Download The Transistor

TRANSISTORS (STAR OF SHOW)
Power amplification comes from transistorother components needed for transistor to work.
Gate
Supporting cast (with symbols):
a. transformers to get different voltages.
b. few other little things not very important.
c.resistors- control currents, divide up voltages as desired.
d. diodes- make AC voltages from transformers into the DC
voltages transistors need.
e. capacitors- pass AC while blocking DC. Also store electric
charges.
Learning goals:
1) Basic physics of d. and e. General idea how they are used.
2) Basic function of transistors as adjustable current gate (voltage controlled
resistance) and how this allows amplification of signals.
3) NOT expected to learn detailed physics of transistor, will cover briefly.
diodes- need to convert AC to DC. Transistors insist.
Pass current in only one direction. Junction of P doped and N doped
semiconductors. anyone understand textbook explanation?
p type semiconductor
pure semiconductor
empty
empty
Si +
tiny fraction
boron
1/boron atom
full
full
Si, 4 electrons go into
valence band, just fill
n type semiconductor
B, 3 electrons go into valence band,
leave level unfilled, tiny
fraction of total (10^16/10^23 /cm^3)
empty
P, 4 electrons go into valence band, 1
extra goes up into cond. band
full
Si + tiny fraction Phosphorus
How will they conduct? a. all about same.
b. pure best, P and N less, c. pure no, P and N
about same, d. only N conducts, e. only P cond.
p type semiconductor
n type semiconductor
empty
empty
full
full
Si + tiny fraction Phosphorus
ans. c. n and p type both conduct ok
(not great), pure Si not conduct.
n- electrons in conduction (top)
band can move.
p- electrons at top of valence (lower) band
can move into empty levels.
DOPING--EXTRA OR MISSING FREE ELECTONS, NOT(!) EXTRA
CHARGE. ARE ALSO EXTRA OR MISSING PROTONS! Book leaves out!!!
pure, no flow when tilt
n, water in top flow
p, water in bottom flows,
but if small bubble, see it.
Doped semiconductors, add or subtract some charges to allow
charges to move.
N type- atoms with extra free electrons
- -- -ee e e e e e e
I
- - - - in nearly empty band move easily
+
V
_
lower level full
DOPING--EXTRA OR MISSING FREE ELECTONS, NOT(!) EXTRA
CHARGE. IS ALSO EXTRA OR MISSING PROTONS! Book leaves out!!!
++ +++
+ ++ + +
+
V
_
I
electrons missing,“holes”,
leave room to move easily
http://www.colorado.edu/physics/phet/projects/semiconductor/v7/semiconductor.jnlp
P- N junction- put together
n type
+ +++ - -- -++ + + - - - - full
+ +++ - -- -++ + + - - - - -
+ ++
++ +
extra neg’s
-- -- - - -
electrons diffuse to left
(lower energy), fill up holes
to annihilate moveables
at border
depletion layer- no
moveable charges
charge buildup--keeps rest of
moveable -’s on right side.
Balances energy they get when
going down.
+ ++
++ +
-- -- - - -
turn on voltage,
what happens?
+
V
_
a. electrons flow from left to
right.
b. electrons will not flow
c. electrons will flow from
right to left
---
+++ + + ++-- - - - - -+++ + + + +- - - - - - --
+
V
I
_
V
+
turn on voltage,
what happens?
+s get shoved into depletion
-s get shoved into depletion
= no more depletion region
moveable charges carry current.
V has to be bigger than 0.6 V to
overcome electrostatic barrier from charge.
+ ++
++ +
-
---
-- -- - - -
turn on opposite voltage,
what happens?
+ ++
++ +
-- -- - - -
turn on opposite voltage,
what happens?
V
+
a. electrons flow from left to
right.
b. electrons will not flow
c. electrons will flow from
right to left
+ ++
++ +
-
V
+
-- -- - big pile
of negative
charges, no moveable
charges
-
turn on opposite voltage,
what happens?
Both +s and -s pulled away
from junction. Depletion
region bigger. Current
can’t flow.
I
So V > 0.6 V in one direction, big current.
V in opposite direction, no current.
Diode is one-way street for current flow.
can also produce light (LED= light emitting diode)- thurs.
Light-emitting diodes
Seen in stoplights, bike lights, some new flashlights
Diodes that emit light
n-doped
-- -- -- -- --
p-doped
http://www.howstuffworks.com/index.htm
BACK TO THE STAR OF THE SHOW: The Transistor•
•
•
like diode, made up of N and P-doped semi-conductors
also has depletion region that controls current flow in transistor
But now we have a separate voltage signal (CD signal) that controls
size of depletion region. We can adjust it up and down, and vary how
much current flows through.
Transistor Amplifies!
A little change in amount of
charge on gate  Makes big
change in current (through
transistor) … varies
resistance.
transistor
Like water faucet knob, valve for current!
Changing signal at Gate … like turning knob …
changes current!.
Gate
(Valve control)
Note: NO direct connection..
NO current flow through here!
+
Gate, change amount
of positive charge at
gate, control valve.
e’s
transistor like plunger valve on spring that blocks flow of water as moved
in and out. Big voltage applied is like pulling plunger back, big current,
zero voltage lets plunger block flow, no current.
water flow
little push of valve in or out changes
big volume of water (current) that
flows.
+++
+++
voltage
off, NO +excess
+
Lots of excess
charges
charges,
plunger closes,
on gate (voltage)
current
plungeroff
out, current on.
Transistor RESISTANCE IS
Transistor RESISTANCE IS
LARGE. (behaving like
SMALL.
insulator now)
What you are required to know about transistors.
Electrically transistor is like a resistance whose value depends on charge voltage
applied to the gate (i.e. plunger). Power amplification. Little work to move plunger
can change large amount of energy flow contained in large water flow or
equivalently large current flow through transistor.
Transistors amplify signals
toward
speaker
+ 10 V
V
lots of power!!
Resistor #1
Tiny
Signal
(Power provided by
10 V power supply)
D
Gate
time
(Resistance of transistor
changes with Gate voltage)
+0V
Voltage at D depends on voltage split between
Resistor #1 and resistance of transistor.
But resistance of transistor depends on voltage on gate!
So voltage at D changes with voltage at Gate.
Vary voltage (positive charges) on A  …. Use resistors to
control voltage at A!
++++9 V
+++
If increase amount of positive charge
on gate, voltage at A will:
a. Decrease
b. Increase
c. Stay the same
50ohms
+++
4.5V
Gate
+
Hint: Think about cause and effect!
A
+++
To speaker
Answer is a. Voltage at A will decrease
(gets closer to 0 Volts). Resistance of
transistor decreases, more current,
bigger voltage change across 50 ohms
than across transistor.
GROUND
short incredibly clear summary of how
transistor works in terms of P-N
semiconductor junctions…
V control
insulator
control electrode (gate)
+ + + + + + - - - - + + + + + + P-N-P doped semicond.
+ + + + + + - - - - + + + + + “P type” transistor(?!)
put it together, free holes and electrons migrate,
big depletion region.
V control =0 V
+++++
+ + + +
+ +++++
++ + +++
V
What happens if
+ voltage on left?
V control =0 V
+++++
+ + + +
+ ++++
++ + +++
+
-
V
+++++
+ + + +
+ +++++
++ + +++
-
V
+
What happens if
+ voltage on left?
+s all move to right,
depletion region shifts
over but still big and
depleted, so no current.
What happens if
+ voltage on right?
v control = 0 V
++++++
+ + + + +
+ +++++
++ + +++
-
V
+
V control =+ big voltage
++++
+++++
+ + + +
+ +++++
++ + +++
V
What happens if
+ voltage on right?
Everything moves to
the left, still depletion,
no current flow.
What happens big
voltage on gate?
V control =+ big voltage
++++
+++++
+ + + +
+ +++++
++ + +++
What happens?
+s repel +s, so depletion
region gets bigger, no I.
I
V
What happens if big negative
control voltage on gate?
sucked up to -s
V control =- big voltage
but stop at insulator
------
++++++
+ +++++++ +++++++
+ + + + + + + ++ + + + + +
+s fill in, depletion
region goes away,
current flows easily
in either direction.
I
V
If moderate negative voltage, just reduce but not remove depletion,
a few ++s to carry charge. Less current (i. e. more resistance.)
So amount of current depends on size of negative control
voltage. If zero or positive no current (like open switch).
If big, like closed switch (no resistance).
In between, current varies as negative voltage varies
but gives amplification.
But what if want to amplify positive voltages?
Reverse the P and Ns
V control
control electrode (gate)
- - - - - - -- - + + + - - - - - - - -------- +++ - - --- --
N-P-N doped semicond.
“N type” transistor(?!)
everything the same with charges and voltages reversed.
television:
camera
electrical
signal broadEM waves received at
cast as radio
antenna. Converted into
wave.
electrical signals (voltage
(low freq.
vs time) Electrical signals
electromagnetic)
converted into image.
image to
electrical
signal
object
Learning goals:
•Be able to explain the basic features of a color TV.
•Be able to design system for controlling energy and pointing of electron
beam.
•Explain how TV signals get to your house and how they are detected.
Guiding principle of TV: the human eye is crummy.
take advantage of this and send as little information as possible
to make viewer think is real color moving picture.
Eye deficiencies:
a. fuzzy- leave out almost all detail from TV picture, just use
bunch of dots.
b. slow- just flash dots up, then change.
c. poor color sense (3 sensors)- just use three colors to trick
into thinking full color range (red, green, and blue).