A little biasing theory, please

[nee]

So we want to bias our semiconductor for 4.5V. This can be done by adjusting either the emitter resistor, or the collector resistor (or both!). When one goes up the other has to go down to maintain 4.5V. How do we choose the correct combination of resistors, the correct "balance"? It can't just be a case of "any two resistors, as long as the result is 4.5V", can it?

Thanks!

IanG

[brett]

Hi
ah.  There's more to it than this.

Consider that the supply-to-base resistor controls the amount of current flowing through the base.  Together with the HFE (current multiplying factor of the BJT) and the resistance of the emitter resistor, this affests the voltage at the emitter.

Here's a link to a handy calculator to "play" with, and discover for yourself the factors involved.  Under "base bias type" we mostly use the voltage divider type.  (?? because it is less prone to effects of variation in hFE ??)

cheers

http://www.daycounter.com/Calculators/Transistor-Bias/NPN-Transistor-Bias-Calculator.phtml

[darrenbkl]

emm,how about more detail explain?? thx 

[DougH]

http://www.tpub.com/content/neets/14179/css/14179_61.htm

[R.G.]

Go search for and read my posts on biasing bipolar transistors.

If you want to bias a bipolar, the big keys are
(a) deciding ahead of time what voltage you want on the collector and emitter and how much current you want through the transistor
(b) assuming that is true, then calculating the resistors on emitter and collector to make the voltages be correct for that current
(c) [the big secret] set your biasing network to make the BASE VOLTAGE be one diode-drop above the emitter voltage you want.

The emitter of a bipolar with any kind of decent gain (over 100, perhaps) will be forever locked to the base VOLTAGE by the forward voltage drop of the base-emitter junction. It cannot be different and have the transistor be operating in a linear manner. So to bias at a certain voltage, you set the stage with resistors and currents you want, then force the base voltage to match with a biasing network.