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2 Νοε 2013 (πριν από 4 χρόνια και 11 μέρες)

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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


.


V
LDR


.


T牡sistor

switches


.


䱅L

=


.

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 .

Opamp

Schematic diagram of lm741

Ideal Opamp

Operational Amplifier (OP AMP)

Basic and most common circuit
building device. Ideally,

1.
No current can enter terminals
V
+

or V
-
. Called
infinite input
impedance
.

2.
V
out
=A(V
+

-

V
-
) with A



3.
In a circuit V
+

is forced equal
to V
-
. This is the
virtual
ground

property

4.
An opamp needs two voltages
to power it V
cc

and
-
V
ee
. These
are called the
rails
.

A

Vo = (A V
-
A V )


= A (V
-

V )

+

+

-

-

INPUT IMPEDANCE

WHY?

For an instrument the Z
IN

should
be very high (ideally infinity) so
it does not divert any current
from the input to itself even if
the input has very high
resistance.

e.g. an opamp taking input from a
microelectrode
.

Input


Circuit

Output

Impedance between
input terminals =
input impedance

OUTPUT

IMPEDANCE

Input Circuit Output

Impedance between output terminals
=

output impedance

WHY?

For an instrument the Z
OUT

should
be very low (ideally zero) so it can
supply output even to very low
resistive loads and not expend
most of it on itself.

e.g. a power opamp driving a motor

OPAMP: COMPARATOR

V
out
=A(V
in



V
ref
)

If V
in
>V
ref
, V
out

= +∞ but practically
hits +ve power supply = V
cc

If V
in
<V
ref
, V
out

=
-
∞ but practically
hits

ve power supply =
-
V
ee

V
cc

-
V
ee

V
IN

V
REF

Application: detection of QRS complex in ECG

A (gain)
very high

OPAMP: ANALYSIS

The key to op amp analysis is simple

1.
No current can enter op amp input terminals.

=> Because of infinite input impedance

2.
The +ve and

ve (non
-
inverting and inverting)
inputs are forced to be at the same potential.

=> Because of infinite open loop gain

3.
These property is called “virtual ground”

4.
Use the ideal op amp property in all your
analyses

OPAMP: VOLTAGE FOLLOWER

V
+

= V
IN
.

By virtual ground, V
-

= V
+

Thus V
out

= V
-

= V
+

= V
IN

!!!!

So what’s the point ? The point is, due to the
infinite input impedance of an op amp, no
current at all can be drawn from the circuit
before V
IN
. Thus this part is effectively
isolated. Very

useful for interfacing to high
impedance sensors such as microelectrode,
microphone…

OPAMP: INVERTING AMPLIFIER

1.
V
-

= V
+

2.
As V
+

= 0, V
-

= 0

3.
As no current can
enter V
-

and from
Kirchoff’s Ist law,
I
1
=I
2
.

4.
I
1
= (V
IN
-

V
-
)/R
1
= V
IN
/R
1

5. I
2
= (0
-

V
OUT
)/R
2
=
-
V
OUT
/R
2

=> V
OUT

=
-
I
2
R
2

6. From 3 and 6, V
OUT

=
-
I
2
R
2

=
-
I
1
R
2

=
-
V
IN
R
2
/R
1

7. Therefore
V
OUT

= (
-
R
2
/R
1
)V
IN

OPAMP: NON


INVERTING
AMPLIFIER

1.
V
-

= V
+

2.
As V
+

= V
IN
, V
-

= V
IN

3.
As no current can
enter V
-

and from
Kirchoff’s Ist law,
I
1
=I
2
.

4. I
1

= V
IN
/R
1

5. I
2
= (V
OUT

-

V
IN
)/R
2
=> V
OUT

= V
IN

+ I
2
R
2

6. V
OUT

= I
1
R
1

+ I
2
R
2

= (R
1
+R
2
)I
1

= (R
1
+R
2
)V
IN
/R
1

7. Therefore
V
OUT

= (1 + R
2
/R
1
)V
IN

SUMMING AMPLIFIER

V
OUT

=
-
R
f
(V
1
/R
1

+ V
2
/R
2

+ … + V
n
/R
n
)

I
f

Recall inverting
amplifier and
I
f

= I
1

+ I
2

+ … + I
n

Summing amplifier is a good example of analog circuits serving as analog
computing amplifiers (analog computers)!

Note: analog circuits can add, subtract, multiply/divide (using
logarithmic components, differentiate and integrate


in real time and
continuously.

DRIVING OPAMPS


For certain applications (e.g. driving a motor or
a speaker), the amplifier needs to supply high
current. Opamps can’t handle this so we modify
them thus

Irrespective of the
opamp circuit, the small
current it sources can
switch ON the BJT
giving orders of
magnitude higher
current in the load.