B

C

E

BIJUNCTION TRANSISTOR

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 N

n p nE

B

C

Cross Section

B

C

E

Schematic Symbol

P N P

p n pE

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

CE

IE IC

IB

-

+

VBE VBC

+

-

+- VCE

B

CEIE IC

IB-

+VEB VCB

+

-

+ -VEC

npnIE = IB + IC

VCE = -VBC + VBE

pnpIE = IB + IC

VEC = VEB - VCB

DC and DC

= Common-emitter current gain = Common-base current gain = IC = IC IB IE

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 OperationMost important mode of operationCentral to amplifier operationEmitter –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 zeroIdeal transistor behaves like an open

switchBoth 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 = VEB & IE

output = VCB & ICCommon-Emitter Biasing (CE)

input = VBE & IB

output = VCE & ICCommon-Collector Biasing (CC)

input = VBC & IBoutput = VEC & IE

BJT Transconductance Curve

Typical NPN Transistor

VBE

IC

2 mA

4 mA

6 mA

8 mA

0.7 V

Collector Current:

Transconductance: (slope of the curve)gm = IC / VBE

IES = The reverse saturation current of the B-E Junction.VT = kT/q = 26 mV (@ T=300K) = the emission coefficient and is usually 1

BEC T ES

T

VI I e

V

Common-BaseCircuit Diagram: NPN Transistor

Region of Operation IC VCE VBE VCB

C-B Bias

E-B Bias

Active βIB =VBE+VCE 0.7V 0V Rev. Fwd.

Saturation Max 0V 0.7V -0.7V<VCE<0 Fwd. Fwd.

Cutoff 0 =VBE+VCE 0V 0V Rev. None/Rev.

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 VEB and input current IE for fixed VCB values

The input circuit in CB configuration involves the emitter-base diode, which is forward biased in active region. Therefore, the relationship between VEB and IE is nothing but the forward characteristics of a diode

Common-Base input characteristicsIn the above characteristics, VCB = Open represents the characteristics of the forward biased emitter

With increase of VCB, the curves shift downwards i.e., we get the same IE with less VEB. This is because, from the early effect increases the IE increases with VEB 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-EmitterCircuit Diagram

+_VCC

IC

VCE

IB

Collector-Current Curves

VCE

IC

Active Region

IB

Saturation RegionCutoff RegionIB = 0

Region of Operation

Description

Active Small base current controls a large collector current

Saturation VCE(sat) 0.2V, VCE increases with IC

Cutoff Achieved by reducing IB to 0, Ideally, IC will also equal 0.

The input quantities for C.E. configuration are base current IB and base emitter voltage VBE

The input characteristics curves are in between IB and VBE for various values of collector to emitter voltage VCE

If VCE = 0 and if the base-emitter junction is forward biased, the input characteristics is the same as the characteristics of forward biased diodeIf VCE is increased then VCB increases By applying KVL around the transistor

If VCE is increased then VCB increases

BE CE CB

CE CB BE

V - V +V =0

V =V +V

Common Emitter Input Characteristics

Common Emitter Input Characteristics

Increase in VCB leads to decrease in effective base width WB

| due to early effect, resulting in decrease of recombination and consequently, decrease in base current due to recombination.

CE CB B

CE

V V I

The curves move right side as V increases

Common Emitter output Characteristics

The output quantities in C.E. configuration are IC and VCE the o/p characteristics gives a relationship between IC and VCE with base current IB as a parameter.This family of curves may be divided into three regions those are active region, saturation region and cutoff region.

Common-CollectorIt 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

VCE

IE

Active

Region

IB

Saturation Region Cutoff Region

IB = 0

The Common-collector biasing circuit is basically equivalent to the common-emitter biased circuit except instead of looking at IC

as a function of VCE and

IB we are looking at IE.Also, since 1, and = IC/IE that means IC

IE

Common collector input Characteristics

Input Characteristics As VCB increases according to early effect base width decreases and IB 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 E C I I

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.

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PARAMETER CB CE CC

Input impedance (Ri)

Output Impedance(Ro)

D.C current gain

Voltage gain Very Large Moderate

Applications For High Frequency

For Audio frequency For impedance matching

Phase relationship between i/p and o/p

In- phase Out-of phase In- phase

EB

E

V (Low) 10

I

( )BE

B

V Moderate 1K

I

( )CB

B

VHigh 100K

I

CB

C

V (High) 100k

I

( )CB

C

V Moderate 10K

I

( ) 10EC

E

V Low

I

C

E

I 1I

C

B

I = High value (50-500)

I

E

B

I = (51-501)

I

very low 1

COMPARISION of CB, CE AND CC PARAMETERS

Transistor summary