One important requirement during amplification.
Vbe at a given collector current has a negative temperature coefficient. The weak signal is given to the base of the transistor and amplified output is obtained in the collector circuit. The term ‘biasing’ is typically used in electronics to describe a fixed DC voltage or current applied to a terminal of an electronic component such as a diode, transistor, or vacuum tube in a circuit with both AC and DC signals present, to establish proper operating conditions. If the resistors are very low value, the bias current variation is defined more by the relationship between Vbe and collector current. In the extreme, R2 is open and R1 + hFE determines the collector current. Figure 1.1 shows the effects of proper and improper DC biasing of an inverting amplifier. If an amplifier is not biased with correct DC voltages on the input and output, it can go into saturation or cutoff when an input signal is applied. If the resistors are very high value then the value of R1 and the transistor hFE characteristic dominates the value of bias current. Bias establishes the DC operating point for proper linear operation of an amplifier. ** There is a degree of freedom with the voltage divider. By keeping R1||R2 low enough, the importance of hFE is reduced (too low and it wastes power). By adding Re, you can make the bias current dependent on Vbe+Vre, and if you make Vee = Re Ie ~= ReIc > Vbe, then the importance of Vbe is greatly reduced. 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. 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. 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. 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.
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