The construction of Darlington pair consists of 2 BJTs connected as shown below in the figure. The emitter current of Q1 becomes the base current of Q2 transistor. The current gain of the pair will be equal to the product of the current gains of the individual transistors.
You may make Darlington pair by wiring 2 individual transistors together. The transistors may also be manufactured as a unit on a single silicon chip. This has three terminals, known as collector, base & emitter. The chip is enclosed in a normal 3-wiredpackage looking just like an ordinary transistor.When the pair is created from the individual transistors,they need not be of similar type. Very often Q1 is a low-voltage low-current type while Q2 is a power transistor.
A Darlington may effectively replace a single transistor used by the BJT common-emitter & common-collector amplifiers. This provides increased sensitivity to small base currents &, in the case of the common-emitter amplifier, may result in increased gain. Darlingtons pairs may also be applied as switching transistors. They need only a very small amount of current to trigger them.The circuit shown below is a very good practical example of a Darlington pair that us being used as a switching transistor.
2 metal plates about 1 cm x 2 cm are mounted side by side leaving a 0.5 mm gap between them. When a finger comes in touch the plates it eventually bridges the gap, & we find a minute current flowing to the base of Q1. This current is only a few tens of microamperes, but the pair of transistors has a current gain of about 1 lac 60 thousands. The resulting collector current through Q2 is sufficient enough for the purpose of powering the solid-state buzzer.
0 comments:
Post a Comment