Chapter 2
ZENER DIODE
Zener diodes
• The breakdown characteristics of diodes can
be tailored by controlling the doping
concentration
–Heavily doped p+ and n- regions result in low
breakdown voltage (Zener effect)
• Used as reference voltage in voltage
regulators
I
Region of
operation
V
ZENER DIODE
Basic Zener regulator
Vi and RL Fixed
RlVi
V  VL 
R  RL
And:
(V i V Z )R L
R
VZ
If V≥VZ and Iz >Izmin that means the Zener diode is on
If V<VZ or Iz <Izmin that means the Zener diode is off
Basic zener regulator
Vi and RL Fixed
2. Substitute the appropriate equivalent circuit
and solve for the desired unknowns.
VL  VZ
IR  IZ  IL
IZ  IR  IL
VL
IL 
RL
PZ  VZ I Z
Vi VL
IR 
R
Fixed Vi, Variable RL
1- Finding RLmin:
Using VDR
RL min
OR: If Izmin is given:
RV Z

Vi  VZ
IZ  IR IL
I L max  I R  I Z min
VR
IR 
R
VR  Vi  Vz
R Lmin
VZ

I Lmax
2- Finding RLmax:
I L min  I R  I Z max
RLmax 
VZ
I L min
Fixed RL, Variable Vi
• The minimum value of Vi must
be sufficiently large to turn the
Zener diode on.
Using VDR
Vi min
( RL  R)VZ

RL
• The maximum value of Vi
limited by maximum Zener
current IZM
I ZM  I R  I L
VR
IR 
R
VR  Vi  Vz
I R max
 I ZM  I L
Vi max  I R max R  VZ
Applications
Simple square-wave generator
Applications
AC Limiter
Protection circuit
Half Wave Voltage Doubler
High Voltage transformers are expensive and
impractical at voltages above 1000V
The peak inverse voltage PIV across each diode is 2Vp
Half Wave Voltage Doubler
First Half Cycle – D1 conducting, C1
charged. C2 remains uncharged
because of the reverse biased D2
Second Half Cycle – D2 conducting, C2
charged by (source + C1) – about
twice the source voltage
Next Half Cycle – Repeat of first stage
with the addition that C2 can now
discharge through RL. Notice that C2
is charged once/cycle, so output has
the same frequency as input.
Full Wave Voltage Doubler
• When the secondary voltage is
positive, D1 is forward-biased and C1
charges to approximately Vp
• During the negative half-cycle, D2 is
forward-biased and C 2 charges to
approximately Vp. The output
voltage 2Vp- is taken across the two
capacitors in series.
The peak inverse voltage PIV across each diode is 2Vp
Voltage Tripler
• The addition of another diode-capacitor section to the half-wave
voltage doubler creates a voltage tripler. The operation is as
follows: On the positive half-cycle of the secondary voltage. C 1
charges to Vp, through D1. During the negative half-cycle,
• C 2 charges to 2Vp through D2 as described for the doubler. During
the next positive half-cycle, C 3 charges to 2Vp through D3 the
tripler output is taken across C 1 and C 3.
Voltage Quadrupler
The addition of still another diode-capacitor section produces an output four
times the peak secondary voltage. C4 charges to 2Vp through D4 on a
negative half-cycle. The 4V p output is taken across C2 and C4 , as shown.
In both the tripler and quadrupler circuits. the PlV of each diode is 2Vp
Voltage Quadrupler
• Can extend this circuit to produce
extremely high voltages (~750kV).
Voltage Quadrupler
Special Purpose Diodes
Varactor Diodes
A varactor diode is best explained as a variable capacitor. Think of the depletion region a
variable dielectric. The diode is placed in reverse bias. The dielectric is “adjusted” by bias
changes.
Varactor Diodes
The varactor diode can be useful in filter circuits as the
adjustable component.
Other Diode Types
The Schottky diode’s significant characteristic is it’s fast switching speed. This is useful for
high frequencies and digital applications. It is not a typical diode in the fact that it does not
have a p-n junction, instead it consists of a heavily doped n-material and metal bound
together.
Other Diode Types
The PIN diode is also used in mostly microwave frequency applications. It’s variable forward
series resistance characteristic is used for attenuation, modulation, and switching. In reverse
bias exhibits a nearly constant capacitance.
Other Diode Types
The tunnel diode is also used for fast switching applications.
This is achieved by reduced doping at the junction.
Other Diode Types
The laser diode (light amplification by stimulated emission of radiation) produces a
monochromatic (single color) light. Laser diodes in conjunction with photodiodes are used
to retrieve data from compact discs.