how do they select the value of resistors!

i want to know that how to they select the value of resistors in amplifier circuits!for example " https://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

We use our knowledge of Ohm's Law and electronics to design circuits.

The values of the resistors are selected in a way that the transistor parameters don't play a major rola in the design anymore. The parameters of a transistor can vary much more than the value of a fixed resistor.

For example if the Hfe of T1 is 200 or 400, the circuit should still work as expected.

Stefaan

Hi aj_silverthunder,

perhaps my answer does not satisfy you because it is a rather general one, but I think it makes no sense to answer specifically to the circuit presented by you.

My answer is very simple: Each resistor should be selected in such a way that he can do his job as good as possible. What does this mean?
That means:
1.) You have to know the task of each resistor within the circuit (not always easy).
2.) You should know and consider that selection of a certain value for a resistor will influence the wanted property of the circuit (for example: gain) as well as other parameters - normally in the unwanted direction (for example: output voltage range).
3.) As a consequence, to find a proper value for each resistor means to find the best compromize between conflicting requirements resp. properties.
4.) Example: The voltage divider at the base of a simple BJT in common emitter configuration should have a very low resistance level (niveau) in order to fix the base voltage - independent on transistor parameters (principle of voltage source).
On the other hand, the resistor values must not be too low because of power consumption. Thus, very often the compromize leads to values in the lower kohm range.
5.) General rule for electronics: By changing the value of one component, one (if you are lucky: two) performance parameter(s) improve - and at the same time some other parameters/properties are becoming worse. And it is the task of a good technician/engineer to find a good compromize.
____________
Regards

The value of resistor is taken by applying the kirchoff's current and voltage loop. For example the Ic= hfe.Ib..we have to take the value of input voltage Vcc ,,and then forming the closed loop equation we get the value of resistors directly.

shashipoddar said:

The value of resistor is taken by applying the kirchoff's current and voltage loop. For example the Ic= hfe.Ib..we have to take the value of input voltage Vcc ,,and then forming the closed loop equation we get the value of resistors directly.

Click to expand...

I am afraid you forgot that the BJT acts as a current source. It isn´t so simple!