Bipolar Junction Transistor (BJT)

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Bipolar Junction Transistor (BJT)CHAPTER 3


IntroductionThe basic of electronic system nowadays is semiconductor device. The famous and commonly use of this device is BJTs (Bipolar Junction Transistors). It can be use as amplifier and logic switches. BJT consists of three terminal:  collector : C  base : B emitter : E Two types of BJT : pnp and npn


Transistor Construction3 layer semiconductor device consisting: 2 n- and 1 p-type layers of material  npn transistor 2 p- and 1 n-type layers of material pnp transistor The term bipolar reflects the fact that holes and electrons participate in the injection process into the oppositely polarized material A single pn junction has two different types of bias: forward bias reverse bias Thus, a two-pn-junction device has four types of bias.


Position of the terminals and symbol of BJT. Base is located at the middle and more thin from the level of collector and emitter The emitter and collector terminals are made of the same type of semiconductor material, while the base of the other type of material


Transistor currents The arrow is always drawn on the emitter The arrow always point toward the n-type The arrow indicates the direction of the emitter current: pnp:E B npn: B EIC=the collector current IB= the base current IE= the emitter current


By imaging the analogy of diode, transistor can be construct like two diodes that connetecd together. It can be conclude that the work of transistor is base on work of diode.


Transistor OperationThe basic operation will be described using the pnp transistor. The operation of the pnp transistor is exactly the same if the roles played by the electron and hole are interchanged. One p-n junction of a transistor is reverse-biased, whereas the other is forward-biased.Forward-biased junction of a pnp transistorReverse-biased junction of a pnp transistor


Both biasing potentials have been applied to a pnp transistor and resulting majority and minority carrier flows indicated. Majority carriers (+) will diffuse across the forward-biased p-n junction into the n-type material. A very small number of carriers (+) will through n-type material to the base terminal. Resulting IB is typically in order of microamperes. The large number of majority carriers will diffuse across the reverse-biased junction into the p-type material connected to the collector terminal.


Majority carriers can cross the reverse-biased junction because the injected majority carriers will appear as minority carriers in the n-type material. Applying KCL to the transistor : IE = IC + IB The comprises of two components – the majority and minority carriers IC = ICmajority + ICOminority ICO – IC current with emitter terminal open and is called leakage current.


Common-Base ConfigurationCommon-base terminology is derived from the fact that the : - base is common to both input and output of the configuration. - base is usually the terminal closest to or at ground potential. All current directions will refer to conventional (hole) flow and the arrows in all electronic symbols have a direction defined by this convention. Note that the applied biasing (voltage sources) are such as to establish current in the direction indicated for each branch.


To describe the behavior of common-base amplifiers requires two set of characteristics: Input or driving point characteristics. Output or collector characteristics The output characteristics has 3 basic regions: Active region –defined by the biasing arrangements Cutoff region – region where the collector current is 0A Saturation region- region of the characteristics to the left of VCB = 0V


The curves (output characteristics) clearly indicate that a first approximation to the relationship between IE and IC in the active region is given by IC ≈IE Once a transistor is in the ‘on’ state, the base-emitter voltage will be assumed to be VBE = 0.7V


In the dc mode the level of IC and IE due to the majority carriers are related by a quantity called alpha = IC = IE + ICBO It can then be summarize to IC = IE (ignore ICBO due to small value) For ac situations where the point of operation moves on the characteristics curve, an ac alpha defined by Alpha a common base current gain factor that shows the efficiency by calculating the current percent from current flow from emitter to collector.The value of  is typical from 0.9 ~ 0.998.


BiasingProper biasing CB configuration in active region by approximation IC  IE (IB  0 uA)


Transistor as an amplifier


Simulation of transistor as an amplifier


Common-Emitter ConfigurationIt is called common-emitter configuration since : - emitter is common or reference to both input and output terminals. - emitter is usually the terminal closest to or at ground potential. Almost amplifier design is using connection of CE due to the high gain for current and voltage. Two set of characteristics are necessary to describe the behavior for CE ;input (base terminal) and output (collector terminal) parameters.


Proper Biasing common-emitter configuration in active region


Input characteristics for a common-emitter NPN transistorIB is microamperes compared to miliamperes of IC. IB will flow when VBE > 0.7V for silicon and 0.3V for germanium Before this value IB is very small and no IB. Base-emitter junction is forward bias Increasing VCE will reduce IB for different values.


Output characteristics for a common-emitter npn transistorFor small VCE (VCE < VCESAT, IC increase linearly with increasing of VCE VCE > VCESAT IC not totally depends on VCE  constant IC IB(uA) is very small compare to IC (mA). Small increase in IB cause big increase in IC IB=0 A  ICEO occur. Noticing the value when IC=0A. There is still some value of current flows.


Beta () or amplification factor The ratio of dc collector current (IC) to the dc base current (IB) is dc beta (dc ) which is dc current gain where IC and IB are determined at a particular operating point, Q-point (quiescent point). It’s define by the following equation: 30 < dc < 300  2N3904 On data sheet, dc=hFE with h is derived from ac hybrid equivalent cct. FE are derived from forward-current amplification and common-emitter configuration respectively.


For ac conditions an ac beta has been defined as the changes of collector current (IC) compared to the changes of base current (IB) where IC and IB are determined at operating point. On data sheet, ac=hfe It can defined by the following equation:


ExampleFrom output characteristics of common emitter configuration, find ac and dc with an Operating point at IB=25 A and VCE =7.5V.

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Last Updated: 8th March 2018

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