Transcript forward-biased

Diodes, Transistors and Tubes
Silicon Semiconductors
Doped Semiconductors
P-N Junction
Extra holes combine with extra electrons to
form depletion zone
Forward Bias
Diode conducts only when it is forward-biased.
Reverse Bias
Diodes
• let current
flow in only
one
direction.
Diodes
• Diode conducts only when it is forward-biased.
• Electrons flow from cathode to anode (ie NOT the
direction of the arrow)
• demodulation is one important application for
diodes in recovering info from transmitted signals
(more about that in the modulation and transmitters
session)
• What else might be a use of a diode?
Rectification
• Diode use: rectification, ie changing alternating current
(AC) to pulsating direct current (DC)
• Diode lets through current only in one direction, so if an
alternating current is applied to the anode of a diode the
cathode would have a pulsating direct current.
• What other diodes are used in power supplies?
Zener diodes
• current can flow in reverse biased direction, but only if it is
over a certain voltage, the breakdown voltage
• Zener diodes work in the forward biased direction as a normal
diode would, but they also allow current to flow in the reverse
way when the voltage is above the breakdown voltage, and
their breakdown voltage is precisely known.
• used as voltage regulators, ie to regulate or maintain a
constant voltage
• What other kind of diodes?
LED - light emitting diode
• Semiconductor device that
glows different colours
depending on chemical
composition
Amplification or Gain
• What is amplification?
• Amplification increases voltage, current, and/or power
• “Amplification” is equivalent to “gain”
• Amplification is opposite of attenuation or loss, and an amplifier
doesn’t have loss (since the exam talks about that a lot)
• A circuit designed to increase the level of its input signal is
called an amplifier
• To increase the level of
- very weak radio signals, use RF amplifier
- very weak signals from a microphone, use an audio amplifier
• What sort of device might be used?
Transistors
Bipolar Transistors
• Transistors can be of PNP or NPN type
• A transistor can amplify a small signal using low voltages.
(tubes can amplify but need high voltages, but tube not
option in exam)
• A basic semiconductor amplifying device is the transistor
(exam throws tubes in there, but they are not semiconductors)
• The three leads are base, collector emitter
• One example bipolar transistor would be a “general purpose
audio NPN device”
• A transistor can be destroyed by excessive heat.
Field Effect Transistors
Field Effect Transistors
• Two basic types are N and P channel
• functions are:
– gate controls the conductance of the channel
– source is where charge carriers enter the channel
– drain is where charge carriers leave channel
• The field-effect transistor is the semiconductor
with the characteristics most similar to a triode
vacuum tube.
• To reduce current, increase reverse-bias voltage.
Triode Vacuum Tube
• Electrons released from cathode
via thermionic emission
• Electrons flow to positively
charged anode
• Negative voltage on grid repels
electrons, decreases flow
• Positive charge on grid attracts
electrons, increases flow
Triode Vacuum Tube
• Inside the envelope is a vacuum.
• Feature common to both transistors and tubes is that they can
amplify signals.
• A triode vacuum tube might be used because instead of a transistor
because it may be able to handle higher power.
• Can amplify a small signal but must use high voltages (transistor
doesn’t need high voltages)
• Parts
– highest positive potential – plate
– cylinder of wire mesh – grid (one grid!)
– element furthest from plate is – filament
• Cathode emits electrons
• Most similar to a Field Effect Transistor
Equivalencies
• The exam has a bunch of “closest” comparisons
Distortion
• If an amplifier becomes non-linear, the output would become
distorted (there are other kinds of distortion…)
The End
Schedule
Date
Topic
Instructor
Feb 20 - 9 AM
Chapter 2: Basic Theory
Rob Striemer (VE4SHS)
Feb-20
Chapter 3: Ohm’s Law & Power
Rob Striemer
Feb-20
Chapter 4: Inductors & Capacitors
Rob Striemer
Feb 20 - 1 PM
Chap. 1: Introduction to Amateur Radio
Dylan Fijal (VA4DFJ)
Feb-20
Chapter 12: Q codes, phonetics, emerg comms, etc.
Dylan Fijal
Feb-20
Chapter 17: Regulations, Towers, etc.
Dylan Fijal
Feb 27 - 9 AM
Chapter 5: Wavelength, Frequency and Bands
Collin Stanners (VE4CST)
Feb 27 - 9 AM
Chapter 6: Propagation
Collin Stanners
Feb 27 - 9 AM
Ch. 7: Transmission Lines
Collin Stanners
Feb 27 - 1 PM
Ch. 10: Power Supplies
Peter Toth (VE4TTH)
Feb 27 - 1 PM
Chapter 11: Setting up an Amateur Radio Station
Peter Toth
Feb 27 - 1 PM
Chapter 12: Operating an Amateur Radio Station
Peter Toth
March 5 - 9 AM
Chapter 8: Antennas
Tina Tenbergen (VA4THN)
Mar-05
Chapter 8: Antennas
Tina Tenbergen
Mar-05
Chapter 9: Diodes, Transistors and Tubes
Tina Tenbergen
March 5 - 1 PM
Chapter 13: Modulation and Transmitters
Jim Townsend (VE4CY)
Mar-05
Chapter 14: Receivers
Jim Townsend
Mar-05
Practice Exam
March 12 – 9 AM
Chapter 15: Radio Frequency Interference
Bob Jacobs (VE4RCJ)
Mar-12
Chapter 16: Safety
Bob Jacobs
Mar-12
Review
March 12 - 1 PM
Review & Practice Exam
Rob Striemer
March 19 – 9 AM
Industry Canada Final Exam 9:00 AM
Adam Romanchuk
March 19 – 11 AM
11:00 AM - workshop: How to get started, tips, etc.