Transcript Basic Electronic Circuits

TDC 311
Basic Electronic Circuits
Voltage
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A battery / generator has + and posts. Electrons flow through a circuit
from the - post to the + post.
The potential difference between the
two posts is V, voltage. V is the push
of electrons.
The higher the voltage, the higher the
push. (Figures 1 and 2)
Current
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The current (I) is the charge passing through a
given cross section of wire per unit time:
I = Δ Q / Δ t = charge / time
Consider a hose versus a fire hydrant.
The direction of a current is in the direction of
positive charge motion. Since only electrons (the
negative charge) move within metal, the direction of
current is opposite the flow of electrons.
For example
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House current
Typical CPU
Lightning strike
Resistance
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All systems experience resistance (R).
Ohm’s Law: V = I*R where R =
Ohms (Ω)
or R = V / I
As temperatures increase in metal,
resistance increases. In
semiconductors, resistance generally
decreases. (Figures 3 and 4)
Watt
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The measure of electric work and
power is the watt.
Power = V*I
A current of 0.50 A flows through a
200 ohm resistor. How much power is
lost in the resistor? (Expression 1)
AC and DC
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All above examples are direct current
(DC). The push of electrons is always
in one direction.
AC - alternating current: The push of
electrons is first from one direction and
then from the opposite direction, over
and over. (Figure 5)
Two ways to control power
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1. Control the amount of power put into the
circuit (harder to do)
2. Control the power at some point other
than at the source (more common). (Figure 6)
Then we can switch it on or off, or
regulate/vary the resistance.
Circuits are made of hundreds / thousands /
... of points that need switching and
regulating. What device does this?
A vacuum tube? Yes, but something better
Transistor
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The transistor is made of either
germanium or silicon and has three
distinct sections (Figure 7)
The NPN transistor can act as a
variable resistor or as a switch
(conduct current, throttle it partially, or
block it entirely).
Consider the following circuit
-
+
Emitter (N)
Base (P)
Microphone
Collector (N)
Figure 8
Base P blocks flow from emitter to collector.
Need to add a wire from base so electrons have some place to flow.
Speaker
More appropriate figure
-
Microphone
+
Speaker
Figure 9
Basic logic gates
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How were relays used to create basic
logic gates? (Figures 11 and 12)
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How are transistors used to create
basic logic gates? (Figures 13 and 14)
Equivalent Transistors
Consider the numbers
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1GB flash drive
230 bits (about 1 billion)
Each bit requires 2 transistors
About 2 billion transistors
Future Trends
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Semiconductors are approaching
fundamental physical size limits
Technologies that may improve
performance beyond semiconductor
limitations
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Optical processing
Hybrid optical-electrical processing
Optical Processing
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Could eliminate interconnection and
simplify fabrication problems; photon
pathways can cross without interfering
with one another
Eliminating wires would improve
fabrication cost and reliability
Not enough economic incentive to be a
reality yet
Electro-Optical Processing
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Devices provide interface between
semiconductor and purely optical
memory and storage devices
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Gallium arsenide (both optical and
electrical properties)
Silicon-based semiconductor devices
(encode data in externally generated
laser light)
Inductor
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An inductor is basically a coil of wire.
While it looks simple, it has some
interesting properties (Figure 15)
If you remove the inductor from the
circuit, this is just a flashlight.
What happens when you insert the
inductor into the circuit?
Inductor
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When you close the switch, the bulb
burns brightly and then gets dimmer.
When you open the switch, the bulb
burns brightly, then quickly goes out.
Why?
Inductor
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The wire in the coil (inductor) has less
resistance than the light bulb. But the coil
wants to build up a magnetic field. While
the field is building, the coil inhibits the flow
of current. Once the field is built, the current
can flow normally through the coil.
When the switch is opened, the magnetic
field around the coil keeps current flowing in
the coil until the field collapses. This keeps
the bulb lit for a short period of time after the
switch is open.
Inductor
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The capacity of an inductor is
controlled by four factors:
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number of turns of wire
material the coils are wrapped around
cross-sectional area of the coil
length of the coil
Inductor
Putting iron in the core of an inductor gives it
much more inductance than air would.
 The standard unit of inductance is the
henry:
H=(4*pi*NumTurns2*AreaOfCoil*m) /
(LengthOfCoil * 10,000,000)
where m=permeability of core (air=2;
steel=2000)
 Common application area: traffic signals
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Capacitor
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A capacitor is a little like a battery. It holds a
charge, but only for a brief moment.
A capacitor has two metal plates separated
by a dielectric (such as air, paper, plastic, or
anything else that does not conduct
electricity).
What happens when you connect a
capacitor to a battery? (Figure 16)
Capacitor
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Once the capacitor is charged, it has
the same voltage as the battery.
Let’s hook up a capacitor, a battery, a
bulb, and a switch in the following
fashion: (Figure 17)
Capacitor
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When the switch closes, the light bulb lights
up as current flows from the battery to the
capacitor and the capacitor charges up.
The bulb gets progressively dimmer and
finally goes out once the capacitor reaches it
full charge.
When you open the switch, the light bulb
brightens momentarily and then dims and
goes out. The capacitor is now discharged.
Capacitor
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The farad is the unit of capacitance.
A farad is one coulomb (6.25e 18 electrons)
of charge at 1 volt. That is a lot of charge!
Most capacitors are rated in microfarads.
Common capacitor applications include
storing charges for high speed use, such as
in a flash of a camera.
Capacitors can also smooth (eliminate)
ripples in current.
A capacitor can block DC voltage and let AC
voltage through.
Diode
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A diode is a simple device that allows
electrons to flow in one direction only
(Figure 18)
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Diodes can be used to make sure
current flows in one direction only, so if
you put your batteries in backwards, it
does not ruin your electronic device.
Diodes can also be used to smooth out
AC voltages (Figure 19)
Serial vs parallel circuits
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Circuits can be either serial or parallel.
Serial circuits: Figures 20 and 21
For example, resistors in series add
directly
Parallel circuits: Figure 22
Resistors in parallel: add their
reciprocals and take reciprocal of total
Magnetic Fields and Wires
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Oersted discovered that currents in wires
produce magnetic fields.
Right-hand rule: Grasp wire with thumb
pointing in direction of current. Fingers
point in direction of magnetic field.
Furthermore, a changing magnetic field
passing through a wire will induce a current
in that wire.
Crosstalk
Memristor?
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Concept introduced in a paper in 1971; first
memristor created in lab April 30, 2008
First basic element since resistor, capacitor
and inductor
Very small; can hold a 1 or 0 with or without
power
Can be fashioned into non-volatile solid
state memory
100 gigabits in a square centimeter at one
tenth the speed of DRAM