SLIDES FOR PN JUNCTION DIODE APPLICATIONS

1. CHAPTER 2:
DIODE APPLICATIONS
MODULE-1
BASIC ELECTRONICS

2. Load Line
The fact that this current and the defined direction of conduction of the diode are a
“match” reveals that the diode is in the “on” state and conduction has been
established. The resulting polarity across the diode will be the forward-bias region.
Applying Kirchhoff’s voltage law to the series circuit - E=VD+ID*R
ID=E/R when VD=0V VD=E when ID=0A

3. Load Line
The Load Line plots all possible current (ID) conditions for all voltages applied to the
diode (VD) in a given circuit. E/R is the maximum ID and E is the maximum VD.
Where the Load Line and the Characteristic Curve intersect is the Q point, which specifies
a particular ID and VD for a given circuit.

4. Diode Approximations
In Forward Bias:
Silicon Diode: VD = .7V
Germanium Diode: VD = .3V
In Reverse Bias:
Both diodes act like opens VD = source voltage and ID =0A

5. DIODE IN DC SERIES CIRCUIT:
FORWARD BIAS
The diode is forward biased.
• VD = .7V (or VD = E if E < .7V) [Formula 2.4]
•VR = E – VD
• ID = IR = IT = VR /R
[Formula 2.5]
[Formula 2.6]
If a diode is in the “on” state, one can
either place a 0.7-V drop across the
element, or the network can be redrawn
with the VT equivalent circuit

6. DIODE IN DC SERIES CIRCUIT:
FORWARD BIAS – EQUIVALENT CIRCUIT

7. DIODE IN DC SERIES CIRCUIT: REVERSE
BIAS
The diode is reverse biased.
• VD = E
• VR = 0V
• ID = IR = IT = 0A
Mentally replacing the
diode with a resistive
element will reveal
that the resulting
current direction does
not match the arrow
in the diode symbol.
The diode is in the
“off” state. Due to the
open circuit, the diode
current is 0 A

8. DIODE IN DC SERIES CIRCUIT: REVERSE
BIAS – EQUIVALENT CIRCUIT

9. Diode in any DC Circuit
Solve this circuit like any Series/Parallel circuit,
knowing VD = .7V (or up to .7V) in forward bias
and as an open in reverse bias.
VD1 = VD2 = Vo = .7V
VR = 9.3V
Diodes in parallel are used to limit current:
IR = E – VD
R
= 10V - .7V = 28mA
.33k
ID1 = ID2 = 28mA/2 = 14mA

10. Diodes in AC Circuits
The diode only conducts when it is in forward bias,
therefore only half of the AC cycle passes through the diode.

11. Diodes convert AC to DC in a process called rectification.
The diode only conducts for one-half of the AC cycle. The remaining half is either all
positive or all negative. This is a crude AC to DC conversion.
The DC Voltage out of the diode :
VDC = 0.318Vm
where Vm = the peak voltage
[Formula 2.7]
Half Wave Rectifier

12. PIV (PRV)
Because the diode is only forward biased for one-half of the AC
cycle, it is then also off for one-half of the AC cycle. It is important
that the reverse breakdown voltage rating of the diode be high
enough to withstand the peak AC voltage.
PIV (PRV) > Vm [Formula 2.9]
PIV = Peak Inverse Voltage; PRV = Peak Reverse Voltage
Vm = Peak AC Voltage

13. FULL WAVE RECTIFICATION
The rectification process can be improved by using more diodes in a Full Wave
Rectifier circuit.
Full Wave rectification produces a greater DC output.

14. Full Wave Rectifier Circuits
There are two Full Wave Rectifier circuits:
• Bridge Rectifier
• Center –Taped Transformer Rectifier

15. BRIDGE RECTIFIER
CIRCUIT
Four diodes are required.
VDC = 0.636 Vm [Formula 2.10]

16. OPERATION OF THE BRIDGE RECTIFIER CIRCUIT
For the positive half of the AC cycle:
For the negative half of the AC cycle:

17. Center–Tapped Transformer Rectifier Circuit
Two diodes and a center-tapped transformer are required.
VDC = 0.636(Vm)
Note that Vm here is the transformer secondary voltage to the tap.

18. OPERATION OF THE CENTER–TAPPED TRANSFORMER
RECTIFIER CIRCUIT
For the positive half of the AC cycle:
For the negative half of the AC cycle:

19. Note: Vm = peak of the AC voltage. Be careful, in the center tapped transformer rectifier
circuit the peak AC voltage is the transformer secondary voltage to the tap.
Rectifier Circuit Summary
Rectifier Ideal VDC Realistic VDC
Half Wave Rectifier VDC = 0.318Vm VDC = 0.318Vm - .7V
Bridge Rectifier VDC = 0.636(Vm) VDC = 0.636(Vm) – 2(.7V)
Center-Tapped Transformer
Rectifier
VDC = 0.636(Vm) VDC = 0.636(Vm) - .7V

20. There are a variety of diode networks called clippers that have the
ability to “clip” off a portion of the input signal without distorting
the remaining part of the alternating waveform.
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.
The series configuration is defined as one where the diode is in
series with the load, while the parallel variety has the diode in a
branch parallel to the load.
Clippers

21. Diodes “clip” a portion of the AC wave. Although first introduced as a half-
wave rectifier (for sinusoidal waveforms), there are no boundaries on the
type of signals that can be applied to a clipper.
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 .7V for a
silicon diode.
Clipper Diode Circuit

22. Steps to analyze the network in presence of DC supply:-
• Make a mental sketch of the response of the network
based on the direction of the diode and the applied
voltage levels.
The direction of the diode suggests that the signal vi must be
positive to turn it on. The dc supply further requires that the voltage vi be
greater than V volts to turn the diode on. The negative region of the input
signal is “pressuring” the diode into the “off” state, supported further by
the dc supply. In general, therefore, we can be quite sure that the diode
is an open circuit (“off” state) for the negative region of the input signal.
Clippers

23. Steps to analyze the network in presence of DC supply:-
• 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 the
point on the characteristics where vd = 0 V and id = 0 A. Applying the
condition id = 0 at vd = 0 to the will result in the configuration of Fig.,
where it is recognized that the level of vi that will cause a transition in
state is vi = V
Clippers

24. Steps to analyze the network in presence of DC supply:-
• Be continually aware of the defined terminals and polarity
of vo.
When the diode is in the short-circuit state, such as shown in
Fig., the output voltage vo can be determined by applying Kirchhoff’s
voltage law in the clockwise direction: vi-V-vo= 0
vo=vi-V
Clippers

25. Steps to analyze the network in presence of DC supply:-
• It can be helpful to sketch the input signal above the
output and determine the output at instantaneous values
of the input.
Keep in mind that at an instantaneous
value of vi the input can be treated as a dc
supply of that value and the corresponding
dc value (the instantaneous value)
of the output determined.
Clippers

26. By adding a DC source to the circuit, the voltage required to forward bias the diode can be
changed.
Variations of the Clipper Circuit

27. By taking the output across the diode, the output is now the voltage when the diode is
not conducting.
A DC source can also be added to change the diode’s required forward bias voltage.
Changing Output Perspective

28. CLIPPER CIRCUITS
SUMMARY

29. CLIPPER CIRCUITS SUMMARY

30. A diode in conjunction with a capacitor can be used to “clamp” an AC signal to a specific
DC level.
CLAMPER DIODE
CIRCUITS

31. A diode in conjunction with a capacitor can be used to “clamp” an AC signal to a specific
DC level.
CLAMPER DIODE CIRCUITS

32. 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

33. SUMMARY OF CLAMPER
CIRCUITS

34. The Zener is a diode operated in reverse bias at the Zener Voltage (Vz).
ZENER
DIODE

35. 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
[Formula 2.16]
The Zener current
2.18]
The Zener Power
IZ = IR - IL [Formula
PZ = VZ IZ [Formula 2.19]
For Vi < Vz:
The Zener acts like an open.

36. ZENER
CALCULATIONS

37. 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.
[Formula 2.25]
[Formula 2.26]
• 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.
[Formula 2.21]
[Formula 2.20]
LOAD RESISTANCE IN A ZENER CIRCUIT
ILmin IR -IZM
RLmax 
VZ
ILmin
RL RLMIN
VZ
ILmax 
VL

ViVZ
RVZ
RLMIN 

38. 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

39. SLIDE 27
Voltage Doubler
This half-wave voltage doubler’s output can be calculated as
Vout = VC2 = 2Vm
Vm = peak secondary voltage of the transformer.

40. 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

41. Voltage Tripler and Quadrupler Circuits
By adding more diode-capacitor networks the voltage can be increased.

42. Practical Applications of Diode Circuits
Rectifier Circuits
Conversions of AC to DC for DC operated circuits
Battery Charging Circuits
Simple Diode Circuits
Protective Circuits against
Overcurrent
Polarity Reversal
Currents caused by an inductive kick in a relay circuit
Zener Circuits
Overvoltage Protection
Setting Reference Voltages