How to select the component

hi all,

What are the specifications i need to consider before selecting a component for FET, OPAMP, Controllers, etc..
Kindly give me a note about the component selection.....

Hi, this is a complex question, and is the reason why engineers spend time studying, then even more time on a factory floor. What is your specific application, what parameters have to be met in the design, what is the target cost, etc etc, and the list goes on.

Richard is correct,
Other questions, what is the temperature range, frequency requirements, second source requirements, the list goes on and on.

You don't say where you are coming from in asking this question, are you hobbyist, training at a technical college or university, starting out in a job, all have a bearing on how a start could be made in answering this question.
There is a free ebook oriented towards someone with some engineering experience at the university level, if that's where you are at, download it from this location.
http://www.eevblog.com/files/seekPDF.pdf
It is oriented towards someone doing volume production, but I believe anybody could get something from looking through this book.
No matter what level you are at, it is always interesting to ask why a design engineer chose what the did in a piece of electronics, I suggest that you do this every opportunity that you get. You will learn a lot that way, and it may result in the person you are asking having to justify themselves, which is an interesting exercise.

Thanks for your reply both of you...

I am a PCB Designer but am not selecting the components based on the reference design and guidance of my senior only am designing the PCB. I want to know how to select the components for a specific design like Analog and digital circuits.

Thanks & Regards,
Santhosh

I am not sure how you will design a PCB without referring to the design (schematic, I presume).

For resistors its usually sufficient to just know the power rating. Not so for capactitors/inductors where you need to know the value, voltage and/or current rating to be able to choose a suitable real-estate and pin-spacing. It gets even more complicated with BJTs - 2 devices with the same (or similar) specs can have different pin configurations (B-C-E or C-B-E or any combination thereof). So once you have selected a particular device outline and placed the PCB tracks as per the schematic, that pretty much ties the particular BJT to the PCB - other replacements or equivalents might not work till they have the same pin-out.

As regards analogue & digital circuits, you need to pay attention to the layout power supply tracks - having tracks that carry power to digital circuits in close proximity to analogue signal lines carries the risk of inducing noise into the analogue sections. Apart from that high-speed digital circuits have their own special rules for PCB layout...

What you said is right, somewhat i know the basic rules and regulations of the Design
in schematic and PCB Layout, i want to know in depth of knowledge about selecting the components thats why am asking...
if i am selecting an Opamp means What are the parameters will differentiate with the various opamps like LM324, LM358, OPA2188,, in that which is the major parameters i need to consider before selecting....

The selection of what op-amp to use falls in the scope of design and this translates to the schematic. I am not sure how you can select an op-amp at the time of PCB design which comes after the circuit design phase, unless I have misread your post.

Anyways, assuming that you do want to directly put an op-amp into the PCB, you need to consider the package that you want - a single, dual or quad op-amp package (again how many you need depends upon design). Most op-amp package (ICs) have standardised pin-outs except certain single op-amp packages and you need to refer to the particular op-amp's datasheet for this. Other considerations are trying to layout the tracks so that any ground-loops are eliminated: Since op-amps have very high input impedance and open-loop gain, careless layout can cause them to oscillate - especially true for high-speed op-amps.