 Induction
 A changing magnetic field causes an induced electric field
 A changing electric field causes an induced magnetic field
 Resonant RLC circuits cause charges to flow back and forth along the antenna
 The Electric and Magnetic fields are perpendicular
 Electromagnetic waves propagate from the antenna

 Travel at the speed of light
 Predicted by Maxwell’s theory
 Verified as light by experiments of Hertz
 c = l f
 Radio operates in the MHz range
 l
= c/f = 3 X 10 8 m/s / 10 6 Hz
 l
= 300 m
 Antenna should be ¼ wavelength for useful transfer of information

 Amplitude Modulation (AM)
A radio wave can be transmitted long distances.
To get our audio signal to travel long distances we piggyback it onto a radio wave.
This process is called MODULATION.
The radio wave is called the CARRIER.
The audio signal is called the
MODULATION.
Amplitude is periodically changed. Information can be sent at a relatively slow rate, using the high frequency as carrier.
Detecting the variation is done by the speaker. It has a high inductance so that it does not respond to high frequency, but only to the average.

 Here the amplitude is constant and the frequency changes
 Free from static since the sources of static
(lightning, car ignitions, etc.) would show up as amplitude changes, and FM responds only to frequency changes

 Video transmitted as AM so that the changing amplitude can control the changing intensity of the electron beam
 Beam sweeps the phosphor screen with 525 lines 50 times/sec.
 Audio transmitted by FM
 Typical frequencies (channel 6)
 Video Carrier 85 MHz
 Audio 98 MHz
 l
= 3.3 m (antenna should be ½ l
)

 720p50
 1280x720 pixels
• 50 full frames/second (Europe)
• 24, 25, 30, 50, 60 fps in US
 1080i25
 1920x1080 pixels
• 25 full frames/s (Europe)
• 50 or 60 fps (US)

 Isolated atoms have energy levels
 The electrons can only be found in these energy states

 Atoms form a lattice structure
The lattice affects the structure of the energy levels of each atom – we now have joint levels for the entire structure

 Three bands of energy levels form
 Valence Band – most of the electrons are here
 Conduction Band – electrons here give the material electrical conductivity
 Forbidden Band – electrons must jump this band to get from the valence to the conduction band

 In order for an electron to become free and participate in current flow, it must gain enough energy to jump over the forbidden band
 For semiconductors at room temperature, there is not enough energy to conduct.
 As temperature increases more electrons have the energy to jump the forbidden band
 Resistivity decreases
 This is the opposite behavior of conductors

R
Semiconductor
Conductor
T

 When an electron becomes free, it creates a “hole” in the lattice structure
A hole is effectively a positive charge

 Elemental or pure semiconductors have equal numbers of holes and electrons
 Depends on temperature, type, and size.
 Compound Semiconductors can be formed from two (or more) elements (e.g., GaAs)

 A pure semiconductors where a small amount of another element is added to replace atoms in the lattice (doping).
 The aim is to produce an excess of either electrons (n-type) or holes (p-type)
 Typical doping concentrations are one part in ten million
 Doping must be uniform throughout the lattice so that charges do not accumulate

 One valence electron too many (n-type)
 Arsenic, antimony, bismuth, phosphorus
 One valence electron too few (p-type)
 Aluminum, indium, gallium, boron

More positive than rest of N
Start with a P and N type material.
Note that there are excess negatives in the n-type and excess positives in the p-type
More negative than rest of P
Merge the two – some of the negatives migrate over to the p-type, filling in the holes. The yellow region is called the depletion zone.

Apply a voltage as indicated. The free charge carriers (negative charges in the N material and positive charges in the P material) are attracted to the ends of the crystal. No charge flows across the junction and the depletion zone grows.
This is called reverse bias .
Switch polarity. Now the negative charges are driven toward the junction in the N material and the positive charges also are driven toward the junction in the
P material. The depletion zone shrinks and will disappear if the voltage exceeds a threshold. This is called forward bias .

P material (anode)
N material (cathode)
Forward Bias
Reverse Bias

Recall Ohm’s Law (V=IR) Put it into slope-intercept form to get I = V/R. The slope of the graph is 1/R. Large slopes mean small R.

 Rectifier Diode
 Used in power supplies
 Signal Diode
 Used in switches, detectors, mixers, etc.
 Zener Diode
 Voltage regulation – operated reverse bias in the avalanche region
 Reference Diode
 Used like zener for voltage regulation

 Varactor Diode
 PN junction exhibits capacitive properties
• Depletion zone acts like a dielectric
• Adjacent material acts like the capacitor plates
 Increasing reverse bias decreases capacitance
– recall
C

 o
A d
 Capacitance Effect is destroyed if the forward bias is great enough to destroy the depletion zone

 Varactor used to tune a harmonic circuit, as before f res

2

1
LC
 Increasing reverse bias decreases C and increases f res

 Conversion of ac to dc.
 Many devices (transistors) are unidirectional current devices
 DC required for proper operation.

Operation

 We have now used diodes to produced a pulsed dc signal.

Most equipment requires “regulated” dc
 We must remove the “ripple”
 Ripple is departure of waveform from pure dc (flat, constant voltage level)
• Frequency – so far we have seen pulsed dc at the same frequency as the input ( ½ wave) or twice the line frequency
(full wave rectifier)
• Amplitude – a measure of the effectiveness of the filter

r

V rms
( ripple voltage out)
V(average out)
Low r indicates better filtering

 Defined also for current

I ac

I dc
= effective value of ac harmonic component
= average of dc component
I rms

I ac

2
I dc
 2
I ac
so,
2
I rms
 2
I dc r

I ac
I dc r
 I rms
I dc
2

1
For ½-wave rectifier r = 1.21
For full-wave rectifier r = 0.48

Z

R
2 
X
C
2 
R
2 


2

1 f C


2
I

V i
Z

V i
R 2 


2

1 f C


2

 I is the same through both R and C
V o

RI

V o
V i

RV i
R
2 


2

1 f C


2
1
1



2

1 f RC


2
When f is small, denominator is large and the voltage ratio is small.
When f is large, denominator is almost one

R = 0.1 M
W
C = 0.16 m
F
Why is it called a high pass filter?

 Note the point at which
V o
V i

1 2
 This will make (V o
/V i
) 2 = ½
 Recall that one form of power is V 2 /R, so the output power is half the input power at this frequency.

I
 V i
Z

R 2 
X
C
2 
R 2 
1

C
2
Z
V o

IZ
C

IX
C

I

C

V o

I

C


C
V o

V o
V i

R

V i

C

2 
1
1

1
 
R C

2
V i
R
2 
1

C
2
Low frequency means denominator is small and V o
/ V i

1
High frequency means denominator is large and V o
/ V i is small

Note the half power frequency again.

 Improving the ripple factor
 During forward bias half-cycle, capacitor is charging
 During the reverse bias half-cycle, the capacitor discharges through the output resistor

 Even better ripple factor.

Full wave rectifier, using ground loop on each half cycle
• The series inductor opposes changes in current
• The coil stores energy when the current is above average and releases energy to the circuit when the current falls below average
 Coil chops off (chokes) the peaks of the ac pulses
 L-section delivers more steady current than the capacitive filter alone
 Used for cases of variable loads where good regulation is required
 Output voltage is less than the capacitive filter

P
Full wave rectifier, using ground loop on each half cycle
Combines the effects of a capacitive and L-section filter. Regulation is poor when load varies, however.

 If the load current is varying, but you require voltage to be constant, R must vary (V=IR).

 Increasing reverse bias on the diode eventually accelerates electrons in the depletion zone so that they ionize other atoms.
 The freed electrons ionize other atoms
 Avalanche occurs
 Zener Diodes are designed so that the transition potential is very steep

For a large change of current, voltage remains at V
Z
Zener acts like an automatically varying resistor.
Can be obtained with V
Z from 2.4 – 200 V.

Since the load is in parallel with the diode, the voltage drop across R
L is always the same as across VR1 and is V constant Zener voltage
Z
=
The input voltage V must be greater than V
Z
.
Zener MUST be operated under load. If not, the zener is still delivering power (more than usual) and may melt. Recall that the zener can draw large currents all at the same voltage.

R protects D from surges
When D is conducting, C charges to V peak
During reverse bias half cycle, C discharges through load at the peak voltage
V out
= V peak
=

2 V rms

First Half Cycle – CCW – D1 conducting, C1 charged. C2 remains uncharged because of the low resistance in the forward biased D1
Second Half Cycle – CW – 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 R
L
. Notice that C2 is charged once/cycle, so output has the same frequency as input.

 Diodes can be used in waveform shaping
 Clippers
 Limit or “clip” portions of the signal
 Used for circuit protection or waveform shaping
 Clampers
 Shift the dc level of the signal

Notice that the dc sources V
1 branch.
and V
2 are reverse biasing the diodes in each
Clockwise half cycle - V
S flows to the output until it becomes equal to V through the first diode branch.
1
. At that point diode 1 can conduct, another path is found, and any voltage above V
1 goes
Counter Clockwise half cycle - V
S flows to the output until it becomes equal to V
Excess voltage flows then through diode 2.
2
.
Output is “clipped” at V
1 and V
2
.

 If V
1
= V
2 and V
S
>> V
1
, then you can make a good square wave output.

 Surge protection
 Noise reduction

V p
0


During the half cycle in which the diode conducts, the capacitor charges to V peak
If the RC time constant is long compared to the input frequency, the capacitor cannot discharge during the other half cycle.
 V o
= V i
+ V peak


Output has the negative peaks “clamped” to zero.
 Could have the positive peaks clamped by just reversing the diode.
 Clamping here is always at zero, independent of the amplitude of the input
 Capacitor always charges to V peak
 Time constant is long

 Insert a dc source
 Capacitor now charges to V peak
+ V
V p
V

Incoming photons collide with electrons in the valence band of the p-type material.
These electrons are promoted to the conduction band, increasing the number of minority carriers.
This increases the reverse bias. Watch the Animation

When curve crosses the horizontal axis (I =
0), we note the open circuit voltage , V
0C
When curve crosses the vertical axis (V = 0), we note the short circuit current , I
SC

SC
0C
V
0C is logarithmic
I
SC is linear

Si is more sensitive in the IR. Se matches the human eye better.

 Semiconductors generally have a negative temperature coefficient – as the temperature increases, the resistance decreases
 This occurs because more electrons can be promoted to the conduction band at higher temperatures. More electron flow means lower resistance.
 Can develop circuits that respond to temperature changes.

 When forward biased, electrons from the N-type material may recombine with holes in the P-type material.
 System energy is decreased
 Excess energy emitted as light
 Indium gallium nitride (InGaN) semiconductors have been used to make colored LEDs
• Stop lights
 Progress toward white LEDs is promising

Since the diode provides light, no external or internal source of light is needed to see the display.
If the diodes are visible when not lit, the result can be hard to read

With no light we have a small
“dark current” (I dark
) due to thermal electron-hole pairs in the depletion zone.
With light the reverse bias current increases (I det
)
Typical:
Dark V r
= 3 V, I = 25 m
A
R = V r
/I = 120 k
W
For Intensity 25000 lumens/m 2
V r
= 3 V, I = 375 m
A
R = 8 k
W
So we can use it as a variable resistor controlled by light

Response is very linear so photodiodes are used to measure light intensity

Typically, more sensitive to longer wavelengths.

Highly
Reflective
Mirror
P material
N material
Partially
Reflective
Mirror
Used in forward bias.
Electrons move into depletion zone and recombine with holes, producing light (like an LED).
More electron-hole recombinations can be stimulated by this photon, producing more photons at the same wavelength.
The mirrors reflect the photons back and forth through the depletion zone, stimulating more photon at each pass.
Eventually, the beam passes out of the right hand mirror.

 Used as CD and DVD detector
Photodiode
Laser Diode
Also used as bar code readers, laser pointers, fiber optics, etc.

 LED-photodiode pairs
 Used in circuits where surges are a problem
 Isolates one part of the circuit from another
• Only connection is light

 Liquid Crystal Display
 State between solid and liquid
 Requires only a little heat to change material into a liquid
• Used as thermometers or mood rings
 Electric currents are used to orient the crystals in predictable ways
• The liquid crystals polarize the light (either internal light source or external) to give light and dark areas on the display