Transistor
BJT Transistors:
NPN
Transistor
PNP
Transistor
Sandwiching a P
-
type layer
between two n
-
type layers.
Sandwiching a N
-
type layer
between two p
-
type layers.
How a “NPN” Transistor works?
Forward
backward
The base
-
emitter diode
(forward) acts as a switch.
when v1>0.7 it lets the
electrons flow toward
collector. so we can control
our output current (Ic) with
the input current (Ib) by using
transistors.
C
B
E
Collector
Emitter
Base
Transistors have three terminals:
Transistors work in 3 regions
Active: Always on
Ic=BIb
Saturation :Ic=Isaturation
On as a switch
Off :Ic=0
Off as a switch
Transistor as a Switch
•
Transistors can be used as switches.
1
•
Transistors
can
either
conduct
or
not conduct
current.
2
•
ie, transistors can either be
on
or
off
.
2
Transistor
Switch
Transistor Switching Example
15
•
When
V
BE
is
less
than
0
.
7
V
the
transistor
is
off
and the lamp does not light.
•
When
V
BE
is
greater
than
0
.
7
V
the
transistor
is
on
and the lamp lights.
X
Variable
Voltage
Supply
12V
Transistor Circuit : Light
-
Controlled Circuit
•
This
transistor
circuit
contains
a Light
-
Dependent Resistor.
•
Because
of
the
LDR,
this
circuit
is
dependent
on
light
.
•
The
purpose
of
this
circuit
is
to
turn
on
the
LED
when
the
light
reaches
a
certain
intensity
.
Input
=
Voltage
Divider
Process
=
Transistor
Output
=
LED
1)
LED
=
Off
.
2)
Cover
LDR
.
3)
R
LDR
.
4)
V
LDR
.
5)
Transistor
switches
on
.
6)
LED
=
On
.
Transistor as an amplifier
:
Transistors are often used as amplifiers to increase input signal in
radios, televisions and some other applications .The circuit may be
designed to increase the current or voltage level.
The power gain is the product of current gain and voltage gain
(P=V*I).
Amplifier example:
As you see, the transistor is
biased to be always on. The input
signal is amplified by this circuit.
The frequency of output is the
same as its input, but the polarity
of the signal is inverted.
The measure of amplification is
the gain of transistor.
Example:
Input Amplitude =0.2v
Output amplitude=10v
Gain=10/0.2=50
Field Effect Transistors
JFET
MOSFET
CMOS
When the gate is negative ,it repels the
electron in the N
-
channel. So there is
no way for electrons to flow from
source to drain
.
When the negative voltage is
removed from Gate ,the electrons
can flow freely from source to drain
.so the transistor is on.
How a JFET transistor works?
When the Gate is positive voltage ,it allows electrons
to flow from drain to source .In this case transistor is
on.
In MosFET, the Gate is insulated from p
-
channel or n
-
channel.
This prevents gate current from flowing, reducing power usage.
How a
MOSFET
Transistor works?
How a CMOS transistor works?
When Gate (input) is high ,electrons can
flow in N
-
channel easily . So output
becomes low. (opposite of input)
When Gate (input) is low ,holes can
flow in P
-
channel easily. So output
becomes high.
(opposite of input)
N
-
channel & P
-
channel MOSFETs can be
combined in pairs with a common gate .
SWITCHING
•
The modern computer is a digital and electronic device.
•
Runs on flow of electrons
-
movement of electrons from outer valence
shells on atoms
•
Requires force/energy to pull an electron out of a shell
-
Electro
-
motive
force (EMF)
-
Voltage
•
How tightly the electrons are held determines how much voltage is
needed and is quantified as resistance
•
Number of Electrons flowing is quantified as Amperes
•
Computers are electronic switching devices
•
Voltage source (battery)
•
Meter to detect voltage differential
0
5
0
5
0
5
Short Circuit
large voltage flow, but no
voltage differential
Digital Switch
-
Transistor
•
Computers are constructed from transistors
•
Transistor are used as on
-
off switches (0
-
off, 1
-
on), hence binary
0
5
0
5
Transistor controlling the circuit
0
5
0
5
Measuring Output before the transistor
•
Acts as an
invertor
•
No voltage on the input causes the meter to read 5 volts
•
A voltage on the input causes the meter to read zero
0
5
0
5
NAND
–
Not AND
A
B
Out
0
0
1
0
1
1
1
0
1
1
1
0
0
5
A
B
0
5
A
B
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