Bipolar Junction Transistors

my previous article was about programming and the uses of microprocessor in mobile phone , laptop and any device which is program , today i will talk about basic circuit elements and its uses in this post and in the next incoming post , from these circuit elements the basic circuit , a skeleton of a device is made, each elements has its own function , before learning modern techtenology it is neccessary to learn these basic ,electrical engineering is impossible without knowing these things, after my study i make a short form of it , which will be easy for the readers and will not consume his valuable time too much , lets have a look of it.

Introduction

A transistor is a device which can be used as either an amplifier or a switch. Let’s first consider it’s operation in a more simple view as a current controlling device

Basic Transistor Operation

Look at this one circuit as two separate circuits, the base-emitter(left side) circuit and the collector-emitter(right side) circuit. Note that the emitter leg serves as a conductor for both circuits.The amount of current flow in the base-emitter circuit controls the amount of current that flows in the collector circuit. Small changes in base-emitter current yields a large change in collector-current.

Transistor Structure

With diodes there is one p-n junction. With bipolar junction transistors (BJT), there are three three layers and two p-n junctions. Transistors can be a either pnp or npn type. 

Transistor Characteristics and Parameters

As previously discussed, base-emitter current changes yields large changes in collector-emitter current. The factor of this change is called beta

beta = IC/IB

There are three key dc voltages and three key dc currents to be considered. Note that these measurements are important for troubleshooting. 

IB: dc base current 

IE: dc emitter current

IC: dc collector current

VBE: dc voltage across base-emitter junction

VCB: dc voltage across collector-base junction

VCE: dc voltage from collector to emitter

For proper operation the base-emitter junction is forward biased by VBB and conducts just like a diode. 

The collector-base junction is reverse biased by VCC and blocks current flow through it’s junction just like a diode.

Remember current flow through the base-emitter junction will help establish the path for current flow from the collector to emitter. 

Analysis of this transistor circuit to predict the dc voltages and currents requires use of Ohm’s law, Kirchhoff’s voltage law and the beta for the transistor. 

Application of these laws begins with the base circuit to determine the amount of base current. Using Kichhoff’s voltage law, subtract the .7 VBE and the remaining voltage is dropped across RB. Determining the current for the base with this information is a mThe collector current is determined by multiplying the base current by beta. atter of applying of Ohm’s law.  VRB/RB = IB

What we ultimately determine by use of Kirchhoff’s voltage law for series circuits is that in the base circuit VBB is distributed across the base-emitter junction and RB in the base circuit. In the collector circuit we determine that VCC is distributed proportionally across RC and the transistor(VCE)

Collector characteristic curves gives a graphical illustration of the relationship of collector current and VCE with specified amounts of base current. With greater increases of VCC , VCE continues to increase until it reaches breakdown, but the current remains about the same in the linear region from .7V to the breakdown voltage.

With no IB the transistor is in the cutoff region and just as the name implies there is practically no current flow in the collector part of the circuit. With the transistor in a cutoff state the the full VCC can be measured across the collector and emitter(VCE)

Current flow in the collector part of the circuit is, as stated previously, determined by IB multiplied by . However, there is a limit to how much current can flow in the collector circuit regardless of additional increases inOnce this maximum is reached, the transistor is said to be in saturation. Note that saturation can be determined by application of Ohm’s law. IC(sat)=VCC/RC The measured voltage across this now seemingly “shorted” collector and emitter is 0V. IB. 

The dc load line graphically illustrates IC(sat) and Cutoff for a transistor

The beta for a transistor is not always constant. Temperature and collector current both affect beta, not to mention the normal inconsistencies during the manufacture of the transistor.

There are also maximum power ratings to consider. 

The data sheet provides information on these characteristics..

in the next article i will let u know about transistor amplifier and transistor switch . be in tough with this blog and learn more and more..................continue