i want to know that how to they select the value of resistors in amplifier circuits!for example " http://www.flickr.com/photos/aj_silverthunder/3817569935/ " in that schematic u can see R,R3,R4 resistors,they resist voltage to the transistor,but on what reason they select the value.what will be the result if a low value resistor is used instead of R,R3,R4, and
what if higher value is selected,what will be the result,on what parameters of the transistor do they select the value! please explain me

 

They are selected to bias the transistor and put it in some working mode (saturated/linear/cutoff for bjs's for example). Look up transistor biasing. It can get quite complex and this is a topic that cannot be fully explained in a single post. There are entire books and courses on this material.

 

As Steinar said, the selection of the components in a circuit (and not only the resistors) determines the input/output voltage range, the frequency response of the circuit, the power levels etc. Depending on the application this can be an easy to a very complicated process. Each application has different requirements thus it is better to post what you want to make and the members of this forum will be glad to help you.