As I stated in a previous post, the common emitter configuration is the most widely used transistor configuration. The main characteristic that makes the common emitter configuration the most used is its high current gain. The applications for the common emitter configuration include amplifiers, phase inverters, and switches.
For the following more specific applications including,
Regulators Power
Audio Amplifier o/p stages
Controlling of motors
Display drivers
Controlling of Solenoid
Light and touch sensors.
the current output supplied from a single transistor being powered by a low voltage source is insufficient. To remedy this problem, two transistors connected in a configuration called a Darlington Pair is used.
This type of connection boosts the overall gain to the product of each of the individual transistor’s gains. In other words, if each transistor had a gain of 50 the Darlington’s overall gain would be 50×50 or 2500. Inputs coming from low current digital sources such as microcontrollers, computer outputs, or switches can control output devices such as motors, relays and solenoids by using a Darlington Pair. Here is an example of a typical application:
Darlington pairs can be made by using two separate NPN or PNP transistors but the they also come already configured in a single package.
