HFE Question + Resistor values and design Question+ Design question

hi,
am not going to talk very much (This is my first post) because I felt from the rules that this is illegal and I will go straight forward for my questions and Am not going to make any "thank you" post so thanks in advance (only this first time) for any reply.

My Questions:
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1) I have known that the dc current gain is const. for the same transistor but when I see the datasheet of 2n3904 I saw a graph between hfe and IC ... how??? , that's mean Beta is not constant ????????

2) As I know ( If wrong tell me please) that in real practical work the resistors have discrete values or series ex: 4.7,47,470,4.7k..... If this true...when someone design a circuit (not analysis) the calculated values are not necessary to belong to a certain resistor value ( I can't get the exact calculated value in reality) and if used the approximated vales the gain will be wrong and the impedance will be wrong....if I am designing black boxes ..(networks) .... the impedance will not be as calculated and the ac signal be be clipped......many problem ????? what is the solution ???? or if am thinking wrong please correct to me...

3) if any one can help me how and where to begin learning design...and what analysis qualifications are required.....and if legal tutorials exist please upload it..

Welcome.

hfe (or beta) is not a constant, as you have seen. Normally you try to make designs insensitive to hfe for that reason.

While components are not available in every value, you have a pretty big choice. If the 1, 1.2, 1.5, 1.8 ... range (E12) isn't good enough then E24 has twice as many values. If that isn't good enough then there is E48, E96 or E192. The tighter the tolerance the more it will cost though.

I am not sure of good online tutorials. I am sure there will be some - hopefully someone else will help.

Keith

So hfe is not const. and changes with Ic .. my multimeter measures the hfe, @ which current this hfe is measured?? and how do I know??? ... I don't have a manual.

As KEITH has mentioned already, hfe is not constant and - more than that - is even not known (unless you measure it) due to rather large tolerances. Therefore, it is an important task of the designer to make a circuit design that is unsensitive (as little as possible) to these uncertainties. In this context, the principle of negative feedback play the major role.

Regarding the resistor values available, you spoke about "wrong" gain values.
Please not and don`t forget that in analog electronics nothing (really nothing) is correct by 100%. Even ohms law neglects some (minor) effects. Therfore, nobody expects that the gain of an amplifier can equal the target gain. Each design goal must inlude and accept some tolerances - depending on the particular application. Thus, a gain cannot be "wrong" - it can possibly be out of the tolerance range only.

Your answers were really very helpful but there is one question .... which is: at which Ic my multimeter measures the hfe ?????......

You would need the multimeter manual to find that out. What is the make and model of the multimeter?

A cheap multimeter is likely to be quite crude and so not done at a specific current. For example, one I have seen uses a 180k base resistor and measures whatever base and collector current that gives so it won't be same for any two transistors.

Keith