Electrical – Emitter resistance in transistor bias


I am having issues with the emitter resistance during biasing of the transistor. Can someone explain me what's the point of using this resistor? It is getting very not intuitive for me.enter image description here

Best Answer

If you are trying to bias the transistor within a specific range of collector currents (regardless of which sample of the type of transistor drops out of the tape, regardless of resistor tolerances, supply tolerances and temperature) you will have a problem meeting that requirement without Re.

You could replace one of the resistors with a trimpot, but if Re = 0, the adjustment will be very fussy and when the transistor warms or cools the collector current will change a lot. As designers, we would like to avoid adjustments or special selection of components where possible, and have the circuit bias work well over a wide range of conditions.

Without Re, the bias current will be very dependent on Vbe and/or hFE**, transistor characteristics that are not tightly specified and which change with temperature. By adding Re, you can make the bias current dependent on Vbe+Vre, and if you make Vee = ReIe ~= ReIc >> Vbe, then the importance of Vbe is greatly reduced. By keeping R1||R2 low enough, the importance of hFE is reduced (too low and it wastes power).

** There is a degree of freedom with the voltage divider. If the resistors are very high value then the value of R1 and the transistor hFE characteristic dominates the value of bias current. In the extreme, R2 is open and R1 + hFE determines the collector current. If the resistors are very low value, the bias current variation is defined more by the relationship between Vbe and collector current. Vbe at a given collector current has a negative temperature coefficient. hFE at a given collector current usually has a positive temperature coefficent so they tend to add (collector current will increase with temperature).