Transcript Phys 345 Electronics for Scientists

Announcements
• Assignment 1 solutions posted
• Assignment 2 due Thursday
• First mid-term Thursday October 27th (?)
Lecture 8 Overview
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Inductors in transient circuits
Semiconductors
Diodes
Rectifying circuits
Other Diode Applications
Transistors
Time response of Inductors
Switch to position a:
  VL  iR  0
 L
dI
 iR  0
dt
iL 
di
dt
1
di
dt 
L
  iR

R
, vL  
  iR  L
i

R
(1  e
vL  e


Rt
L
Rt
L
)
)
Integrate and apply boundary condition t=0, i=0
i

R
(1  e

Rt
L
)
Time constant τ=L/R.
Switch to position b:
i

R
e

Rt
L
Talk about "Charging a capacitor"
"Current build-up" in an inductor
Time response of Inductors
A battery is connected to an inductor. When the switch
is opened does the light bulb:
1.Remain off
2.Go off
3. Slowly Dim out
4. Keep burning as brightly as it did before the switch
was opened
5. Flare up brightly, then dim and go out
Answer 5
Semiconductors
Simple review of basic concepts: What is a semiconductor?
(for more detail see e.g. Simpson Ch. 4)
Elements such as Silicon and
Germanium have 4 valence
electrons in their outer shell
They form covalent bonds
with neighbouring atoms to
form strong crystal lattice
structures.
In pure silicon, all valence
electrons are bound in the
lattice structure
Semiconductors
The addition of impurities
("doping"), such as Sb(Antimony)
with 5 valence electrons, leaves
one electron unbound and free to
move and create a current flow
(n-type semiconductor).
Alternatively, an impurity with 3
valence electrons can be used to
create positive "holes".
When a p-type and an n-type are
joined (p-n junction), mobile
electrons diffuse from the n-type to
the p-type, forming positive and
negative ions at fixed positions in a
state of equilibrium which inhibit
further transfer of electrons
(depletion region)
-
depletion region E-field
+
(~0.2V Ge, ~0.5V Si)
What happens when you apply a voltage?
For the device to conduct, electrons from the n-type region must cross the junction
Depletion region E-field
Reverse bias: Apply an electric field
in this direction, mobile electrons
are driven away from the junction
(unlike fixed charged ions). Mobile
holes are also driven away in the
opposite direction. Depletion region
acts like an insulating slab - No
current flows
Forward bias: Helps electrons
overcome the depletion region.
Current flows easily
applied E-field
Depletion region E-field
applied E-field
Ideal diode
• A diode is a non-linear circuit element
• Only passes current in one direction
• Constructed from a p-n semiconductor junction
Real diode
Diode law:
IS = reverse-leakage current
v = voltage across the diode
kB = Boltzmann's constant
e- = electron charge
T = Temperature (K)


i  I S e v vT  1
k BT
vT  
e
Strong dependence on T
IS is small ~ 10-6A (Ge), ~10-8(Si)
Diode Circuit
• Diodes are non-linear; how do we
calculate the operating conditions?
(Can’t easily use V=IR)
•Consider the simplest diode circuit
I D  I S eVD VT
KVL:
VDD - ID R -VD = 0
VDD  VD
ID 
R
When ID=0; VD=VDD
When VD=0; ID=VDD/R
Must satisfy both equations:
Operating point can be
calculated by seeing when
diode law line intersects
load line
Rectifying Circuit
Ideal transformer: VS/VP=NS/NP
Real transformers are ~98% efficient
One of the most important applications of a diode is in rectifying circuits: used
to convert an AC signal into the DC voltage required by most electronics
Half-wave rectifier
• Only lets through
positive voltages.
Rejects negative
voltages
Full-wave rectifier
• To use both halves of the input sinusoid,can use a
centre-tapped transformer:
-
+
-
+
+
e.g. Battery Charger
or use a Bridge rectifier
• Does not require centre-tapped transformer
• Requires 2 diodes in each direction – cheap, but
voltage drop is double
Bridge rectifier
• Current flow in the bridge
-
vO
+
-
vO +
Peak rectifier
• Most devices need steady DC
• To smooth out the peaks and obtain a DC voltage
When source voltage < capacitor voltage
Diode is reversed biased
Capacitor discharges through resistor
Another diode application: Voltage doubler
• High Voltage transformers are expensive and impractical at
voltages above a few thousand Volts. How do we get higher?
C2 charges to Vsec
C1 charges to 2Vsec
Voltage doubler
• Can extend this circuit to produce
extremely high voltages (~750kV).
Voltage Quadrupler
• Cockroft-Walton voltage multiplier
•1932, Cavendish Labs
• reached 250 kV
• Accelerated protons onto a Lithium target
• Split the atom!
•http://www.youtube.com/watch?v=RdBqMtioh6U