Chapter 4

Bipolar Junction Transistors (BJTs)

Contents

• Bipolar Junction transistors• Construction• Circuit diagrams• BJT biasing circuits• Q-point• BJT as a switch• BJT as an Amplifier

• Transistor is a combination of two words i.e. transfer and resistor. It is because a transistor is basically a resistor that amplifies electrical impulses as they are transferred through it from its input to output terminal.

Transistor

• A transistor has three doped regions. • The bottom region is called the emitter • The middle region is the base• And the top region is the collector. • In an actual transistor, the base region is much thinner as compared to the collector

and emitter regions. • The transistor Shown in figure (b) is an npn device because there is a p region between

two n regions. Recall that the majority carriers are free electrons in n-type material and holes in p-type material.

• Transistors shown in figure (c) is an pnp. A pnp transistor has an n region between two p regions. To avoid confusion between the npn and the pnp transistors, our early discussions will focus on the npn transistor.

Architecture of a Transistor

• There are two types of BJTs, the npn and pnp.• The two junctions are termed the base-emitter junction and the base-

collector junction• The term bipolar refers to the use of both holes and electrons as

charge carriers in the transistor structure• In order for the transistor to operate properly, the two junctions must

have the correct dc bias voltages– the base-emitter (BE) junction is forward biased(>=0.7V for Si,

>=0.3V for Ge)– the base-collector (BC) junction is reverse biased

Architecture of a BJTs

• An integrated circuit (IC) consists of transistors, resistors, diodes and capacitors combined together in one wafer-thin chip of silicon. This one wafer-thin chip is called a microchip. The microchip is only a few millimeters square with a thickness of 0.5 mm.

IC?

EmitterIt is the most heavily doped part of the transistor. Its major function is to supply the majority charge carriers to base.BaseIt is the smallest part of the transistor with 10.6mm area and it is lightly doped.CollectorIt is physically the largest part of the transistor. Its major function is to collect the charge carriers.

Parts of a Transistor

Diode equivalent of a Transistor

The minus signs represent free electrons.The heavily doped emitter has the following job: to emit or inject its free electrons into the base. The lightly doped base also has a well-defined purpose: to pass emitter-injected electrons on to the collector. The collector is so named because it collects or gathers most of the electrons from the base.The left source VBB of forward-biases the emitter diode, and the right source VCC reverse-biases the collector Diode.VBB forward-biases the emitter diode, forcing the free electrons in the emitter to enter the base. The thin and lightly doped base gives almost all these electrons enough time to diffuse into the collector. These electrons flow through the collector, through RC, and into the positive terminal of the VCC voltage source.

Basic Circuit of a BJT

Symbolic Representation

• Recall Kirchhoff’s current law. It says that the sum of all currents into a point or junction equals the sum of all currents out of the point or junction. When applied to a transistor, Kirchhoff’s current law gives us this important relationship:

IE = IC + IB

IC >> IB

IE = IC

• alpha (DC)IC = DCIE

• beta (DC)IC = DCIB

– DC typically has a value between 20 and 200.

DC Analysis of BJTs (Transistor Currents)

Operation of BJTs

Examples

DC Analysis of a TransistorAs the base-emitter junction is forward bias so,

VBE ≈0.7 VSince the emitter is at ground (0V), by Kirchoff’s Voltage law

VRB=VBB – VBE

Also by OHM’S law VRB = IBRB

Substituting for VRB yields IBRB = VBB – VBE

Solving for IB IB = VBB – VBE

RBThe voltage at the collector with respect to the grounded emitter is

VCE = VCC – VRC

Since the drop across RC VRC=ICRC

VCE = VCC – ICRC

Where IC = DCIB

The voltage across the reverse-biased collector-base junction is VCB = VCE - VBE

DC Analysis of BJTs

• DC voltages for the biased transistor:• Collector voltage

VC = VCC - ICRC

• Base voltageVB = VE + VBE

– for silicon transistors, VBE = 0.7 V– for germanium transistors, VBE = 0.3 V

DC Analysis of BJTs

Example

• The base current, IB, is established by the base bias.• The point at which the base current curve intersects the

dc load line is the quiescent or Q-point for the circuit.

Q-Point

• The voltage divider biasing is widely used

• Input resistance is:RIN DCRE

• The base voltage is approximately:VB VCCR2/(R1+R2)

DC Analysis of BJTs

• When used as an electronic switch, a transistor normally is operated alternately in cutoff and saturation.– A transistor is in cutoff when the base-emitter junction is

not forward-biased. VCE is approximately equal to VCC

– When the base-emitter junction is forward-biased and there is enough base current to produce a maximum collector current, the transistor is saturated.

BJTs as a Switch

BJTs as a Switch

BJT Operating Regions

Operation Region

IB or VCE

Char. BC and BE Junctions

Mode

Cutoff IB = Very small

Reverse & Reverse

Open Switch

Saturation VCE = Small Forward & Forward

Closed Switch

Active Linear

VCE = Moderate

Reverse &Forward

Linear Amplifier

Break-down

VCE = Large Beyond Limits

Overload

•Common emitter mode•Linear Active Region•Significant current Gain

Example:•Let Gain, = 100

•Assume to be in active region -> VBE=0.7V

•Find if it’s in active region

BJT as Amplifier

V

VRIRIVVmAII

mARRVVI

IIIIVV

BEEECCCCCB

BC

EB

BEBBB

BCBE

BE

93.37.0)0107.0*101)(2()07.1)(3(10

**07.10107.0*100*

0107.04027.05

101*

)1(7.0

VCB>0 so the BJT is in active region

BJT as Amplifier