Fundamentals of Electricity Flashcards
Matter
- Occupies space
- Has weight
Elements
- Basic building block of nature
2.Can not be reduced to a simpler substance by chemical means - Over 100 known elements
Parts of an atom
- Nucleus: located at the centre of the atom, it is a place
- Protons: Positively charged particles inside the nucleus
- Neutrons: Uncharged particles inside the nucleus
- Electrons: Negatively charged particles that orbit the nucleus
Size of protons and electrons
Imagine a cathedral
In the middle, there is a grapefruit, that is roughly the size of protons and neutrons. And electrons would be like a laser point around the out walls
Atomic number
The number of protons in the nucleus of the atom
Each element has its unique number of protons
Atomic weight
- The mass of an atom
- Determined by the total number of protons and neutrons in the nucleus.
Each element has its unique weight
Shell
- Electrons orbit here
- Concentric circles around nucleus
- Filled in sequence
- Label from K to Q
Valence Shell
The outermost shell, the Q shell
Valence
The number of the electrons contained in the valence shell
Conductors
- Materials that contain a large number of free electrons
- 3 or fewer electrons in the valence shell is potentially a good conductor
High to low
Silver > Copper > Gold (least oxidized) > Aluminum > Tungsten > Iron > Nichrome
Insulators
- Prevent the flow of electricity
- Stabilized by absorbing valence electrons
Mica > Glass > Teflon > Paper(Paraffin) > Rubber > Bakelite > Oil > Procelain > Air
5 or more electrons in the valence shell
Semiconductors
- Can be altered to function as either a conductor or insulator
4 electrons in the valence shell
Negative and positive Ion
A negatively charged or positively charged atom
Ionization
The process of gaining or losing electrons
Significant in current flow
e.g rubbing something to get static electric
Current
Movement of electrons from negatively charged atoms to positively charged atoms at the speed of light.
Represented as I
Coulomb
Unit adopted for measuring charges
6.24 * 10 18 electrons
Represented as C
Ampere
One coulomb moving past a single point in one second
Named for French Physicist Andre Marie Ampere
Current measured in ampers
Represented by A
Potential
The ability of the source to perform electrical work
Difference of Potential
- Causes electrons to move or flow in a circuit
- Referred to as electromotive force (emf) or voltage
Voltage
- The force that moves the electrons in the circuit
- The pressure or pump that moves electrons
- Represented by E or V
- Unit of measure called the volt
Resistance
- Opposition to the flow of electrons
- Degree of resistance depends on size, shape or temperature
- Measured in Ohms, named for George Simon Ohm
- Represented by Greek letter Omega Ω
Everything has resistance, except super conductors
First low of electrostatic charges
Like charges repel each other
Second law of electrostatic charges
Unlike charges attract each other
The relationship between amperes and coulombs pers second can be expressed as
I = Q / t
I = current measured in amperes
Q = quantity of electrical charge in coulombs
t = time in seconds
Hole
The movement of an electron from one atom to the next, creating the appearance of a positive charge moving in the opposite direction
Flow
Negative to Positive: electron flow and current flow
Positive to Negative: hole flow
Voltage source
- Supplies electrons from one end of the conductor
- Removes electrons from the other end of the conductor
- Can be thought of as a kind of a pump
Voltage sources
- Friction
- Magnetism
- Chemicals
- Light
- Heat
- Pressure
Voltage sources - Friction
Van de Graaff generator, lightning storm
Voltage sources - Magnetism
- Most common method of producing electrical energy
- Produced using a generator
- Powered by steam from nuclear power or coal, water, wind or gasoline or diesel engines.
Voltage sources - Chemicals
- Cell (1.5 V)
- Second most popular method of producing electrical energy
- Consists of two metals
- Copper
- Zinc
- Many cells can be connected to form a battery
Voltage sources - Light
- Photovoltaic cell
- A single cell can produce a small voltage
- Many cells must be linked to produce a usable voltage and current
- Primarily used in satellites and cameras
- Cost is high but is declining
Voltage sources - Heat
- Thermocouple
- Two dissimilar wires twisted together
- Voltage is directly proportional to the amount of heat applied
- Used in thermometers
- Also called a pyrometer
Voltage sources - Pressure
- Piezoelectric effect
- Voltage is small; must be amplified to be useful
- Used in crystal microphones, phonograph pickups (crystal cartridges) and precision oscillators
Direct current
Electrons flow in only one direction
Alternating current
Electrons flow in one direction then in the opposite direction
Battery
A combination of two or more cells
Primary cells
- Cells that can not be recharged
- Leclanche cell or dry cell; also referred to as a carbon-zinc cell
- Alkaline cell
- Lithium cell (last longer)
Secondary cells
- Cells that can be recharged
- Lead-acid battery or wet cell
- Nickel- Cadmium cell or Ni-Cad
Connecting Cells and Batteries
- Series
- Series
- It = I1 = I2 = I3 (current)
- Vt = V1 + V2 + V3
- Series opposing
- Little practical value
- Series
- Parallel
- It = I1 + I2 + I3
- Vt = V1 = V2 = V3
Voltage rise and drop
Voltage rise: Potential energy or voltage introduced into a circuit
Voltage drop: The energy given up as electrons encounter resistance in the circuit
Ground
- Earth
- Used to prevent electric shock.
- Electrical
- Provides a common reference point.
Resistance
- Opposition to electron flow in a circuit
- Expressed by the symbol R
- Measured in ohms
- Abbreviated with Greek symbol Ω
- Varies from material to material
- Silver is the best
- Copper is the most common
- Gold doesn’t tarnish
- Affected by temperature
- Affected by the size diameter of the conductor
Resistivity
The resistance of a material to current flow
- Resistivity is different for different materials
- Even good conductors have different levels of resistivity
Compare everything to silver
Conductance
- The ability of a material to pass electrons
- Expressed as G
- Unit known as Mho (ohm backwards)
- Abbreviated with the inverted Greek symbol ℧
- is derived by
R = 1/G or G = 1/R
Resistors
- Components manufactured to possess a specific value of resistance to the flow of current
- Come in two classifications:
- Fixed value
- Variable
- Variety of shapes and sizes to meet specific circuit, space and operating requirements
Tolerance
- The amount that the resistor may vary and still be acceptable
- The larger the tolerance the cheaper it is to manufacture
- Resistors are available with tolerances of +-20%, 10%, 5%, 2% and 1%
Molded Carbon resistor
- The most commonly used
- Inexpensive
- Manufactured in standard resistor values
Wirewound
- Used in high-current circuits
- Resistance varies from a fraction of an ohm to several thousand ohms
Film resistors
- Becoming increasingly popular
- Three types : carbon film, metal film and tin oxide film
Surface mount resistors
- Ideal for small circuit applications
- Available in both thick and think films
Variable resistors
- Allow the resistance to vary
- Vary linearly or logarithmically
- Called a potentiometer when used to control voltage
- Called a rheostat when used to control current
Resistor identification
- Alphanumeric
- Example of an alphanumeric code
- RN60D5112F
RN60 = Resistor style (composition, wirewound, film)
D = Characteristics (effects of temperature)
5112 = Resistance value (2 represents the number of zeros)
F = Tolerance
- RN60D5112F
- EIA (Electronic Industries Association) Color code
Resistors in circuits
- Resistors are typically configured in a circuit in one of three different ways
- Series circuit configuration (voltage divider)
- Parallel circuit configuration (current divider)
- 1/Rt = 1/R1 + 1/R2 + … + 1/Rn
- 1/Rt is always less than the least Rn
- Compound circuit configuration
Electric circuits
- The path that the current follows is called an electric circuit
- All electric circuits consist of
- A voltage source
- A load (convert energy to different form)
- A conductor
Ohm’s law
V = I * R
Kirchohoff’s Law
In GR Kirchhoff extended Ohm’s Law with two important statement
Kirchhoff’s current law
The Algebraic sum of all the current entering and leaving a junction is equal to zero
Kirchhoff’s voltage law
The Algebraic sum of all the voltages around a closed circuit equals zero
Power
- Relates to the rate at which work is being done.
- Work is done when a force (voltage) causes motion (current)
- Voltage creates current, causing electrons to move in a circuit
- The total power dissipated in a series or parallel circuit is equal to the sum of the power dissipated by the individual components
Electric power rate
- The instantaneous rate at which work is done
- Measured in watts
- Wattage is greater when work is done in a short period of time than when the same amount of work is done in a longer period of time.
Watt
- The basic unit of power
- Equal to the voltage across a circuit, multiplied by the current through the circuit
- Represents the rate at which work is being done
P = I * E
The power dissipated in a circuit is often less than 1 watt
Circuit analysis
To determine the power dissipated by a component
- Voltage drop times current
Voltage Dividers
Used to set a bias or operating point of various active electronic components
- Transistors
- Integrated circuits
Used to divide a higher voltage to a lower voltage
Ofen referred to as scaling
Current Division
Current is directly proportional to the voltage across the circuit
The voltage drop is equal to the percentage of the dropping resistor to the sum of the dropping network
E (drop) = E (source) * R (drop) / R (total)
Bleeder Current
The current flow into the first component in the circuit to start the voltage division process
Should have 1/10 of the amp of the overall load current (rule of thumb)
Magnets
- Natural magnet
- Derived from magnetite
- Artificial magnet
- Created by ribbing a piece of soft iron with a piece of magnetite
- Electromagnet
- Created by current flowing through a coil of wire
The earth is a huge magnet
The north magnetic pole is different from the north geographic pole
The colour code for magnets
Red for the north pole
Blue for the south pole
Magnetism
Can be traced to the atom
As electrons orbit the nucleus, they also spin on their axis
This electrostatic charge produces a magnetic field
The direction of the magnetic field is the same as the electron’s direction of spin
Ferromagnetic materials
Materials that respond to magnetic fields, like our fridge. when it is ferrie, it would rust
Atoms combine into domains or groups
When unmagnetized, the domains are random
When magnetized, the domains align in a common direction and the material becomes a magnet
Magnetic field
The invisible lines of force surround a magnet. These are called flux lines
Flex lines
- Have polarity from North to South
- Always form a complete loop
- Do not cross each other
- Tend to form the smallest possible loop
Permeability
The ability of a material to accept magnetic lines of force
If a magnets stick to it, it has a high level of permeability
Electricity and Magnetism
A magnetic field is generated when current flows through a wire.
Electronmagnets
Composed of many turns of wire close together
The principle of the electronmagnet
- When a wire is twisted into a loop
- The flux lines are brought together
- The flux lines are concentrated at the center of the loop
- A north and south pole are established
The strength of the magnetic field can be increased three ways
- The more turns of wire, the more flux lines are added together
- The greater the current, the greater the number of flux lines generated
- A ferromagnetic core is inserted into the center of the coil, usually iron
Magnetic induction
The effect a magnet has on an object without physical contact
Residual magnetism
The magnetic field that remains when an object is separated from a magnet.
Retentivity
The ability of a material to retain its magnetic field after the magnetizing force is removed
Magnetic shields
Low reluctance materials
Used to protect electronic equipment from magnetic flux lines
Electromagnetic induction
The principle behind the generation of electricity
- A current is produced when a conductor passed or is passed by a magnetic field
- As the conductor passes through the magnetic field, a deficiency of electrons is created
- This results in a difference of potential between the ends of the conductor
- When the conductor is removed from the magnetic field, the free electrons return to their parent atoms.
Faraday’s law
The induced voltage in a conductor is directly proportional to the rate at which the conductor CUTS the magnetic lines of force.
The left hand rule
Thumb Motion
Index Flux (North to South)
Middle Current (Negative to positive)
Magnetic and Electromagnetic Applications
- AC generator: converts mechanical energy to electrical energy by utilizing the principle of electromagnetic induction.
- DC generator: functions like an AC generator with the exception that it converts the AC voltage to DC voltage.
Relay
An electromagnetic switch that opens and closes with an electromagnetic coil
- Used where it is desirable to have one circuit control another circuit
- it electrically isolates the two circuits
- Also used to control several circuits some distance away
- Doorbell
Solenoid
A coil, when energized, pulls a plunger that does some mechanical work
- Door chimes
- Automotive starters
Phonograph pickups
Used the electromagnetic principle
- A magnetic field is produced by a permanent magnet attached to the stylus
- The stylus tracks through the groove of a record in response to the audio signal recorded
- The movement induces a small voltage that varies at the audio signal response
- The induced voltage is amplified and used to drive a loudspeaker, rpoducing the audio signal
Loudspeaker
Constructed of a moving coil around a permanent magnet
- The magnet produces a stationary magnetic field.
- The current passes through the coil, producing a magnetic field that varies at the rate of the audio signal
- The magnetic field of the coil is attracted and repelled by the field of the magnet
- The coil is attached to a cone that moves in response to the audio signal
- The cone reproduces the audio signal
Magnetic recording
Uses the electromagnetic principle to store information
- A signal is stored on tape or disk with a record head, to be read back later with a playback head.
– Some are combined in one package
– They may be on the same head
- The record and playback heads are a coil of wire with a ferromagnetic core.
- A tiny gap between the ends of the core is a magnetic field
- A piece of material covered with iron oxide, is pulled across the record head, magnetizing it.
– Information is written in a magnetized pattern
- To play back or read the information, the material is moved past the gap in the playback head
- The magnetic field induces a small voltage into the coil winding
- When amplified, the information is reproduced.
Examples
- Floppy disk drives
- Hard disk drives
- Reel to reel recorders
- Cassette recorders
- Video recorder
DC motor
Operation depends on the principle that a current-carrying conductor, placed in and at right angles to a magnetic field, tends to move at right angles to the direction of the field.
TV, radar, computer terminals
Uses the current-carrying principle
- The conductor carrying current is deflected by a magnetic field
- The electrons travel through a vacuum to strike a phosphor screen where they emit light
- By varying the electron beam over the surface of the picture screen, a picture can be created.
- Two magnetic fields deflect the beam
– One field moves the beam from side to side
– One field moves the beam up and down
Inductance
The characteristic of an electrical conductor that opposes a change in current flow
The symbol for inductance is L
Once current is moving through a conductor. Inductance helps to keep it moving
As the magnetic flux lines build up. They create opposition to the flow of current
Is measured by the henry
- Named after Joseph Henry
- Represented by H
- The amount of inductance required to induce an emf of 1 volt when the current in a conductor changes at the rate of 1 ampere per second
- Most commonly used are the millihenry (mH) and the microhenry(uH)
Inductor
A device that stores energy in a magnetic field
Designed to have a specific inductance
Consist of a conductor coiled around a core
Classified by the type of core material, magnetic or nonmagnetic
When calculate inductance, it works just like resistance in series and parallel
Lenz’s law
An induced emf in any circuit is always in a direction to oppose the effect that produced it. The amount of counter emf is in proportion to the rate of change. The faster the rate of change, the greater the counter emf
Inductor - time constants
The time required for current through a conductor to increase to 63.2% or decrease to 36.8% of the maximum current
Expressed as t = L / R
t = time in seconds
L= Inductance in henries
R = Resistance in ohms
5 time constants required to fully build up or collapse the magnetic field of an inductor
Types of inductors
- Air core
- Fernte or pawdered Ion core
- Toroid core
- Shielded core
- Laminated core
Capacitance
The ability of a device to store electrical energy in an electrostatic field
The letter C stands for capacitance
Similar to storage cells
The basic unit of capacitance is the farad (F)
A farad is the amount of capacitance that can store 1 coulomb (C) of charge when the capacitor is charged to 1 volt
Capacitor
A device that possesses a specific amount of capacitance
Made of two conductors separated by an insulator
- The conductors are called plates
The insulators are called dielectric
Treat all capacitors as though they were charged
- Never touch both leads of a capacitor with your hands
- A capacitor can hold a potential indefinitely if it does not have a discharge path
Factors affecting capacitance
- Area of the plate
- Distance between the plates
- Type of dielectric material
- Temperature
Dielectric constant
A measure of the effectiveness of a material as an insulator
Some examples
Paper: 2~3
Meca: 5~6
Electrolytic capacitors
Large capacitance for size and weight
Consists of two metal foils separated by fine gauze saturated with a chemical paste called an electrolyte
Polarized, having a positive and negative lead
How to calculate the capacitance
The opposite as resistance in series and parallel
RC time constants
Reflects the time required for a capacitor to charge up to 63.2% of the applied voltage or to discharge down to 63.2%
t =RC
t = time in seconds
R = resistance in ohms
C = capacitance in farads
Alternating Current (AC)
The father of AC is Nikola Tesla
AC generator
Cycle: each time the AC generator completes on revolution, its output voltage is referred to as one cycle of output voltage. It produces one cycle of output current in a complete circuit
The two halves of a cycle are called alternations
Two complete alternations make up a cycle
One cycle per second is called a hertz (Hz)
Armature
The rotating loop of wire
AC values
Each point on a sine wave has two numbers associated with it
- The degree of rotation: the angle to which the armature has rotated
- The amplitude: The maximum departure of the value of an alternating current or wave from the average value
Effective value
The amount that produces the same degree of heat in a given resistance as an equal amount of direct current
Can be determined by the root mean square (rms) process
Also called the rms value
Erms = 0.707E(peak)
Period
The time required to complete one cycle of a sine wave
Measure in seconds
The letter t is used to represent period
Frequency
The number of cycles that occur in a specific period of time
Expressed in terms of cycles per second
Unit of frequency is called hertz
One hertz equals one cycle per second
Harmonics
Higher frequency sine waves that are exact multiples of the fundamental frequency (Represents the repetition rate of the waveform)
Odd harmonics are odd multiples of the FF, Even …
AC Meters
Moving coil meter movement
- Referred to as D’Arsonval meter movement
- Designed to measure DC current
- AC current must be converted to DC current to be measured
- The process is called rectification
- The rectifiers covert the sine wave into a pulsating DC current
Oscilloscopes
Most versatile piece of test equipment available for working on electronic equipment and circuits
Provide a visual display of what is occurring in the circuit
Oscilloscope provides
- The frequency of a signal
- The duration of a signal
- The phase relationship between signal waveforms
- The shape of a signal’s waveform
- The amplitude of a signal
The basic parts of an oscilloscope are
- A cathode ray tube (CRT)
- A sweep generator
- Horizontal and vertical deflection amplifiers
- Power supplies
Resistive circuit
Just resistive load NO inductor and Capacitor
Capacitive AC circuits
Capacitors in AC circuits
- AC voltage applied to a capacitor, gives the appearance that electrons are flowing in the circuit
- Current and voltage do not flow in phase with each other
– 90 degrees out of phase
– Current leads voltage
Capacitive reactance/resistance
The opposition that capacitor offers to the applied AC voltage
Represented by Xc
Measured in ohms
Calculated by using the formula
Xc = 1 / 2π * fc
f stands for frequency
c stands capacitance
Can be used alone or combined with resistors to form RC (resistor-capacitor) networks
Used for filtering, phase shifting, coupling and decoupling
A filter
A circuit that discriminates among frequencies, attenuating (weakening) some while allowing others to pass
Low pass filter and high pass filter
Inductance in AC circuits
Inductors offer opposition to the current flow
- Voltage placed across an inductor creates a magnetic field
- When AC voltage changes polarity, it causes the magnetic field to expand and collapse.
- Voltage is induced in the inductor coil called a counter-electromotive force (CEMF)
– 180 degrees out of phase with the applied voltage
– Opposes the applied voltage
– Opposition is as effective in reducing current flow as a resistor
Voltage leading current, ELI, 90 degrees
Inductive reactance
The opposition offered to current flow by an inductor
Measured in ohms
Depends on its inductance and the frequency of the applied voltage
Expressed by the symbol Xl
Xl=2π * fl
l = inductance in henries
Applications of inductive circuits
Compete with capacitors for filtering and phase shift application
Have fewer applications than capacitors because they are larger, heavier and more expensive
Inductor vs Capacitors
Inductors provide a reactive effect while still completing a DC circuit path
Capacitors provide a reactive effect, but block the DC elements
Inductors and capacitors are sometimes combined to improve the performance of a circuit
The effects of pure inductance or capacitance cause the voltage and current to be 90 degrees out of phase.
ELI (inductor) the ICE (capacitor) dude
Cut off frequency
The frequency above or below the frequencies passed or attenuated is called the cut-off frequency
Symbol is fco
can be determined by the formula
fco = R / 2π * fL
L = inductance in Henries
Reactance in Series Circuits
When an AC voltage source is applied, the instantaneous value of current flowing through the resistor varies with the alternating output voltage applied.
Ohm’s law applies
Peak current calculated from the source peak voltage
Rms current calculated from rms voltage
Voltage: Vt2 = Vr2 + Vl2
Resistance: Zt2 = R2 + Xl2
Power: Pappt2 = Pr2 (true power) + Pl2
Impedance
The combined effect of resistance and reactance
Must be used to calculate current in a reactive circuit when the applied voltage is known
Represented by the symbol Z
The impedance of a parallel RL or RC circuit is smaller than both the individual resistance or reactance
Reactance in parallel circuits
Parallel circuits containing inductors and capacitors maybe analyzed with vector diagrams
- Use current vectors
- Voltage across each component must be equal and in place
Power consumption
In a purely resistive AC circuit
- Obtain the average power
– Calculate the product of the rms current and rms voltage
Power factor
- The ratio of true power in watts to apparent power in volt-amps
Resonance
A resonance device produces a broadening and dampening effect
Resonant circuits pass desired frequencies and reject others
Occurs when a circuit’s inductive and capacitive reactance are balanced Xl = Xc
Resonant circuit
- Both inductive and capacitive components at the same frequency
– Reactances are equal but opposite
- Used with radio frequencies in tuning receivers and transmitters
- Not used in the audio bands of frequencies
Transformer
The action caused when two electrically isolated coils are placed next to each other and an AC voltage is put across one coil. Resulting in a changing magnetic field which induces a voltage into the second coil.
The device used to create this action is called a transformer
The coil containing the AC voltage is the primary winding. The coil in which the voltage is induced is the secondary winding
The design of a transformer is determined by
- The frequency at which it will be used
– Low -frequency, iron cores
– High frequency, air cores
- The power it must handle
- The voltage it must handle
Transformers are rated in volt-amperes
Transformers are wound with tapped secondaries
- Center tapped secondary is equal to two secondary windings
- Used for power supply to convert AC voltages to DC voltages.
Coefficient of coupling
A number from 0 to 1
The coupling coefficient is defined as the fraction of the magnetic flux produced by the current in one coil that links with the current in the other coil.
Turns ratios
Determines whether a transformer is used to step up (greater than one), step down (less than one) or pass voltage unchanged (Isolated transformer)
The number turns in the secondary winding divided by the number of turns in the primary winding
Expressed as turns ratio = Nsec / Nprim
Where N = number of turns
Either step up or step down, power is the same.
Impedance ratio is equal to the turns ratio squared
- Zp / Zs = Np2 / Ns2
Semiconductor materials
Three pure semiconductor elements
- Carbon C
- Germanium Ge
- Silicon Si most commonly used
Covalent bonding
The process of sharing valence electrons, resulting in the formation of crystals
Negative temperature coefficient
As the temperature increases, its resistance decreases
For silicon, resistance cuts in half every 60 C rise in temperature
Hole
The absence of an electron
Represents the loss of a negative charge
Therefore, it has the characteristics of a positively charged particle
Each corresponding electron and hole are referred to as an electron-hole pair
Holes constantly drift toward the negative terminal of the voltage source.
Electrons flow toward the positive terminal
Current flow in a semiconductor consists of the movement of both electrons and holes
The amount of current flow is determined by the number of electron-hole pairs
Doping for semiconductors
A process to increase conductivity
Doping is the process of adding impurities to a semiconductor material
- Pentavalent, atoms with five valence electrons
– N-type material
— Has more electrons than holes
— Negative charge is the majority carrier
— Free electrons flow toward the positive terminal
- Trivalent, atoms with three valence electrons
– P-type material
– Has more holes than electrons
— Positive charge is the majority carrier
— The holes move toward the negative terminal
Diodes, created by joining N and P-type materials together
PN Junction Diodes
Junction diodes are created by joining N-type and P-type materials together
The depletion region is the area near the junction
The charge at the junction creates a voltage called a voltage barrier
Current flows through a diode only when the external voltage is greater than the barrier voltage
A diode that is forward-biased conducts current
A diode that is reverse-biased conducts only a small leakage current
A diode is a one-directional device
The arrow on a diagram means the flow of holes
P-Type is called Anode
N-type is called Cathode
Transistor construction
A bipolar transistor is produced when a third layer is added to a semiconductor
It can amplify power, current or voltage
Also called a junction transistor or transistor
Can be constructed of germanium or silicon (more popular)
Consists of three alternately doped regions
The regions are arranged two ways
- P-type material is sandwiched between two N-type materials NPN transistor
- N-type material is sandwiched between two P-type materials PNP transistor
It has three points, emitter, base and collector
current flows 100% in emitter and 99% in collector
Transistors are identified by a number
- Begin with 2N and up to four more digits
- Identify the device as a transistor
- indicate that it has two junctions
The basic functions of a transistor are
- to provide current amplification of a signal
- to switch a signal
A transistor must be properly biased
- The emitter junction is forward-biased
- The collector junction is reverse-biased
Junction field effect transistors (JFETs)
A unipolar transistor that functions using only majority carriers
Voltage operated
Constructed from N-type and P-type semiconductor materials
Capable of amplifying electronic signals
JFETs have three electrical connections
- One lead is connected to the substrate to form the gate (G)
- One lead is connected to each end of the channel to form the source (S) and the drain (D)
JFET requires external 2-bias voltages to operate
- One is connected between the source and the drain, forcing the current to flow through the channel
- One is connected between the gate and the source, controlling the amount of current flowing through the channel
The side of the depletion region is controlled by the voltage gate-source
- As the voltage increases, so does the depletion region
- As the depletion region increases, the size of the channel is reduced, thus reduces the amount of current flow and can be used to control drain flow
The amount of gate to source voltage required to reduced the drain current to zero is the gate to source cut off voltage
The drain to source voltage has control over the depletion region within the JFET
- As the voltage increases, so does the current
- A point is reached where current levels off, even though voltage still increases
- The value of the drain source voltage required to pinch off or limit the drain current is called pinch off voltage
Metal oxide semiconductor field effect transistors (MOSFETS)
Do not use PN junction
Use a metal gate
- electronically isolated from the semiconductor channel by a thin layer of oxide
- Two important types of MOSFETs
– N-type units with N channels
— Called depletion mode devices
— They conduct when zero bias is applied
– P-type units with N channels
— Called enhancement mode devices
— Electron flow is cut off until it is aided or enhanced by the bias voltage on the gate
Silicon controlled rectifiers (SCR) AKA Thyristors
Three terminals
- anode
- cathode
- gate
Used primarily as switches
Controls current in only one direction
A power transistor would require ten times the trigger signal of an SCR to control the same amount of current
Constructed of four alternately doped semiconductor layers
- Three junctions are formed
A small gate current can control a large load current
Amplifier
Electronic circuits used to increase the amplitude of an electronic circuit
Amplifier configuration
- The transistor can be connected in 3 different circuit configurations
– The common base circuit
– The common emitter circuit
– The common collector circuit
- One lead serves as a common reference point and the other two leads serve as input and output connections
- Each configuration can be constructed using NPN or PNP transistors
- The transister’s emitter base junction is forward biased
- The collector base junction is reverse biased
- The common emitter circuit is the most widely used, 180 degree phase shift
