Thank you.. 80% of my doubt is cleared..First you go to shop and buy a BJT. For example you buy BC548B.
Next you look into the data sheet and check for Hfe_min = 200 and Hfe_typical = 250.
http://www.b-kainka.de/Daten/Transistor/BC548.pdf
Now you start design your circuit.
For example we have Vcc = 10V and we want Vce = 5V.
So we choose Ic = 10mA
Rc = (10V - 5V)/10mA = 500Ω so we use 470Ω or 510Ω resistor.
We calculate the base current
Ib = Ic/Hfe = 10mA/250 = 40μA
Therefore
Rb = (Vcc - Vbe)/40μA = (10V - 0.65V)/40μA = 233KΩ so we use 220KΩ or 330KΩ.
Next we build the circuit and we test it.
I build this circuit on the breadboard and use two different BC548B (form different manufacture ).
And the results of measurements are :
Vce = 5.7V and Vce = 5.1V for Rb = 330KΩ and Rc = 510Ω.
As you can see the result are not so bad as for such a simple and bad circuit.
We have a simpler situation in circuit when BJT need to work as switch.
The simply we select Ic/Ib = 20 ... 10 and we have the end of a story.
No, the load determines the Ic current that we choose in a particular application.So you say that ...we randomly assume some Ic current ??
THANKS for replies...Normally, the transistor is driving a load, another stage, a speaker, a soleniod, etc., and the load determines the Ic, although sometimes indirectly.
thank you..it is true. Most circuits are designed from the finish to the beginning. The final load demands a particular ic from its driver, and you are designing a driver. The final load might be a speaker, another transistor stage, the input to an integrated chip, a length of shielded cable (at a particular frequency), or many other things. One form of thinking that helped me is to assume the circuit will work correctly, then calculate backwards to see what it needs.
Of course, you will try circuits that seem right, then find out it will get too hot or fail at the highest frequency you need. Then you change your design. Almost nobody gets it right the first time. Besides that, as a project gets larger, more parameters must be balanced from stage to stage and compromises must be made. Right now, you are doing one stage at a time. You are supposed to discover what happens when you focus on one aspect then calculate what will happen to other aspects because of what you chose at first. This is the experience that will eventually teach you how to guess very close to what you need the next time you need a circuit like this.
As audioguru said, this circuit is useless in a production line design, but that is one of the things you are supposed to learn from this exercise.
by Jake Hertz
by Aaron Carman
by Aaron Carman