CHAPTER 5 TRANSISTOR BIAS CIRCUITS - HBCC

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PHYS 162
-

Chapter 5


Transistor Bias Circuits

Prepared By: Syed Muhammad Asad



Semester 102

Page
1


Figure
1

Example of linear and nonlinear opera
tion

CHAPTER 5

TRANSISTOR BIAS CIRCUITS

5
-
1
THE DC OPERATING POINT

-

A transistor must be properly biased with a DC voltage to operate in the linear region.

-

It ensures an amplified and accurate signal production at the output.

-

The DC operating point is often refe
rred as Q
-
point.

-

The DC parameters that need to be found to determine the Q
-
point are collector current I
C

and
collector
-
emitter voltage V
CE
.

5.1.1 DC Bias

-

If an amplifier is no
t biased with the correct DC voltages, it can go into saturation and cutoff.

-

F
igure
1(a)
shows the correct
linear operation with amplified
output.

-

Figure 1(b) shows nonlinear
operation where the amplifier is in
cutoff. The clipping in the positive
cycle is always due to cutoff.

-

Figure 1(c) shows nonlinear
operation where the amplifi
er is in
saturation. The clipping in the
negative cycle is always due to
saturation.

5.1.1.1 Graphical Analysis

-

In
Figure
2, we chose three values
of I
B

and observe what happens to
I
C

and V
CE
.

o

For
𝐼

=
200
𝜇
,
𝑉

=
5
.
6
𝑉

o

For
𝐼

=
300
𝜇
,
𝑉

=
3
.
4
𝑉

o

For
𝐼

=
400
𝜇
,
𝑉

=
1
.
2
𝑉

-

The corresponding Q
-
points can be seen on the graph.



PHYS 162
-

Chapter 5


Transistor Bias Circuits

Prepared By: Syed Muhammad Asad



Semester 102

Page
2



5.1.1.2 DC Load Line

-

The DC operation of a transistor circuit can be described graphically u
sing a DC load line
.

-

It is a straight line connecting
𝐼

=
𝐼





on the y
-
axis to
𝑉

=
𝑉


on the x
-
axis.

-

At saturation
𝐼




=
𝑉


𝑉







and at cutoff
𝑉

=
𝑉

.

-

Figure 3 shows the t
hree Q
-
points.

Figure
2

Q
-
point adjustment

Figure
3

The Dc load line

PHYS 162
-

Chapter 5


Transistor Bias Circuits

Prepared By: Syed Muhammad Asad



Semester 102

Page
3


5
.
1
.
1
.
3

Linear Operation

-

All point along the DC load line between saturation and cutoff is the linear region of operation for a
transistor
.

-

Figure 4 is an example of linear operation.

-

AC voltage V
in

produces an AC base current
𝐼

(
𝑝𝑒 𝑘
)
=
100
𝜇


above and below the Q
-
point.

-

This produces an AC collector current
𝐼

(
𝑝𝑒 𝑘
)
=
10
𝑚

above and below the Q
-
point.


-

This change in th
e collector current changes the collector
-
emitter voltage
𝑉
𝑒
(
𝑝𝑒 𝑘
)
=
2
.
2
𝑉
.

-

Th
is changing
V
ce

is the r
equired voltage amplification at the output of the transistor.

NOTE: REFER EXAMPLE
5
-
1

PAGE
221

5
-
2 VOLTAGE
-
DIVIDER BIAS

-

Voltage
-
divider bias is one of the widely used biasing techniques for a
transistor.

-

It uses a single power source and a voltage
-
divider

to attain the voltage
b
ase bias voltage.

-

For circuit analysis, it is assumed that the base current I
B

is small enough
to be neglected.

-

There are two types of voltage
-
dividers.

o

Stiff voltage divider where

𝑉

=


2

1
+

2

𝑉


If

𝐼𝑁

 


10

2

o

Non Stiff voltage divider where

𝑉

=


2
|
|

𝐼𝑁

 


1
+

2
|
|

𝐼𝑁

 


𝑉


If

𝐼𝑁

 

<
10

2

-


𝐼𝑁

 

=
𝛽




Figure
4

Variation in
AC current and voltage

Figure
5

Voltage
-
divider bias

PHYS 162
-

Chapter 5


Transistor Bias Circuits

Prepared By: Syed Muhammad Asad



Semester 102

Page
4


NOTE: REFER EXAMPLE
5
-
2

PAGE
224

5
-
3 OTHER BIAS METHODS

-

Other types of biasing methods are

o

Emitter Bias



Excellent Q
-
point stability.



Uses two voltages sources instead of one.

o

Base Bias



Mainly used for switching circuits.



Not suitable for linear amplifier

because of poor Q
-
point stability.

o

Emitter
-
Feedback Bias



Adding an RE in Base bias circuits gives emitter
-
feedback bias.



Better Q
-
point stability than the base bias but still not
well

enough
for linear operation.

o

Collector
-
Feedback Bias



Better Q
-
point sta
bility than emitter
-
feedback bias.



Can be used in linear amplifier circuits.

-

A summary of all the equations is given in Table 1.

Table
1

Transistor Bias Circuit Formula Sheet

Voltage
-
Divider Bias

Emitter Bias

Base Bias

Emitter
-
Feedb
ack
Bias

Collector
-
Feedback Bias

Stiff voltage
-
divider

𝑉

=


2

1
+

2

𝑉


If

𝐼𝑁

 


10

2

Non Stiff voltage divider

𝑉

=


2
|
|

𝐼𝑁

 


1
+

2
|
|

𝐼𝑁

 


𝑉


If

𝐼𝑁

 

<
10

2

𝑉

=
𝑉

+
𝑉






𝐼𝑁




=
𝛽








𝑉

=
𝑉


𝑉


𝑉



1
𝑉

(neglecting
effect of
𝛽

)


𝑉

=
𝑉

+
𝐼



(taking
𝛽


into account)




𝐼


𝐼

=
𝑉




Without
𝛽


𝐼


𝐼

=

𝑉


1
𝑉



With
𝛽


𝐼


𝐼

=

𝑉


𝑉



+


/
𝛽


𝐼

=
𝛽


𝑉


𝑉






𝐼


𝐼

=
𝑉


𝑉



+


/
𝛽


𝐼

=
𝑉


𝑉



+


/
𝛽


𝑉

=
𝑉


𝐼




𝑉

=
𝑉


𝐼







𝑉

=
𝑉


𝑉


𝑉

=
𝑉


𝑉


𝑉

=
𝑉


𝐼




𝑉

=
𝑉


𝐼




+




𝑉

=
𝑉


𝐼





NOTE: REFER E
XAMPLE
5
-
6,

5
-
7,

5
-
8,

5
-
9,

5
-
10

PAGE
230
-
236