Semiconductors/Transistors as Amplifiers

When the first transistor was invented, the scientists used it as an amplifier first, just to prove the mathematical concept behind it. Using transistors as amplifiers is an incredibly important use. From this model of transistors we get such things as "Transistor Radios" which rely on transistors for signal amplification. This page is going to discuss the basic mathematical model behind transistors and their use as amplifiers.

Base, Collector, Emitter

In a typical Bipolar-Junction Transistor (BJT), there are three layers upon which it is constructed. These layers are known as the emitter, base, and collector. The emitter and collector make up most of the mass of the semiconducting layers while the base is a much thinner layer between them. Current direction depends on whether the transistor a NPN or PNP-type device. The emitter provides the charge carriers to the transistor, base controls the device current and collector collects the charge carriers.

NPN And PNP

NPN:a n-type,a p-type,a n-type properly doped semiconductors are joined respectively in other words are sandwiched. the first n-type semiconductor is called as collector.the next p-type semiconductor is called as base.the next n-type semi- conductor is called emitter. PNP:A p-type,a n-type,a p-type properly doped semiconductors are joined respectively in other words are sandwiched. the first p-type semiconductor is called as collector.the next n-type semiconductor is called as base.the next p-type semi- conductor is called emitter.

         The emitter region is heavily doped. The base region is lightly doped. The collector region is moderately doped.
          This condition is required for transistor to act as amplifier.Hence, it is doped so.

Amplifier Rules

The following are the amplifier rules.

  1)the base region should be very thin compared to collector and emitter
  2)the emitter is heavily doped,the collector is moderately doped,the base is very lightly doped relatively.
  3)for transistor to function as an amplifier the collector-base junction should be forward biased,the emitter-base junction
    should be reverse biased

input /output relation: There is no particular input-output relation because the input and output are not fixed. They vary according to the configuration of transistor. There are 3 types of transistor configurations

 1)common emitter configuration
 2)common collector configuration
 3)common base configuration

then by applying Kirchhoff voltage and current rule in input and output circuits relation between input and output circuits can be found depending upon configuration.