chapter five diode circuit applications · the three basic types of rectifier circuits are: 1. the...
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Chapter five diode circuit applications
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Chapter five diode circuit applications
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.
Peak Inverse Voltage
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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Peak Inverse Voltage
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.