Sunday, October 26, 2008

Transistors

Transistors


Transistors are basically can be categorise into two as BJTs (Bipolar Junction Transistors) and FETs (Field Effect Transistors). In this section describes about Bipolar Junction Transistors only. The BJTs can be also categorise into two as NPN and PNP. These BJTs are basically a combination of two diodes. The application of them is using as an amplifier or as a switch.




The first transistor invented by John Bardeen and Walter Brattain at Bell Laboratories.

The transistors are consists of two PN junctions, each are back to back and manufactured as a single piece of semi-conductors. The following diagram shows how a NPN and PNP transistors consist of the two junctions.







The middle part of the transistor is called as the Base and the other two around the base are called the collector and the Emitter. The Emitter contains more impurities than the collector.



Schematic diagram of PNP Transistor Schematic diagram of NPN Transistor


To make a transistor to a working state, it must be supplied the necessary voltages. The E-B junction must be forward biased and the C-B junction must be reversed biased. And also the common point must be grounded.



Using a Transistor as a Amplifier

There are three methods using a transistor as an amplifier. They are as,
ü Common Base Configuration
ü Common Collector Configuration
ü Common Emitter Configuration



Common Base Configuration

This configuration of amplifiers has a power gain and a voltage gain. But it has a disadvantage as it doesn’t have a current gain. In this configuration the input signal and the output signal are in a same phase angle. But due to the disadvantage this configuration doesn’t commonly use.

Common Collector Configuration




In this configuration, there is a current gain and a small power gain. But there isn’t any voltage gain. Therefore this configuration is also not commonly used.


Common Emitter Configuration

This is the most commonly used transistor amplifier configuration. There is a good current gain, good voltage gain and good power gain. The output signal has a phase angle of 1800 with the input signal.

The current gain of this configuration is called β.
β = IC/Ib
As Ib is a very small value, the current gain of this configuration is a higher value.


This is a typical common emitter configuration transistor amplifier circuit and it is a silicon transistor circuit as it has shown typical voltage values. When the forward base/emitter voltage is 0.6 to 0.7 V, the transistor is silicon. Germanium transistors will have a forward base/emitter bias voltage of 0.2 to 0.3 V This is a silicon transistor because 2.6 base volts minus 1.9 emitter volts equal a forward bias of 0.7 volts indicating a silicon transistor.

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