Transcript Lab #1: Ohm’s Law (and not Ohm’s Law)

Lab Report
Don’t forget: this is one of the 3 labs that
requires a formal lab report!
Lab 2
Fabula
Brower
Fowler
Flair
Gima
Gallagher
Germain
Garhart
Loman
Henry
Quelland
Lambert-Brown
Robinson-Tillenburg
Reiser
Shetty
San Miguel
Someshwar
Rogers
Stillwell
Stahl
Abdulkadir
Stefany
Belenky
Whittemore
Lab #2: Diode and Rectifier Circuits
• learn what diode is and how it works
• Measure the “turn-on voltage” for some
different diode types
• learn what rectification is and how to make a
rectifier with diodes
• learn the relationship between AM and
rectification
Crystal radio (AM radio)
Where is the
crystal? No
longer in there.
Modern crystal
radios use
diodes instead.
Will start
discussion today
on purpose of the
diode.
•Energy levels of electrons in
atoms are quantized (1s, 2s, 2p,
etc)
Diode
•When these atoms are brought
together in crystals, these energy
levels become energy bands with
gaps between them.
• Because of the “Pauli exclusion
principal” no 2 electrons can have
the same quantum numbers.
• The distributions of electron
energies depends on the
temperature. The highest energy
when T=0 is called the Fermi
Energy (EF).
• some materials can be made into
semi conductors by adding small
amounts of impurities. p-type (one
fewer valence e than Si) and ntype (one more valence than Si).
• semi conductors do not conduct
at T=0.
1 eV
Diode
104 – 106 V/cm, 0.3-0.6 V
Electrons migrate from n to the holes in p, giving a net
charge to each side (neg on p side, pos on n side)
Current will flow from p to n with a little help to
get across that potential barrier at the
junction.
Diode
+
-
Here, the external bias aids the
electrons trying to go from n-p and you
get a net current in the p to n direction.
(when the voltage is larger than the
gap voltage, typically 0.6 V) Holes in p
move towards junction, depleting it.
When it becomes small enough,
electrons cross into the p type region
and combine with holes there. Holes
flow in the opposite direction and
combine with electrons on the n side.
current flows.
-
+
Here, the external voltage stops the
electrons going from n to p. The
ones going from p-n can still go and
you get a (small) net current from n
to p (the prob to get enough thermal
energy to break the bond is
unchanged)
Current-Voltage (IV) Characteristic
of a Diode
qV /kT
I = I 0 (e
- 1)
V /VTH
- 1)
I = I 0 (e
VTH has strong
dependence on
temperature
Diode Current-Voltage Characteristic
Not to Scale
In the first part of this lab we will use data in the low V region to get VPN and Rf
Half-wave rectifier
In the second part of the lab, we’ll look at a simple
useful circuit using diodes.
Full wave rectifier
Vsource
Vsource
Vload
New equipment
Instrumentation (differential) amplifier:
• Your scope probe, like a volt meter, has two
connections. However, for your scope, one of
these is hardwired to ground.
• If you want to measure the voltage across
something, and neither side is at ground, you can:
• Use the AC coupling on the scope
• Use the instrumentation amplifier.
Amplitude Modulation (AM)
Carrier:
Signal (for now, a pure tone):
Modulate the amplitude of the carrier with the signal.
Modulated wave:
Reference: Elements of Television Systems, George E.
Anner, Prentice-Hall Electrical Engineering Series,
Englewood Cliffs, N.J., 1951
Modulated wave:
Some math to rearrange to a more useful form:
Remember that
3 frequencies: carrier, upper side-band, lower side-band
The bandwidth required is
encoding
http://www.radio-electronics.com/info/rf-technology-design/am-amplitudemodulation/what-is-am-tutorial.php
decoding
Due to bandwidth limitations of the ear phone. It
averages over the fast oscillation and gets zero
without the diode. Modulating frequency is
“slow”. Carrier is “fast” compared to the
response of the headset.