# Prelab 5

Ηλεκτρονική - Συσκευές

2 Νοε 2013 (πριν από 4 χρόνια και 6 μήνες)

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ELECTRONICS LABORATORY

EXPERIMENT 5 PRELAB

BJT BIASING

THEORY

BJT

The Bipolar Junction Transistor (BJT) is a nonlinear three terminal device with
following dc models:

a)

Cutoff mode: V
BE
< V
F
, I
B
=0, I
C
=0

b)

Active mode: V
BE

V
F
, V
CE

V
CESAT
, I
C

F
I
B
+I
CEO

(I
CEO
is usually negligibly small)

c)

Saturation mode: V
BE

V
F
, V
CE
=

V
CESAT
, I
C
< ß
F
I
B

I
C

is determined by the external circuit.

(a)

(b)

(c)

Figure1: NPN BJT dc equivalent circuits: a) Cutoff mode b)Active mode

c)Saturation mode. For PNP BJT circuits, currents and voltages

change direc
tions

In analog circuits, the BJT is operated in the active mode. For this purpose a
biasing circuit is employed. Figure

(
2
) has various biasing circuits that use only one
power supply.

The bias circuit without feedback is the simplest of all: The base c
urrent of the BJT
is set by the resistor R
B
. In the design, R
B

is set to be (V
CC
-
V
BE
)/I
B
.

For the emitter feedback bias circuit, the resistor R
E

causes negative feedback and
reduces the changes
in the ope
r
a
ting

point
.

In the collector feedback bias cir
cuit, the resistor R
C

has essentially the same effect
as the R
E

in the emitter feedback bias circuit.

The most common and stable circuit among single source biasing circuits is the
voltage divider biasing circuit. In this circuit, the current in the resis
tors R
1
and R
2
is set to a value more than 10 times the base current. This keeps the voltage drop
caused by the base current on these resistors negligibly small.

Hence, the
voltage
divider formed by R
1

and R
2

is considered as an independent voltage source

to set
the voltage at the base of BJT. The resistor R
E

provides negative feedback and sets
the emitter current.

(a)

(b)

(c)

(d)

Figure 2
: BJT Biasing circuits: a)

Without feedback b) Emitter feedback

c)

Collector feedback d) Voltage divider

The operating point of a transistor in a circuit depends not only on the voltage
sources and resistors, but also on V
F
, ß
F

and I
CO
of the transistor.
The BJT
parameters may vary due to production li
ne
spread or temperature. As the variation

of these parameters will affect the operating point
,
the sta
bility of the ope
r
a
ting
point should be considered
during the bias circuit design.
To observe this variation,
the following stability

factors should be i
nvestigated:

PRE
-
LAB

1)

Learn the
ß
F

values of the transistors and use these values in your design.

2)

For the general purpose transistor, design 4 bias circuits that are given above
so that I
C
=1
mA

and V
CE
=3 V.

3) Determine the operating point (I
C

and V
CE
) for each circuit you have designed
by using P
S
pice simulation program.

4) Replace general purpose transistor with the power transistor

and determine
the operating point in each circuit.

Important note
s
:

1
)

Those who did not do the prelab will n
ot be accepted to the lab.
You must do
first two design(a and b) until 07.04.2010 and bring your simulation prints to the
lab. The rest two will be used on 14.04.2010, so you can bring c and d at that date.

2
)
Bring the transistors with you. You have to b
uy the transistors beforehand.

3
)