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Diode Applications
• Introduction to diode circuits
– DC and AC diode circuits
• Diode applications
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Clippers
Clampers
Limiters
Peak rectifiers
Voltage multipliers
Voltage regulators (with Zener diodes)
Rectifiers
• Properties of electrical signals
– Average value (DC)
– Effective value (RMS)
• Half-wave rectifier
• Full-wave rectifier
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Diode Approximations
In Forward Bias:
Silicon Diode:
VD(ON) = 0.7V
Germanium Diode: VD(ON) = 0.3V
In Reverse Bias:
Both diodes act like opens VD = source voltage and ID = 0A
Diode in DC Series Circuit: Forward Bias
The diode is forward biased.
• VD = 0.7V
• VR = E – VD
• ID = IR = VR /R
(or VD = E if E < 0.7V)
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Diode in DC Series Circuit: Reverse Bias
The diode is reverse biased.
• VD = E
• VR = 0V
• ID = IR = 0A
Diode in any DC Circuit
Solve this circuit like any Series/Parallel circuit,
knowing VD = 0.7V (or up to 0.7V) in forward bias
and as an open in reverse bias.
VD1 = VD2 = VD(ON) = 0.7V
VR = 9.3V
Diodes are used in parallel to limit current:
IR = E – VD = 10V - 0.7V = 28mA
R
0.33k
ID1 = ID2 = 28mA/2 = 14mA
What will happen if D2 is replaced by a Germanium diode?
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Clipper Diode Circuits
Positive Half Cycle (using ideal diode model)
Negative Half Cycle (using ideal diode model)
The diode only conducts when it is in forward bias, therefore only half of the AC
cycle passes through the diode.
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Let us also plot the voltage transfer characteristics (VTC), i.e., vo vs. vi for the
clipper circuit above.
We already found that
Thus, diodes “clip” a portion of the AC wave.
The diode “clips” any voltage that does not put it in forward bias. That
would be a reverse biasing polarity and a voltage less than 0.7V for a
silicon diode.
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Variations of the Clipper Circuit
By adding a DC source to the circuit, the voltage required to forward bias the diode can be
changed.
Changing Output Perspective
By taking the output across the diode, the output equals to the input voltage when the
diode is not conducting.
A DC source can also be added to change the diode’s required forward bias voltage.
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Clipper Examples
Series clipper circuits
A parallel clipper circuit and its output
Series Clipper Circuits Summary
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Parallel Clipper Circuits Summary
Clampers (DC Restorers)
A diode in conjunction with a capacitor can be used to “clamp” an AC signal to a
specific DC level.
 Shifts the signal in the direction of the diode arrow by (Vp-V0)
 Discharge constant: 5 
>> T / 2 (T is the period of the input signal and capacitor
is discharging fully in five time constants,  = R C )
• e.g.  = R C > 5T
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Operation in a clamper circuit
Input signal
Diode “on” and
the capacitor
charging to V volts
Determine vo with the
diode “off.”
Output signal
Operation in a clamper circuit -2
A clamper circuit and its output
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Variations of Clamper Circuits
The input signal can be any type of waveform: sine, square, triangle wave, etc.
You can adjust the DC camping level with a DC source.
Variations of Clamper Circuits
A clamper circuit and its output
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Summary of Clamper Circuits
Diode Limiter
(Dual-Diode Parallel Clipper)
Input
Output
Output vs Input
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Peak Rectifier
Input
When a load RL is connected, the discharge
constant  = RLC should have a very high
value compared to the half period of the
signal, otherwise ripples are observed at
the output voltage, i.e. RL should have a
very large value.
Output
Voltage Multiplier Circuits
Voltage multiplier circuits use a combination of diodes and
capacitors to step up the output voltage of rectifier circuits.
• Voltage Doubler
• Voltage Tripler
• Voltage Quadrupler
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Voltage Doubler
This half-wave voltage doubler’s output can be calculated as
Vout = VC2 = 2Vm
Vm = peak secondary voltage of the transformer.
Operation of a Voltage Doubler Circuit
The 1st capacitor charges up to Vm during the positive half of the cycle,
then the 2nd capacitor charges up to Vm in the same polarity as the 1st capacitor,
finally the output is the sum of the voltages across both capacitors:
Vout = 2Vm
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Voltage Tripler and Quadrupler Circuits
By adding more diode-capacitor networks the voltage can be increased.
Zener Diode
The Zener is a diode operated in reverse bias at the Zener Voltage (Vz).
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Zener Calculations
Determine the state of the Zener:
if Vi  VZ, then the Zener is biased “on” ; the Zener is at VZ
if Vi < VZ, then the diode is biases “off” ; VZ = Vi
For Vi  VZ:
The Zener voltage
The Zener current IZ = IR - IL
The Zener Power
PZ = VZ IZ
For Vi < VZ:
The Zener acts like an open.
Load Resistance in a Zener Circuit
The size of the load resistor affects the current in the Zener.
• RL is too large
Not enough current through the Zener and it is biased “off”.
The minimum current for a Zener is given as IZK in the data sheets.
• RL is too small
Too much current in the Zener and it avalanches and is quickly destroyed.
The maximum current for a Zener is given as IZM in the data sheets.
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Finally, we arrive at the following equations
where
where
Exercise (Midterm 1, 2004-2005)
•
In the figure, Vs is an unregulated voltage that varies between
6V and 7V while the Zener diode voltage is VZ = 5V. The load
resistor RL can have a value from 100  to  (i.e. open circuit).
Also you can take IZmin  0 A.
a)
b)
c)
Find the maximum value of R1 so that the load voltage VL would be
still kept constant at 5V for all values of RL and Vs.
Provide a symbolic expression for the maximum power dissipated
by the Zener diode.
Determine the minimum value of R1 so that the power dissipated by
the Zener diode does never exceeds 1W for all values of RL and Vs.
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Other Zener Regulators
•
A single Zener diode can limit one side of a sinusoidal waveform to the zener
voltage while clamping the other side to near zero. With two opposing zeners, the
waveform can be limited to the zener voltage on both polarities.
Zener limiter circuits
Zener Diode Parameters
•
The basic parameters of a zener diode are:
a)
Obviously, the zener voltage must be specified. The most common range of
zener voltage is 3.3 volts to 75 volts, however voltages out of this range are
available.
b)
A tolerance of the specified voltage must be stated. While the most popular
tolerances are 5% and 10%, more precision tolerances as low as 0.05 % are
available . A test current (Iz) must be specified with the voltage and
tolerance.
c)
The power handling capability must be specified for the zener diode.
Popular power ranges are: 1/4, 1/2, 1 , 5, 10, and 50 Watts.
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