Capacitors, diodes, transistors

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2 Νοε 2013 (πριν από 3 χρόνια και 7 μήνες)

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1  
Capacitors,  diodes,  transistors  
CHM6158C  -­‐  Lecture  
3
 
 


capacitors


charging and time response


filters


(impedance)


semi-conductor diodes


rectifiers


transformers


transistors
2  
Capacitors  
Symbol
3  
Capacitors  
Device consisting of two conducting plates
separated by an insulating material

By e.g. pulling away electrons from one
side (Q+), build up electric field, which pulls
electrons onto Q-.

Unit of capacitance is Farad (F) = C/V
(electrical charge that can be built up for a
particular voltage)

Typically

F (10
-6
), nF (10
-9
), pF (10
-12
)
Dielectric material = insulating
material that contains polar
molecules that orient with the
electric field (energy storage)
What is the resistance
of a capacitor?
4  
RC  circuit  
Charging up a capacitor takes time
Every circuit has a
resistance, which
slows down the
flow of electrons
))
/
exp(
1
(
RC
t
V
V
i



Why?
What will the current
plot as a function of time
look like?
What will be the
DC potential
here?
5  
Capacitors  in  parallel  
Capacitances in parallel add up like resistors in series.
b
Electron flow from the same circuit has access to both
sides of the capacitors
`

e.g. C
1
= 0.100

F, C
2
= 0.010

F and C
3
= 0.001

F,
what is the total capacitance?
)
(
1


N
i
total
C
C
6  
Capacitors  in  series  
Capacitances in series add up like resistors in parallel.
Electron flow from the same circuit do NOT have access
to both sides of the capacitors
e.g. C
1
= 0.100

F, C
2
= 0.010

F, what is the total
capacitance?


N
i
total
C
C
1
1
1
Total capacitance
lower than sum of
capacitances.

capacitors
adversely affect
each others
`
ability
to store charge.
7  
AC  and  capacitors  
In alternating current, orientation of
electron flow keeps changing.
Note that current leads
voltage by 90
o
.
What will happen if RC time constant
is slower than AC frequency?
dt
dv
C
i

8  
Voltage  response  
At any moment in time,
total voltage V
r
+ V
c
= E,
but relative voltages for
V
r
and V
c
change as a
function of time.
What is RC in this case?
Remember that V
c
as a
function of time looks like this:
What will be corresponding
voltage across resistor as a
function of time?
9  
Voltage  response  
http://www.ac.wwu.edu/~vawter/PhysicsNet/
Topics/DC-Current/gifs/Circuits50.gif
Initially, total voltage on resistor – why?
What will happen for an ac voltage source, if frequency
is low, frequency is high?
10  
High-­‐  and  low-­‐pass  filter  
For high-frequency signal, where will
signal be transmitted?
For low-frequency signal, where will signal
be transmitted?
More on noise and filters later
11  
Note  on  
impedance
:  AC  equivalent  of  DC  resistance  
Impedance = measure of opposition to alternating current
AC voltage source
What does voltage response
compared to current look like?
12  
Impedance  
Pure resistance circuit:
For capacitance-only circuit:
Current leads voltage
dt
dv
C
i

Does this make sense based
on formula?
More on impedance in
lecture 17
13  
(Semi-­‐conductor)  Diodes  
+
-


doped material causes local charge
imbalance between P (holes) and N
(electrons)


in
l
forward
z
bias, holes and electrons
combine at P-N junction


in
l
reverse
z
bias, a depletion region is
created, which impedes conductance

current flow occurs in one direction
only
What
happens
here?
14  
ApplicaQon:  RecQfier  circuits:  AC  →  DC  


Only ac phase in forward
direction leads to flow of current
Draw which way does electricity flows
(i.e., holes)
What will the DC voltage look
like? How can this voltage be
made more continuous?
15  
RC  circuit  to  create  smoother  DC  
RC >> 1/f
16  
Transformer:  converQng  ac  voltage  
115 ac
P
S
P
S
N
N
V
V

No. of coils
Principle of
…………………….
115 V ac converted to 12 VDC:
Notice that current in
secondary winding flows in
opposite direction
17  
BJT  (Bipolar  JuncQon  Transistor)  
NPN transistor = two diodes with a
shared anode (+)

Transistor = device that
switches

or
amplifies
electronic signals
18  
Bipolar junction transistor (BJT):
pnp
or
npn
Common
emitter
circuit
:
Base current I
B

controls collector
current I
C
. I
C
=
β
I
B
Note design of
pnp junction to
allow much
larger current
flow from
emitter to
collector (i.e.,
very narrow n
layer)
6th Ed., Douglas A. Skoog, F. James Holler and
Stanley R. Crouch, Thomson Brooks/Cole, 2007.
Gain
BJT  (Bipolar  JuncQon  Transistor)  
19  
NPN  transistor:  common  base  
Source: Wikipedia
Gate: base-emitter

Larger current: collector-emitter
Smaller current (base-emitter) acts as a gate to allow a larger current
(collector-emitter). Collector-emitter current
α
base-emitter current
Draw this circuit diagram
20  
Transistor  amplifies  current  
I
C
= h
FE
I
B
Collector current I
C
is
h
FE
times
larger

than base current I
B
l
Art of Electronics
z

21  
Other  examples  
Which way does
I
B
flow?
Common collector circuit
Transistors are essential units in integrated
circuits (IC
`
s), which are at the basis of
computer operation.