

reverse-voltage breakdown point.

5. Light Emitting Diodes (LED)

Light-emitting diodes are designed with a very large bandgap so movement of carriers

across their depletion region emits photons of light energy. Lower bandgap LEDs

(Light-Emitting Diodes) emit infrared radiation, while LEDs with higher bandgap

energy emit visible light. Many stop lights are now starting to use LEDs because they

are extremely bright and last longer than regular bulbs for a relatively low cost.

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LEDs are pn junction diodes that emit light at various frequencies. The light could be

visible or infrared. The LED is active when it is forward biased. The i-v characteristics

of an LED diode are similar to that of a regular diode except that the forward voltage

may vary from 0.5 Volts to 2.5Volts depending on the type of semiconductor used.

The value of the current limiting resistor is determined by the forward voltage drop of

the diode, Vg, the maximum current through it, Imax, and the source voltage Vs. The

relationship is obtained by applying KVL and it is

Advantages of LED

The light-emitting diode (LED) is a solid-state light source. LEDs have replaced

incandescent lamps in many applications because they have the following advantages :

i. Low voltage

ii. Longer life (more than 20 years)

iii. Fast on-off switching

LED diode circuit.

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6. Photodiodes:

While LEDs emit light, Photodiodes are sensitive to received light. They are

constructed so their pn junction can be exposed to the outside through a clear window

or lens.

In Photoconductive mode the saturation current increases in proportion to the intensity

of the received light. This type of diode is used in CD players.

In Photovoltaic mode, when the pn junction is exposed to a certain wavelength of light,

the diode generates voltage and can be used as an energy source. This type of diode is

used in the production of solar power.

Photodiodes

When the pn junction of a diode is exposed to light of sufficiently high frequency the

energy of the photons causes the electrons to move, thereby creating electron-hole

pairs.The motion of these pairs results in a current through the diode. The symbol, for

the photodiode is Consider the circuit on Fig. where the photodiode is reverse biased.

When the light intensity is zero, the current that flows through he diode is the reverse –

saturation current which is typically very low. When the energy of the light increases,

electrons are separated and there is a current flowing in the reverse bias direction. The

output voltage Vo is proportional to the current Ip

Photodiode symbol

Photodiode circuit

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Photo-diode

A photo-diode is a reverse-biased silicon or germanium pn junction in which reverse

current increases when the junction is exposed to light. The reverse current in a photo-

diode is directly proportional to the intensity of light falling on its pn junction. This

means that greater the intensity of light falling on the pn junction of photo-diode, the

greater will be the reverse current.

Principle. When a rectifier diode is reverse biased, it has a very small reverse leakage

current. The same is true for a photo-diode. The reverse current is produced by

thermally generated electron-hole pairs which are swept across the junction by the

electric field created by the reverse voltage. In a rectifier diode, the reverse current

increases with temperature due to an increase in the number of electron-hole pairs. A

photo-diode differs from a rectifier diode in that when its pn junction is exposed to

light, the reverse current increases with the increase in light intensity and vice-versa.

This is explained as follows. When light (photons) falls on the pn junction, the energy

is imparted by the photons to the atoms in the junction. This will create more free

electrons (and more holes). These additional free electrons will increase the reverse

current. As the intensity of light incident on the pn junction increases, the reverse

current also increases. In other words, as the incident light intensity increases, the

resistance of the device (photo-diode) decreases.

zener diode voltage regulators

when the zener diode is operated in the breakdown or zener region, the voltage across it

is constant for a large change of current through it. This characteristic permits it to be

used as a voltage regulator

Fig. shows the circuit of a zener diode regulator

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Operation. The zener will maintain constant voltage across the load in spite of changes

in load current or input voltage. As the load current increases, the zener current

decreases so that current through resistance RS is constant. As Vout = Vin –IRS, and I is

constant, therefore, output voltage remains unchanged. The reverse would be true

should the load current decrease. The circuit will also correct for the changes in input

voltages. Should the input voltage Vin increase, more current will flow through the

zener, the voltage drop across RS will increase but load voltage would remain constant.

The reverse would be true should the input voltage decrease.

Limitations. A zener diode regulator has the following drawbacks :

(i) It has low efficiency for heavy load currents.

(ii) The output voltage slightly changes due to zener impedance as Vout = VZ + IZ RZ.

Changes in load current produce changes in zener current.

Conditions for Proper Operation of Zener Regulator

When a zener diode is connected in a circuit for voltage regulation, the following

conditions must be satisfied :

(i) The zener must operate in the breakdown region or regulating region i.e. between IZ

(max) and IZ (min). The current IZ (min) (generally 10 mA) is the minimum zener current to

put the zener diode in the ON state i.e. regulating region. The current IZ (max) is the

maximum zener current that zener diode can conduct without getting destroyed due to

excessive heat.

(ii) The zener should not be allowed to exceed maximum dissipation power otherwise it

will be destroyed due to excessive heat. If maximum power dissipation of a zener is PZ

(max) and zener voltage is VZ, then,

(iii) There is a minimum value of RL to ensure that zener diode will remain in the

regulating region i.e. breakdown region. If the value of RL falls below this minimum

value, the proper voltage will not be available across the zener to drive it into the

breakdown region.

Example Fig. shows the zener regulator. Calculate (i) current through the series

resistance (ii) minimum and maximum load currents and (iii) minimum and maximum

zener currents. Comment on the results.

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Solution.

Comments. The current IS through the series resistance RS is constant. When load

current increases from 0 to 60 mA, the zener current decreases from 75 mA to 15 mA,

maintaining IS constant in value. This is the normal operation of zener regulator i.e. IS

and Vout remain constant in spite of changes in load current or source voltage.

Example . A zener regulator has VZ = 15V. The input voltage may vary from 22 V to

40 V and load current from 20 mA to 100 mA. To hold load voltage constant under all

conditions, what should be the value of series resistance ?

Solution. In order that zener regulator may hold output voltage constant under all

operating conditions, it must operate in the breakdown region. In other words, there

must be zener current for all input voltages and load currents. The worst case occurs

when the input voltage is minimum and load current is maximum because then zener

current drops to a minimum

Example Determine the minimum acceptable value of RS for the zener voltage

regulator circuit shown in Fig. The zener specifications are :

VZ = 3.3V ; IZ (min) = 3 mA ; IZ (max) = 100 mA

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Solution. When load RL goes open (i.e. RL → ∞), the entire line current IS will flow

through the zener and the value of RS should be such to prevent line current IS from

exceeding IZ (max) if the load opens.

Example Determine the maximum allowable value of RS for the zener voltage

regulator circuit shown in Fig.

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4.7 V zener diode is used as a voltage regulator as shown in the diagram below. The

input voltage comes from a rectifier and has a ripple that ranges between 8.5 V and 9.3

V. The resistor R1 has a resistance of 100 Ω.

(a) Calculate the peak to peak value of the ripple voltage.

(b) What is the value of the voltage drop across the load for this input voltage signal?

(c) What happens to the magnitude of the current through resistor R1?

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The diode i-v characteristic with the breakdown region shown in some detail.

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clamping circuits and waveform generation

clipper and clamper circuits

Clippers

Clipping circuits (also known as limiters, amplitude selectors, or slicers), are used to

remove the part of a signal that is above or below some defined reference level. We‘ve

already seen an example of a clipper in the half-wave rectifier – that circuit basically cut

off everything at the reference level of zero and let only the positive-going (or negative-

going) portion of the input waveform through.

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A clipper is a type of diode network that has 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 a diode.

Depending on the orientation of the diode, the positive or negative region of the input

signal is ―clipped‖ off.

There are two general categories of clippers: series and parallel.

Series clipper:- A series clipper and its response for two types of alternating

waveforms are provided.

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Key points

1. The first step is to find out in which interval of the input signal the diode is in

forward-bias

2. The direction of the diode suggests that the signal vi must be positive to turn it

on. The dc supply further requires the voltage vi to be greater than v volts to turn the

diode on. The negative region of the input signal turns the diode into the OFF state.

Therefore, in the negative region the diode is an open circuit.

3. Determine the applied voltage (transition voltage) that will cause a change in state

for the diode. For the ideal diode the transition between states will occur at that point

on the characteristics where vd = 0 V and id = 0 A. Applying this condition, it is

recognized that the level of vi that will cause a transition in state is:

For an input voltage greater than V volts, the diode is in the short-circuit state, while

for input voltage less than V volts it is in the open-circuit or OFF state (as it is

reverse-biased).

4. Be continually aware of the defined terminals and polarity of vo. When the diode is in

the short-circuit state, the output voltage vo can be determined by applying KVL in the

clock-wise direction:

5. It can be helpful to sketch the input signal above the output and determine the

output at instantaneous values of the input. It is then possible to sketch the output

voltage from the resulting data points of vo.

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For Vm > V, the diode is in the short-circuit state and vo = Vm – V. At vi = V, the diode

changes state and vi = – Vm, vo = 0 V. The complete curve for vo can be sketched.

Parallel clipper:- Input vi is applied for the output vo. The analysis of parallel

configuration is very similar to the series configuration.

Parallel clipper

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clamping circuits and waveform generation

• Clamper is a network constructed of a diode, a resister and a capacitor that shifts a

waveform to a different level without changing the appearance of the applied signal.

• A clamper adds a dc voltage to the signal

• A positive clamper shifts its input waveform in a positive direction, so that it lies

above a dc reference voltage.

• A negative clamper shifts its input waveform in a negative direction, so that it lies

below a dc reference voltage.

Response of parallel clipper

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Working

• On the first negative half cycle of the input the diode is turned on.

• At negative peak capacitor is fully charged to Vp

Beyond negative peak

• Diode is off

• RLC is made much larger than the time period of the signal.

• Stiff Clamper RLC > 100T

• Due to this reason capacitor remains fully Charged During off time of diode

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Ideal Positive Clamper

Negative Clamper

Application

• Clampers are used in

• test equipment

• radar systems,

• electronic counter measure systems

• sonar systems.

• These are commonly used in analog television receivers to restore the DC component

of the video signal.

Documents

Chapter five diode circuit applications · The three basic types of rectifier circuits are: 1. The Half Wave Rectifier 2. The Full Wave Rectifier 3. The Bridge Rectifier Half-wave