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:
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
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
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
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: = = + 1 1 - Note: and are sometimes referred to as dc and dc because the relationships being dealt with in the BJT are DC.
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
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
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
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.
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
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
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
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
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
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.
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.
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.
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
Common collector input Characteristics Input Characteristics As V CB increases according to early effect base width decreases and I B decreases.
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
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
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