Transistors
Part 1
Transistors
• Transistors are used for amplification.
• They have been referred to as a ThreeTerminal 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 collector 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 IB 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 reduction 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.