Transcript Transistor - Downend School

Transistors
Introduction
• Transistors are :
– Electronic switches
– Amplifiers of electrical current
• Many sensors (e.g. LDR, Thermistor, etc) only provide a
very small output current or voltage.
• To provide a practical working circuit, it may be
necessary for a designer to amplify (e.g. make bigger)
this current or voltage in order to drive an output device
(e.g. Bulb, LED, Buzzer, etc).
There are two types
• Bipolar Transistors (NPN or PNP)
• Field Effect Transistors (FET)
Note …
We have only got to know about Bipolar NPN
Transistors.
Transistors are semiconductor devices. An NPN
transistor is made from three layers of n-type
and p-type semiconductor material.
Case Appearance
Transistors come in a variety of cases:
Transistor Leads
•
A transistor has three leads
which are called:
i.
The Base (b)
(responsible for
activating the
transistor)
ii. The Collector (c)
(The positive lead)
iii. The Emitter (e)
(The negative lead)
•
It is important that each of
the leads is correctly
identified
E
B C E
B
C
Transistor circuit symbol
How does a Transistor work?
• For a transistor to work it has to receive a small
voltage (e.g. 0.6 - 0.7 Volts) at its base.
• This small voltage will result in a small electrical
current to flow into the base and out of the
emitter.
• The resistance between the collector and emitter
will fall.
• A larger current will then flow from the collector
to the emitter.
Amplification of current
• The amplification of a
transistor is known as the
GAIN.
• Two current paths flow through
a transistor:
– The base current which is
small
– The collector current which is
large
• The gain of a transistor is
found by dividing the collector
current by the base current
and is represented by the
symbol hFE.
Transistor Calculations
Example 1
Calculate the gain of
a transistor if the :
collector current is
100mA when the
base current is 0.5
mA
Example 2
The hFE gain of a
transistor is 40 when
the collector current is
150 mA.
Calculate the base
and the emitter
current
A Transistor Circuit
E
G
A
F
D
B
C
Questions on the transistor circuit
1. Which of the components is the INPUT?
2. Which components are the output?
3. Which two components make up the
potential divider?
4. What does component C do?
5. What is component D and how does it
work?
6. What does component E do?
Single transistor circuit
Two transistor circuit
• By adding a second transistor, the performance of the circuit is
improved.
• The second transistor benefits from the gain of the first transistor
and the circuit becomes more sensitive to changes at the input.
Darlington Pair
• A common method of connecting two transistors is called a
DARLINGTON PAIR or DARLINGTON DRIVER. The total gain is
the gain of the first transistor multiplied by the gain of the second
transistor.
Two transistor circuit
(Darlington Pair)