Diode Applications

Dr. Upanith S. Liyanaarachchi

Diode Clipping circuits
Diode Clipper
• 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.
• Diode Clipping Circuits can be used a variety of applications to modify
an input waveform using signal and Schottky diodes or to provide
over-voltage protection using zener diodes to ensure that the output
voltage never exceeds a certain level protecting the circuit from high
voltage spikes.
• Also, it can be used in voltage limiting applications.
 Positive Diode Clipping Circuits

• During the positive half cycle, diode become forward biased.

• It must have the input voltage magnitude greater than +0.7 volts
• 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.
Positive Diode Clipping Circuits

• During the negative half cycle, the diode is reverse biased.

• Block 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

• 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.
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.
Positive Biased Diode Clipping Circuits

• 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 V, then the sinusoidal
voltage at the diode’s anode terminal must be greater than, 4.0 + 0.7
= 4.7 V for it to become forward biased. Any anode voltage levels
above this bias point are clipped off.
Negative 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.
Diode Clipping of Different Bias levels

• 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.
In Summary
• As well as being used as rectifiers, diodes can also be used to clip the
top, or bottom, or both of a waveform at a particular dc level and pass
it to the output without distortion.
• The most common application of a “diode clipping” is as a flywheel or
free-wheeling diode connected in parallel across an inductive load to
protect the switching transistor form reverse voltage transients.
• 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.
Diode Clamping circuits
Diode clamping circuits
• A clamp circuit adds the positive or negative dc component to the
input signal so as to push it either on the positive side or negative
side.
• Basically, there are two types of diode clampers known as,
I. Positive clamper
II. Negative clamper
Positive clamper

• The circuit will be called a positive clamper, when the signal is pushed
upward by the circuit.
• When the signal moves upward, the negative peak of the signal
coincides with the zero level.
Positive clamper cont…

• Initially when the input is given, the capacitor is not yet charged and
the diode is reverse biased. The output is not considered at this point
of time.
• During the negative half cycle, at the peak value, the capacitor gets
charged with negative on one plate and positive on the other. The
capacitor is now charged to its peak value Vm
• The diode is forward biased and conducts heavily.
Positive clamper cont…

• During the next positive half cycle, the capacitor is charged to +Vm
while the diode gets reverse biased and gets open circuited. The
output of the circuit at this moment will be, 𝑉0 = 𝑉𝑖 + 𝑉𝑚
• Hence the signal is positively clamped as shown in the above figure.
The output signal changes according to the changes in the input, but
shifts the level according to the charge on the capacitor, as it adds the
input voltage.
Positive Clamper with Positive Voltage (Vr)
• A Positive clamper circuit if biased with some positive reference
voltage (Vr), that voltage will be added to the output to raise the
clamped level.
• Using this, the circuit of the positive clamper with positive reference
voltage is constructed as below.
Positive Clamper with Positive Voltage (Vr) cont…
• During the positive half cycle, the reference voltage is applied through
the diode at the output and as the input voltage increases, the
cathode voltage of the diode increase with respect to the anode
voltage and hence it stops conducting.
• During the negative half cycle, the diode gets forward biased and
starts conducting. The voltage across the capacitor and the reference
voltage together maintain the output voltage level.
Negative Clamper

• The circuit will be called a negative clamper, when the signal is

pushed downward by the circuit.
• When the signal is pushed on the negative side, the positive peak of
the input signal coincides with the zero level.
Diode Rectifier circuits
 Use to obtain “steady and smooth” DC
Rectifiers voltage

Half Wave rectifier Full Wave Rectifier

Using Center-tap Transformer

Using Diode Bridge

Half Wave Rectifier
• Simplest of all the rectifier
circuits is the Half Wave
Rectifier.
• The power diode in a half
wave rectifier circuit
passes just one half of
each complete sine wave
of the AC supply in order
to convert it into a DC
supply.
DC output voltage

𝑉𝑚𝑎𝑥
𝑉𝐷𝐶 = = 0.318 𝑉𝑚𝑎𝑥
𝜋

Where,
VDC – DC output voltage

Vmax – Maximum input voltage

Full Wave Rectifier
• Full wave rectifier circuit produces an output voltage or current which
is purely DC or has some specified DC component.

• Advantages over Half wave rectifier

• average (DC) output voltage is higher than for half wave
• much less ripple than that of the half wave rectifier producing a smoother
output waveform
Full Wave Rectifier
Using center-tap transformer
• two diodes are used, one for each half
of the cycle
• multiple winding transformer is used
whose secondary winding is split
equally into two halves with a
common center tapped connection (C)
• producing an output during both half-
cycles, twice that for the half wave
rectifier so it is 100% efficient
Full Wave Rectifier
Using diode bridge
• This type of single phase rectifier uses
four individual rectifying diodes
connected in a closed loop “bridge”
configuration to produce the desired
output
• The main advantage of this bridge
circuit is that it does not require a
special centre tapped transformer,
thereby reducing its size and cost
Full Wave Rectifier
The positive half-cycle
• During the positive half cycle of the
supply, diodes D1 and D2 conduct in
series while diodes D3 and D4 are
reverse biased and the current flows
through the load as shown.
Full Wave Rectifier
The negative half-cycle
• During the negative half cycle of the
supply, diodes D3 and D4 conduct in
series, but diodes D1 and D2 switch
“OFF” as they are now reverse biased.
• The current flowing through the load
is the same direction as before.
Full-wave Rectifier with Smoothing Capacitor

• The smoothing capacitor

converts the full-wave rippled
output of the rectifier into a
more smooth DC output
voltage.
DC output voltage

2𝑉𝑚𝑎𝑥
𝑉𝐷𝐶 = = 0.637 𝑉𝑚𝑎𝑥 = 0.9 𝑉𝑅𝑀𝑆
𝜋

Where,
VDC – DC output voltage
Vmax – Maximum input voltage
VRMS – Root Mean Square voltage
Done…