chapter 3. bipolar junction transistorsjpkc.xidian.edu.cn/ac/uploads/ppt/chapter3.pdf · chapter 3....

Bipolar Junction Transistors

Outline:

Fundamental of Transistor

Chapter 3. Bipolar Junction Transistors

Common-Base Configuration

Common-Emitter Configuration

Common-Collector Configuration


The transistor is a three-layer semiconductor device consisting of two n- and one p-typelayers of material or two p- and one n-typelayers of material.

Introduction

The former is called an npn transistor, and the latter is called a pnp transistor.


The terminals have been indicated by E for emitter, C for collector and B for Base.

This three-terminal device is often referred to as bipolar junction transistor.

The term bipolar reflects the fact that holes and electrons involve in the current flow.

They have different thickness and degrees of doping.


Figure: Types of Transistor


Advantages over Vacuum Tubes

Semiconductor devices:

of smaller size, lightweight, rugged, efficient no warm-up period

no heater requirement

lower operating voltages.


Transistor Operation

The biasing of the two types of transistor has been illustrated in the figures.

The operation of npn and pnp transistors are same if the roles played by the electrons and holes are interchanged.One p-n junction of a transistor is reverse-biased, whereas the other is forward-biased.


Just like the role played by external potential to diode, the applied voltages influence the depletion regions of the transistors.

Thus the flow of holes and electrons has been manipulated.

So for both types of transistors, we obtain:

IE = IC +IB


Figure: Biasing of pnp transistor

IEIB

IC


Figure: Biasing of npn transistor

IEIB

IC


The common-base configuration with pnpand npn transistors are shown in the figures.

Common-Base Configuration

The term common-base is derived from the fact that the base is common to both the input and output sides of the configuration.

The arrow in the symbol defines the direction of emitter current through the device.


That is, direction of IE is the same as the polarity of VEE and IC to VCC .

Also, the equation IE = IC + IB still holds.

The applied biasing are such as to establish current in the direction indicated for each branch.


Figure: Common-base configuration of pnp transistor

IE

IB

ICIE

IB

IC


Figure: Common-base configuration of npn transistor

IE

IB

ICIE

IB

IC


An input current (IE) is a function of an input voltage (VBE) for various of output voltage (VCB ).

The driving point or input parameters are shown in the figure.

This closely resembles the characteristics of a diode.

Input characteristics


As an approximation, the change due to changes in VCB can be ignored.The characteristics can be shown in orange curve.

If piecewise-linear approach is applied, the blue curve is obtain.

Furthermore, ignoring the slop of the curve and the resistance results the red curve.


It is this red curve that is used in the dc analysis of transistors.

Once a transistor is in “on” state, the b-evoltage is assumed to be 0.7V.

And the emitter current may be at any level as controlled by the external network.


Figure: Input characteristics for common-base transistor


Figure: Equivalent model for b-e junction


The output set relates an output current (IC) to an output voltage (VCB) for various of level of input current (IE ).There are three regions of interest:

Output characteristics

Active regionIn the active region, the b-e junction is forward-biased, whereas the c-b junction is reverse-biased.


The active region is the region normally employed for linear amplifier.

Cutoff region

Also, in this region,

I C IE

The cutoff region is defined as that region where the collector current is 0A.


In the cutoff region, the b-e and c-b junctions of a transistor are both reverse-biased.

Saturation region:

It is defined as that region of the characteristics to the left of VCB= 0 V.

In saturation region, the b-e and c-bjunctions of a transistor are both forward-biased.


Figure: Output characteristics for common-base transistor

Saturation Region

Cutoff Region

Active Region


Alpha (α)

In the dc mode, the levels of IC and IE at the operation point are related by:

Normally, α 1.

αdc = IC / IE

For practical devices, α is typically from 0.9 to 0.998.


Common-Emitter Configuration

The common-emitter configuration with npnand pnp transistors are shown in the figures.The term common-emitter is derived from the fact that the emitter is reference to both the input and output terminals.

The current relations are still applicable, i.e.,

IE = IC + IB and IC =α IE


Figure: Common-emitter configuration of npn transistor


Figure: Common-emitter configuration of pnp transistor


An input current (IB) is a function of an input voltage (VBE) for various of output voltage (VCE ).

The characteristics of the input or base-emitter circuit is shown in the figure.

The magnitude of IB is in μA and not as horizontal as IE in common-base circuit.

Input characteristics


Figure: Input characteristics for common-emitter transistor


The output set relates an output current (IC) to an output voltage (VCE) for various of level of input current (IB ).There are three portions as shown:

Output characteristics

Active regionThe active region, located at upper-right quadrant, has the greatest linearity.


The curve for IB are nearly straight and equally spaced.

In active region, the b-e junction is forward-biased, whereas the c-b junction is reverse-biased.

The active region can be employed for voltage, current or power amplification.


For linear amplification, cutoff region should be avoided.

Saturation region:The small portion near the ordinate, is the saturation region, which should be avoided for linear application.

Cutoff regionThe region below IB = 0μA is defined as cutoff region.


Figure: Output characteristics for common-emitter transistor

Saturation Region

Cutoff Region

Active Region


Beta (β)In the dc mode, the levels of IC and IB at the operation point are related by:

Normally, β ranges from 50 to 400.

βdc = IC / IB

For ac situations, β is defined as

constant

CEVB

Cac I

I


The proper biasing is essential to place the device in the active region.

A common-emitter amplifier of a pnptransistor is shown in the figure.

Biasing

1. The first step is to indicate the direction of IE as established by the arrow in the transistor symbol.


3. The supplies are introduced with polarities that will support the resulting directions of IB and IC .

2. The other current , IB and IC , are introduced, satisfying IC + IB = IE .

4. If the transistor is a npn transistor, all the current and polarities would be reversed.


Figure: Biasing for common-emitter pnp transistor


Common-Collector Configuration

The common-collector configuration with npn and pnp transistors are shown in the figures.

It is used primarily for impedance-matching purpose since it has a high input impedance and low output impedance.


The collector is tied to ground and the circuit resembles common-emitter circuit.

The load resistor is connected from emitter to ground.

The output set relates an output current (IE) to an output voltage (VCE) for various of level of input current (IB ).


This is almost the same as the output characteristics of common-emitter circuit, which are the relations between IC and VCEfor various of level of input current IB.

Since that: I E IC .

The input characteristic of common-emitter are sufficient for requirement of common-collector circuit.


Figure: Common-collector configuration of npn transistor


Figure: Common- collector configuration of pnp transistor


Figure: Common-collector circuit used for impedance-matching purpose


Summary of Chapter 3• Three-terminal devices, transistor

• Three types of configurations:

• Proper biasing of the three configurations.

common-base, common-emitter and common-collector.

• Input and output characteristics of the three configurations.

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