1. Course Outline 1. Chapter 1: Signals and Amplifiers
2. Chapter 3: Semiconductors
3. Chapter 4: Diodes
4. Chapter 5: MOS Field Effect Transistors (MOSFET)
5. Chapter 6: Bipolar Junction Transistors (BJT)
6. Chapter 2 (optional): Operational Amplifiers

2. Chapter 4: Diodes Part II

3. Figure 4.20: Block diagram of a dc power supply
4.5 Rectifier Circuits
Figure 4.20: Block diagram of a dc power supply
The primary application of diode is the rectifier –
Electrical device which converts alternating current (AC) to direct current (DC)
One important application of rectifier is dc power supply.

4. step #1: increase / decrease rms magnitude of AC wave via power transformer
step #2: convert full-wave AC to half-wave DC (still time-varying and periodic)
step #3: employ low-pass filter to reduce wave amplitude by > 90%
step #4: employ voltage regulator to eliminate ripple
step #5: supply dc load

5. 4.5.1 The Half-Wave Rectifier
Half-wave rectifier – utilizes only alternate half-cycles of the input sinusoid
Constant voltage drop model is employed.

6. 4.5.1 The Half-Wave Rectifier
Small inputs? Regardless of the model employed, one should note that the rectifier will not operate properly when input voltage is small (< 1V)
Those cases require a precision rectifier (diode with op amps).

7. 4.5.2 Full-Wave Rectifier
Center-tapping of the transformer, allowing “reversal” of certain currents…

8. Full-Wave Rectifier
When instantaneous source voltage is positive, D1 conducts while D2 blocks…

9. 4.5.2 Full-Wave Rectifier
when instantaneous source voltage is negative, D2 conducts while D1 blocks

10. Center-tapped Transformer

11. 4.5.3 Bridge Rectifier
An alternative implementation of the full-wave rectifier is bridge rectifier
Does not require center-tapped transformer
Four diodes instead of two

12. 4.5.3 Bridge Rectifier
when instantaneous source voltage is positive, D1 and D2 conduct while D3 and D4 block

13. 4.5.3 Bridge Rectifier
when instantaneous source voltage is negative, D3 and D4 conduct while D1 and D2 block

14. 4.5.4. The Rectifier with a Filter Capacitor
Why is this example unrealistic?
Because for any practical application,
the converter would supply a load
(which in turn provides a path for capacitor discharging)

15. 4.5.4. The Rectifier with a Filter Capacitor

16. 4.5.4. The Rectifier with a Filter Capacitor

17. 4.5.4. The Rectifier with a Filter Capacitor
Precision rectifier – is a device which facilitates rectification of low-voltage input waveforms
How?

18. 4.6: Limiting and Clamping Circuits
Q: What is a limiter or clamping circuit?
A: One which limits voltage output.

19. single limiters employ one diode
double limiters employ two diodes of opposite polarity
linear range may be controlled via string of diodes and dc sources
zener diodes may be used to implement soft limiting

20. soft vs. hard limiter 4.6: Soft vs Hard limiter
Q: How are limiter circuits applied?
A: Signal processing, used to prevent breakdown of transistors within various devices.
Why use soft?

21. 4.6.2 The Clamped Capacitor or DC Restorer
Q: What is a DC restorer?
A: Circuit which provides the dc component of an AC without DC value.
Q: Why is this ability important? A:
1) Average value of the output is effective way to measure duty cycle
2) Duty cycle is modulated to carry digital data (PWM): use DC restorer followed by RC low pass filter

22. Q: What is a voltage doubler?
4.6.3 The Voltage Doubler
dc restorer
peak rectifier
Q: What is a voltage doubler?
A: One which multiplies the amplitude of a wave or signal by two.
How?

23. 4.7 Special Diodes Schottky-Barrier Diode or Schottky Diode
metal and moderately doped semiconductor junction
current flows from metal to semiconductor
current is conducted by majority carriers: can switch it on and off faster than p-n junction
forward voltage drop is lower than p-n junction
( V)

24. reverse-biased p-n junction
4.7 Special Diodes
Varactors
reverse-biased p-n junction
junction capacitance is a function of reverse bias voltage
how?
voltage variable capacitor
tuning of receivers, Phase locked loops
Anode
Cathode

25. 4.7 Special Diodes Photodiodes reverse-biased p-n junction
expose to light: covalent bonds break, electron-hole pairs are created
free electrons sweep to n side and holes to p side
reverse current is created
photocurrent is proportational to intensity of incident light
convert light into electric signal
applications: CD-ROM, fiber-optic
what happens when you don't reverse bias the photodiode and expose it to light?
Anode
Cathode
Po...P2= light levels

26. 4.7 Special Diodes LEDs convert forward current into light
forward bias region: when minority carriers diffuse into p and n sides, they recombine with majority carriers, e.g. electrons with holes.
recombination: emission of light
special semiconductor material: direct band-gap
known spectra of light when electrons leave orbit
emitted light is proportional to number of recombinations which is proportional to the forward current
Anode
Cathode