DARLINGTON PAIR
The Darlington pair was invented by
Sidney Darlington in 1953. He was an American electrical
engineer working at Bell Laboratories.
A Darlington pair is a configuration of two bipolar transistors
connected in such a way that the current amplified by the
first transistor is further amplified
by the second one. This results in a significantly higher
current gain than a single transistor, making it useful for
applications that require high current amplification with low
input current.
Working of Darlington pair:
A Darlington pair is a configuration
of two bipolar transistors connected in such a way that the
current amplified by the first transistor is further amplified 1
by the second one. This results in a significantly higher
current gain than a single transistor.
Here's a breakdown of how it works:
1. Base Current: A small base current (Ib) is applied to the
first transistor (Q1).
2. Collector Current of Q1: This small base current causes a
much larger collector current (Ic1) to flow through Q1,
due to its transistor action.
3. Base Current of Q2: The collector current of Q1 (Ic1) is
fed into the base of the second transistor (Q2).
4. Collector Current of Q2: Due to the amplified current
from Q1 acting as the base current for Q2, Q2 amplifies
this current further, resulting in a very large collector
current (Ic2) flowing through Q2.
Key characteristics of a Darlington pair:
High current gain: The combined current gain of the two
transistors is the product of their individual current
gains, resulting in a very high overall current gain.
Low input impedance: The input impedance of a
Darlington pair is relatively low, which can be
advantageous in some applications but can also be a
disadvantage in others.
High output impedance: The output impedance of a
Darlington pair is relatively high, which can be beneficial
for driving high-impedance loads.
Slower switching speed: Due to the additional
capacitance and inductance in the circuit, Darlington
pairs tend to have slower switching speeds compared to
single transistors.
Applications of Darlington pairs:
High-current switching: Darlington pairs are commonly
used in applications that require high current switching,
such as motor control, power supplies, and relay drivers.
Amplification of weak signals: The high current gain of
Darlington pairs makes them suitable for amplifying
weak signals, such as those from sensors or
microphones.
Interface circuits: Darlington pairs can be used to
interface low-power circuits with high-power devices.
Advantages of Darlington pairs:
High current gain
High output impedance
Low input impedance
Simple to implement
Disadvantages of Darlington pairs:
Slower switching speed
Higher voltage drop across the transistors
More complex biasing requirements
Overall, Darlington pairs are a versatile and useful circuit
configuration that can be used in a variety of applications.
However, it is important to consider their limitations, such as
slower switching speed and higher voltage drop, when
selecting them for a particular application.
Darlington Pair
Aim: To show the working of Darlington pair circuit using an
LED and switch.
Materials Required:
1. Two Bipolar Junction Transistors (BJTs):
o These should be of the same type (NPN or PNP).
o Common choices include BC547, 2N2222, or other
suitable transistors. We have taken BC547.
2. Resistors:
o A few resistors of different values are needed for
biasing the transistors and limiting the current.
o Common values include 1kΩ, 2.2kΩ, and 10kΩ
resistors. We have used 2.2kΩ in our circuit.
3. Power Supply:
o A DC power supply is required to provide the
necessary voltage for the circuit.
o A 5V or 9V power supply is suitable for most
applications. We have taken 9V in our case.
4. Breadboard:
o A breadboard is a convenient platform to build and
test the circuit without soldering.
5. Connecting Wires:
o To connect the components on the breadboard.
6. LED: To visually observe the output of the circuit.
7. Switch: A switch is required to ON and OFF the circuit.
Optional Components:
Potentiometer: To control the input signal or bias
current.
Multimeter: To measure voltages and currents in the
circuit.
Procedure:
1. Carefully study the circuit and make start
making connections on breadboard.
2. After creating the circuit from the shown
figure, add a 9V battery to the circuit in order
to provide it power supply.
3. Connect the LED to check if the circuit is
working.
Precautions:
1. Make sure connections made are right and
tight.
2. Make sure all the component are fully
functioning using Multimeter.
Result: The circuit of Darlington Pair is working.
