Transistors Part I 4-27-2011

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Transistors

Part 1

Transistors


Transistors are used for amplification.


They have been referred to as a Three
-
Terminal Amplifier with one terminal being
common to the other two terminals and is
usually grounded.


Transistors behave as a resistor whose ohmic
value is varied by the AC input signal.

Transistors


A transistor is made up of diodes: NPN type


Collector




Base





Emitter

Transistors


There are two types of transistors we will look
at; they are the NPN and the PNP.



Transistors


Current flows in a transistor from emitter to
collector with a small portion flowing into the
base. (NPN)



Transistors


This also applies to PNP transistors.



Transistors


Designation of transistor terminals are the
Collector (output), Base (input) and Emitter
(common).


Transistors

THE
BASE
CURRENT CONTROLS THE COLLECTOR
CURRENT


The
coll
ector current
Ic

is forced to
flow
through the base region
on its way between emitter and collector. The transistor is so
constructed
that the base current
I
B

can control the resistance
around the base region and thus control
Ic
.
Regardless of the
type of transistor (NPN or PNP), an increase in base current
reduces the resistance of the transistor, resulting in a larger
value of collector current. A reduc
tion in base current increases
the transistor resistance, and a smaller value of
collector
current
will flow.


Transistors

Basics of DC Biasing


Transistors

Basics of DC Biasing



Transistors


Transistor schematic symbols



Transistors

The transistor as an amplifier








The DC bias currents are the solid dark arrows and the
mini
-
sine waves is the AC signal. This is a Common
Emitter PNP type as the arrow on the emitter point
in

to the base.

Transistors


The DC bias currents remain constant in the circuit to operate
the transistor. Without them the transistor will not function.
Thus, the AC signal will
ride

on top of the DC values. The
dashed lines represent DC and the sine wave represents AC.



Transistors

AC Current Input on Base and Collector

Output. Beta (
β
) is the transistor gain.

Equation shown at bottom of chart ↘




Transistors


Amplifying Characteristics of an NPN type



Transistors


We have 10
-
mVp
-
p as input signal on the
base. We develop 20
-
uAp
-
p of input signal
current and since our Beta is 200 our output
signal should be 10
-
mVp
-
p x 200 = 4
-
Vp
-
p and
20
-
uAp
-
p x 200 = 4
-
mAp
-
p.


The individual voltage drops and rises are the
DC biasing of the circuit to allow the
amplifying transistor to operate.

Transistors


In Transistors Part 2 we will discuss:


Transistor configurations


Transistor voltage feedback


Transistor current feedback


Two
-
Stage, Push
-
Pull, Darlington and other
coupling circuits.