1. Special Purpose Diodes
Chapter 5 & Lecture 5 & 6

2. Graph of zener current versus voltage
-VZ (Volts) 6 4 2 20 40 60 VZ
-IZ (mA) 80
100
120
140
Graph of zener current versus voltage

3. Power supply A zener diode voltage regulator RS VS VZ RL
This circuit will regulate when the Thevenin voltage
facing the zener diode is greater than the zener voltage. RL
VTH = VS
RS + RL

4. Power supply RS VS VZ RL Assuming the zener is conducting: VS - VZ VZ
IL =
IZ = IS - IL RS RL

5. A zener waveshaping circuit
Power
supply RL
VZ V
-VZ V

6. Power
supply RZ
Zener diode
second approximation VZ

7. Power supply Applying the second approximation RS RZ VS RL VZ
The deviation in load voltage from the ideal case:
DVL = IZRZ

8. Power supply The zener regulator also reduces ripple RS RZ VR(in) RLVZ
Assuming that both RL and RS >> RZ: RZ
VR(in) RS

9. Zener diode ratings Maximum power = PZ(max) = VZIZ(max)
Available tolerances: 20, 10 and 5 percent
Zener resistance, RZT, increases at the knee of the characteristic curve
A derating factor such as 3.2 mW per degree for temperatures above 50 degrees Celsius is typical.

10. What happens to VZ when VS varies from 20 to 30 volts?
1 kW
-30
-20
-10 RS Q1 VS
-10 mA Q2
VS = 20 V
-20
30 V
-30
What happens to VZ when VS varies from 20 to 30 volts?
Load lines provide a graphical solution.

11. Power supply RS VS Light-emitting diode VS - VD IS = RS
The typical voltage drop for most LEDs is from 1.5 to 2.5 V.

12. Photodiodes are reverse biased and conduct in the presence of light.
Power
supply VS
Photodiode
Photodiodes are reverse biased and
conduct in the presence of light.

13. Optocoupler combines an LED and a photodiode.
Signal
source
Power
supply V1 V2
Optocoupler combines an LED and a photodiode.

14. cause poor performance
High-frequency
rectification
Vin
Vout
Charge storage can
cause poor performance
at high frequencies.
Vin
Vout
Tails

15. Schottky diodes eliminate tails at high frequencies.
Hot-carrier
rectifier
Vin
Vout
Schottky diodes eliminate
tails at high frequencies.
Vin
Vout

16. Less reverse bias
(more capacitance)
More reverse bias
(less capacitance)
Varactor symbol

17. A varistor diode can be used to protect
line-operated equipment from voltage surges.

18. Power supply RS VS Current-regulator diode
RS can vary over a wide range and the current stays the same.

19. Other diode types Laser: emits coherent light
Step-recovery: snaps off when reverse biased
Back: conducts better when reverse biased
Tunnel: has a negative resistance region

20. Other diode applications
Laser: CD players, communications
Step-recovery: Frequency multipliers
Back: Small-signal rectifiers
Tunnel: High-frequency oscillators