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B B C C E E BIJUNCTION TRANSISTOR.  A transistor is a semiconductor device used to amplify and switch electronic signals.  A transistor is made up of.

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Presentation on theme: "B B C C E E BIJUNCTION TRANSISTOR.  A transistor is a semiconductor device used to amplify and switch electronic signals.  A transistor is made up of."— Presentation transcript:

1 B B C C E E BIJUNCTION TRANSISTOR

2  A transistor is a semiconductor device used to amplify and switch electronic signals.  A transistor is made up of three layers – an ‘n’ layer sandwiched between two ‘p’ layers or a ‘p’ layer between two ‘n’ layers.  Doping of each layer is different and that is what is responsible for the operation(amplification). Transistor

3 The BJT – Bipolar Junction Transistor : The Two Types of BJT Transistors: N P Nnpn E B C Cross Section B C E Schematic Symbol P N Ppnp E B C Cross Section B C E Schematic Symbol  Collector is moderately doped  Base is lightly doped  Emitter is heavily doped Click to view NPN Transistor Click to view PNP Transistor

4 BJT Relationships - Equations B C E IEIE ICIC IBIB - + V BE V BC V CE B C E IEIE ICIC IBIB - + V EB V CB V EC npn I E = I B + I C V CE = -V BC + V BE pnp I E = I B + I C V EC = V EB - V CB

5 DC  and DC   = Common-emitter current gain  = Common-base current gain  = I C  = I C I B I E The relationships between the two parameters are:  =   =    Note:  and  are sometimes referred to as  dc and  dc because the relationships being dealt with in the BJT are DC.

6 Modes of Operation  Most important mode of operation  Central to amplifier operation  Emitter –Base junction Forward biased and Collector –base Reverse Biased Active: Saturation:  Barrier potential of the junctions cancel each other out causing a virtual short  Ideal transistor behaves like an closed switch  Both junction are Forward biased Cutoff:  Current reduced to zero  Ideal transistor behaves like an open switch  Both junction are Reverse biased

7 Three Types of BJT Configurations Biasing the transistor refers to applying voltage to get the transistor to achieve certain operating conditions. Common-Base Biasing (CB) input = V EB & I E output = V CB & I C Common-Emitter Biasing (CE) input = V BE & I B output = V CE & I C Common-Collector Biasing (CC) input = V BC & I B output = V EC & I E

8 BJT Transconductance Curve Typical NPN Transistor V BE ICIC 2 mA 4 mA 6 mA 8 mA 0.7 V Collector Current: Transconductance: (slope of the curve) g m = I C /  V BE I ES = The reverse saturation current of the B-E Junction. V T = kT/q = 26 mV T=300K)  = the emission coefficient and is usually 1

9 Common-Base Circuit Diagram: NPN Transistor The Table Below lists assumptions that can be made for the attributes of the common-base biased circuit in the different regions of operation. Given for a Silicon NPN transistor.

10 Common-Base input characteristics  Input characteristics for the CB configuration gives relation between the input quantities, input voltage V EB and input current I E for fixed V CB values  The input circuit in CB configuration involves the emitter-base diode, which is forward biased in active region. Therefore, the relationship between V EB­ and I E is nothing but the forward characteristics of a diode

11 Common-Base input characteristics  In the above characteristics, V CB = Open represents the characteristics of the forward biased emitter  With increase of V CB, the curves shift downwards i.e., we get the same I E with less V EB. This is because, from the early effect increases the I E increases with V EB held constant

12 Common-Base out put characteristics. Although the Common-Base configuration is not the most common biasing type, it is often helpful in the understanding of how the BJT works. Emitter-Current Curves

13 Common-Emitter Circuit Diagram +_ V CC ICICICIC V CE IBIBIBIB Collector-Current Curves V CE ICICICIC Active Region IBIBIBIB Saturation Region Cutoff Region I B = 0

14 The input quantities for C.E. configuration are base current I B and base emitter voltage V BE The input characteristics curves are in between I B and V BE for various values of collector to emitter voltage V CE If V CE = 0 and if the base-emitter junction is forward biased, the input characteristics is the same as the characteristics of forward biased diode If V CE is increased then V CB increases By applying KVL around the transistor If V CE is increased then V CB increases Common Emitter Input Characteristics

15 Increase in V CB leads to decrease in effective base width W B | due to early effect, resulting in decrease of recombination and consequently, decrease in base current due to recombination.

16 Common Emitter output Characteristics  The output quantities in C.E. configuration are I C and V CE the o/p characteristics gives a relationship between I C and V CE with base current I B as a parameter.  This family of curves may be divided into three regions those are active region, saturation region and cutoff region.

17 Common-Collector  It is often called an emitter follower since its output is taken from the emitter resistor.  Is useful as an impedance matching device since its input impedance is much higher than its output impedance.  It is also termed a "buffer" for this reason and is used in digital circuits with basic gates.

18 Common-Collector Emitter-Current Curves V CE IEIE Active Region IBIB Saturation Region Cutoff Region I B = 0  The Common-collector biasing circuit is basically equivalent to the common-emitter biased circuit except instead of looking at I C as a function of V CE and I B we are looking at I E.  Also, since  1, and   = I C /I E that means I C I E

19 Common collector input Characteristics Input Characteristics As V CB increases according to early effect base width decreases and I B decreases.

20 Common collector output Characteristics Output Characteristics: The common-collector circuit is basically same as the common-emitter, with the exception that the load resistor is in the emitter circuit, the output characteristics are similar to that of CE configuration. It is because

21 Transistor as amplifier  Transistor amplifies current as well as voltage and is a current operated device.  The CE configuration is widely used as it amplifies current and voltage unlike the other configurations. Click to view Image

22 PARAMETERCBCECC Input impedance (R i ) Output Impedance (R o ) D.C current gain Voltage gainVery LargeModerate ApplicationsFor High Frequency For Audio frequencyFor impedance matching Phase relationship between i/p and o/p In- phaseOut-of phaseIn- phase COMPARISION of CB, CE AND CC PARAMETERS

23 Transistor summary


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