There are basically two types of transistors
Bipolar junction transistor (BJT) :- a current controlled device in which output characteristics are controlled by base current
Field effect transistor (FET) :- output characteristics controlled by input voltage i.e. electric field
Types of FET
1. Junction Field Effect Transistor (JFET)
2. Metal oxide semiconductor Field Effect Transistor (MOSFET)
The two types of transistor differ in both their operating characteristics and their internal construction.
BJT is so called because both holes and electrons plays part in the conduction process
Principal Disadvantages Of The BJT
It has low input impedance because of the forward biased emitter junction. Although low input impedance problem may be improved by careful design and used of more than one transistor.
it has a considerable noise level.
it has a considerable noise level.
Difference Between JFET and BJT
- Only one type of carrier, holes in p – type channel and electron in n- type channel in JFET WHILE electron and holes takes part in conduction in BJT
- JFET has high input impedance since the input circuit is reversed biased WHILE input circuit for BJT is forward biased hence it has low input impedance
- No current enters gate of JFET i.e. IG = 0 WHILE BJT base current is of few µA
- JFET uses voltage applied on gate to control current between drain and source WHILE BJT uses current applied to base to control a large current between the collector and the emitter
- In JFET there is no junction as in BJT. Conduction through an n –type or p – type semi conductor material for this reason noise level in BJT is very small
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