DIODE CLIPPING CIRCUITS

 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.
 The Diode Clipper, also known as a Diode Limiter, is a wave shaping circuit
that takes an input waveform and clips or cuts off its top half, bottom half or
both halves together.
 This clipping of the input signal produces an output waveform that resembles a
flattened version of the input. For example, the half-wave rectifier is a clipper
circuit, since all voltages below zero are eliminated.

Positive Diode Clipping Circuits

 In this diode clipping circuit, the diode is forward biased (anode more positive
than cathode) during the positive half cycle of the sinusoidal input waveform.
For the diode to become forward biased, it must have the input voltage
magnitude greater than +0.7 volts (0.3 volts for a germanium diode).
 When this happens the diodes begins to conduct and holds the voltage across
itself constant at 0.7V until the sinusoidal waveform falls below this value.
Thus the output voltage which is taken across the diode can never exceed 0.7
volts during the positive half cycle.
 During the negative half cycle, the diode is reverse biased (cathode more
positive than anode) blocking current flow through itself and as a result has no
effect on the negative half of the sinusoidal voltage which passes to the load
unaltered. Thus the diode limits the positive half of the input waveform and is
known as a positive clipper circuit.
Negative Diode Clipping Circuits

Here the reverse is true. The diode is forward biased during the negative half cycle of
the sinusoidal waveform and limits or clips it to –0.7 volts while allowing the positive
half cycle to pass unaltered when reverse biased. As the diode limits the negative half
cycle of the input voltage it is therefore called a negative clipper circuit.

Clipping of Both Half Cycles

 If we connected two diodes in inverse parallel as shown, then both the positive
and negative half cycles would be clipped as diode D1 clips the positive half
cycle of the sinusoidal input waveform while diode D2 clips the negative half
cycle. Then diode clipping circuits can be used to clip the positive half cycle,
the negative half cycle or both.
 For ideal diodes the output waveform above would be zero. However, due to
the forward bias voltage drop across the diodes the actual clipping point occurs
at +0.7 volts and –0.7 volts respectively. But we can increase this ±0.7V
threshold to any value we want up to the maximum value, (VPEAK) of the
sinusoidal waveform either by connecting together more diodes in series
creating multiples of 0.7 volts, or by adding a voltage bias to the diodes.

Biased Diode Clipping Circuits

 To produce diode clipping circuits for voltage waveforms at different levels, a
bias voltage, VBIAS is added in series with the diode to produce a combination
clipper as shown.
 The voltage across the series combination must be greater
than VBIAS + 0.7V before the diode becomes sufficiently forward biased to
conduct. For example, if the VBIAS level is set at 4.0 volts, then the sinusoidal
 voltage at the diode’s anode terminal must be greater than 4.0 + 0.7 = 4.7 volts for
it to become forward biased. Any anode voltage levels above this bias point are
clipped off.

Positive Bias Diode Clipping

 Likewise, by reversing the diode and the battery bias voltage, when a diode
conducts the negative half cycle of the output waveform is held to a level –VBIAS –
0.7V as shown.

Negative Bias Diode Clipping

 A variable diode clipping or diode limiting level can be achieved by varying the
bias voltage of the diodes. If both the positive and the negative half cycles are to
be clipped, then two biased clipping diodes are used.
 But for both positive and negative diode clipping, the bias voltage need not be the
same. The positive bias voltage could be at one level, for example 4 volts, and the
negative bias voltage at another, for example 6 volts as shown.

Diode Clipping of Different Bias levels

 When the voltage of the positive half cycle reaches +4.7 V, diode D1 conducts and
limits the waveform at +4.7 V. Diode D2 does not conduct until the voltage
 reaches –6.7 V. Therefore, all positive voltages above +4.7 V and negative
voltages below –6.7 V are automatically clipped.
 The advantage of biased diode clipping circuits is that it prevents the output signal
from exceeding preset voltage limits for both half cycles of the input waveform,
which could be an input from a noisy sensor or the positive and negative supply
rails of a power supply.
 If the diode clipping levels are set too low or the input waveform is too great then
the elimination of both waveform peaks could end up with a square-wave shaped
waveform.
Application:

Clippers

 Used for the generation and shaping of waveforms

 Used for the protection of circuits from spikes
 Used for amplitude restorers
 Used as voltage limiters
 Used in television circuits
 Used in FM transmitters
 Sinusoidal to square-wave conversion.

CLAMPER CIRCUITS
Definition
 A clamper is an electronic circuit that changes the DC level of a signal to the
desired level without changing the shape of the applied signal. In other words, the
clamper circuit moves the whole signal up or down to set either the positive peak
or negative peak of the signal at the desired level.
 The dc component is simply added to the input signal or subtracted from the input
signal. A clamper circuit adds the positive dc component to the input signal to
push it to the positive side. Similarly, a clamper circuit adds the negative dc
component to the input signal to push it to the negative side.
 If the circuit pushes the signal upwards then the circuit is said to be a positive
clamper. When the signal is pushed upwards, the negative peak of the signal
meets the zero level.

 On the other hand, if the circuit pushes the signal downwards then the circuit is
said to be a negative clamper.
 When the signal is pushed downwards, the positive peak of the signal meets the
zero level.
 The construction of the clamper circuit is almost similar to the clipper circuit. The
only difference is the clamper circuit contains an extra element called capacitor.
 A capacitor is used to provide a dc offset (dc level) from the stored charge.
Types of clampers
Clamper circuits are of three types:
 Positive clampers
 Negative clampers
 Biased clampers
Positive clamper
 The positive clamper is made up of a voltage source Vi, capacitor C, diode D,
and load resistor RL.
 In the below circuit diagram, the diode is connected in parallel with the output
load. So the positive clamper passes the input signal to the output load when
the diode is reverse biased and blocks the input signal when the diode
is forward biased.

 The peak to peak amplitude the input signal is 2Vm, similarly the peak to peak
amplitude output signal is 2Vm. Therefore, the total swing of the output is same
as the total swing of the input.
 The basic difference between the clipper and clamper is that the clipper removes
the unwanted portion of the input signal whereas the clamper moves the input
signal upward or downwards
During positive half cycle:

 During the positive half cycle of the input AC signal, the diode is forward biased
and hence no signal appears at the output. In forward biased condition, the diode
allows electric current through it. This current will flows to the capacitor and
charges it to the peak value of input voltage in inverse polarity -Vm.

 As input current or voltage decreases after attaining its maximum value Vm, the
capacitor holds the charge until the diode remains forward biased.

During negative half cycle:

 During the negative half cycle of the input AC signal, the diode is reverse biased
and hence the signal appears at the output. In reverse biased condition, the diode
does not allow electric current through it. So the input current directly flows
towards the output.
 When the negative half cycle begins, the diode is in the non-conducting state and
the charge stored in the capacitor is discharged (released). Therefore, the voltage
appeared at the output is equal to the sum of the voltage stored in the capacitor (-
Vm) and the input voltage (-Vm) {I.e. Vo = -Vm- Vm = -2Vm} which have the same
polarity with each other. As a result, the signal shifted downwards.
Biased clampers

 Sometimes an additional shift of DC level is needed. In such cases, biased

clampers are used. The working principle of the biased clampers is almost similar
to the unbiased clampers. The only difference is an extra element called DC
battery is introduced in biased clampers.

Positive clamper with positive bias

 If positive biasing is applied to the clamper then it is said to be a positive clamper
with positive bias. The positive clamper with positive bias is made up of an AC
voltage source, capacitor, diode, resistor, and dc battery. During positive half
cycle:

During positive half cycle:

 During the positive half cycle, the battery voltage forward biases the diode when
the input supply voltage is less than the battery voltage. This current or voltage
will flows to the capacitor and charges it.
 When the input supply voltage becomes greater than the battery voltage then the
diode stops allowing electric current through it because the diode becomes reverse
biased.
During negative half cycle:

 During the negative half cycle, the diode is forward biased by both input supply
voltage and battery voltage. So the diode allows electric current. This current will
flows to the capacitor and charges it.

Positive clamper with negative bias

During negative half cycle:

 During the negative half cycle, the battery voltage reverse biases the diode when
the input supply voltage is less than the battery voltage. As a result, the signal
appears at the output.
 When the input supply voltage becomes greater than the battery voltage, the diode
is forward biased by the input supply voltage and hence allows electric current
through it. This current will flows to the capacitor and charges it.

During positive half cycle:

 During the positive half cycle, the diode is reverse biased by both input supply
voltage and the battery voltage.
 As a result, the signal appears at the output. The signal appeared at the output is
equal to the sum of the input voltage and capacitor voltage.

Negative clamper with positive bias

During positive half cycle:

 During the positive half cycle, the battery voltage reverse biases the diode when
the input supply voltage is less than the battery voltage.
 When the input supply voltage becomes greater than the battery voltage, the diode
is forward biased by the input supply voltage and hence allows electric current
through it. This current will flows to the capacitor and charges it.

During negative half cycle:

 During the negative half cycle, the diode is reverse biased by both input supply
voltage and battery voltage. As a result, the signal appears at the output.

Negative clamper with negative bias

During positive half cycle:

 During the positive half cycle, the diode is forward biased by both input supply
voltage and battery voltage. As a result, current flows through the capacitor and
charges it.
During negative half cycle:

 During the negative half cycle, the battery voltage forward biases the diode when
the input supply voltage is less than the battery voltage.
 When the input supply voltage becomes greater than the battery voltage, the diode
is reverse biased by the input supply voltage and hence signal appears at the
output.

Application

Clampers

 Used as direct current restorers

 Used to remove distortions
 Used as voltage multipliers
 Used for the protection of amplifiers
 Used as test equipment
 Used as base-line stabilizer