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In the Common Emitter or grounded emitter configuration, the input signal is applied between the base, while the output is taken from between the collector and the emitter as shown. This type of configuration is the most commonly used circuit for transistor based amplifiers and which represents the "normal" method of bipolar transistor connection.
The common emitter amplifier configuration produces the highest current and power gain of all the three bipolar transistor configurations. This is mainly because the input impedance is LOW as it is connected to a forward-biased PN-junction, while the output impedance is HIGH as it is taken from a reverse-biased PN-junction. 

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  1. How the common emitter configuration works ?
  2. Why of this type produces the highest current and power gain of all the three bipolar transistor configurations ?

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As its name suggests, in the Common Base or grounded base configuration, the BASE connection is common to both the input signal AND the output signal with the input signal being applied between the base and the emitter terminals. The corresponding output signal is taken from between the base and the collector terminals as shown with the base terminal grounded or connected to a fixed reference voltage point.
The input current flowing into the emitter is quite large as its the sum of both the base current and collector current respectively therefore, the collector current output is less than the emitter current input resulting in a current gain for this type of circuit of "1" (unity) or less, in other words the common base configuration "attenuates" the input signal.

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  2. Why current gain is low of this type ?

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When a transistor is used as a switch it must be either OFF or fully ON. In the fully ON state the voltage VCE across the transistor is almost zero and the transistor is said to be saturated because it cannot pass any more collector current Ic. The output device switched by the transistor is usually called the 'load'.The power developed in a switching transistor is very small:
  • In the OFF state: power = Ic × VCE, but Ic = 0, so the power is zero.
  • In the full ON state: power = Ic × VCE, but VCE = 0 (almost), so the power is very small.
This means that the transistor should not become hot in use and you do not need to consider its maximum power rating. The important ratings in switching circuits are the maximum collector current Ic(max) and the minimum current gain hFE(min). The transistor's voltage ratings may be ignored unless you are using a supply voltage of more than about 15V. 

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In the Common Collector or grounded collector configuration, the collector is now common through the supply. The input signal is connected directly to the base, while the output is taken from the emitter load as shown. This type of configuration is commonly known as a Voltage Follower or Emitter Follower circuit. 
The emitter follower configuration is very useful for impedance matching applications because of the very high input impedance, in the region of hundreds of thousands of Ohms while having a relatively low output impedance.

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The invention of the transistor represents a major milestone in electronics and science in general. Today transistors power everything from cell phones to supercomputers, all of which take advantage of their numerous advantages.

  • Size :
One of the biggest advantages of using transistors is their small size. Even though vacuum tubes have continued to grow smaller over the years, transistors have decreased to literally microscopic proportions .

  • Weight :
In addition to being smaller, transistors are lighter than their vacuum tube counterparts. This has all of the advantages of a smaller size and also makes transistors perfect for use in mobile electronic devices or scientific instruments.

  • Heat
Transistors produce far less heat than vacuum tubes .
  • .Power Consumption .
Transistors also have the advantage of consuming less power than vacuum tubes.This means that they are more efficient and cost less to operate. 

  • Durability
Transistors are very durable, in part because of their simple design and metallic construction. Vacuum tubes use a glass bulb that is subject to breakage and must be handled carefully.

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The common-emitter amplifier is designed so that a small change in voltage (Vin) changes the small current through the base of the transistor; the transistor's current amplification combined with the properties of the circuit mean that small swings in Vin produce large changes in Vout.

Various configurations of single transistor amplifier are possible, with some providing current gain, some voltage gain, and some both.

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1.      How do we use the transistor as an amplifier?
2.      How the various configurations of single transistor amplifier are possible  ?


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As the Bipolar Transistor is a three terminal device, there are basically three possible ways to connect it within an electronic circuit with one terminal being common to both the input and output. Each method of connection responding differently to its input signal within a circuit as the static characteristics of the transistor vary with each circuit arrangement.
  • Common Base Configuration.
  • Common Emitter Configuration.
  • Common Collector Configuration.

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  2. How does the common basic configurations work ?

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There are two main types of transistors-junction transistors and field effect transistors. Each works in a different way. But the usefulness of any transistor comes from its ability to control a strong current with a weak voltage. For example, transistors in a public address system amplify (strengthen) the weak voltage produced when a person speaks into a microphone. The electricity coming from the transistors is strong enough to operate a loudspeaker, which produces sounds much louder than the person's voice. 

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 and PNP ?

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John BardeenWilliam Shockley, and Walter Brattain, were all scientists at the Bell Telephone Laboratories in Murray Hill, New Jersey. They were researching the behavior of germanium crystals as semi-conductors in an attempt to replace vacuum tubes as mechanical relays in telecommunications. The vacuum tube, used to amplify music and voice, made long-distance calling practical, but the tubes consumed power, created heat and burned out rapidly, requiring high maintenance.

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  2. Where was discover the transistor? 

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Transistors have infiltrated virtually every area of science and industry, from the family car to satellites. Even the military depends heavily on transistors.The ever increasing uses for transistors have created an urgent need for sound and basic information regarding their operation.


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