Transistor Emitter Bias

Emitter bias provides excellent bias stability in spite of changes in β or temperature. It uses both a positive and a negative supply voltage. To obtain a reasonable estimate of the key dc values in an emitter-biased circuit, analysis is quite easy. In an npn circuit, such as shown in Figure, the small base current causes the base voltage to be slightly below ground.

The emitter voltage is one diode drop less than this. The combination of this small drop across R_B and V_BE forces the emitter to be at approximately  -1v. Using this approximation, you can obtain the emitter current as

V_EE is entered as a negative value in this equation.

You can apply the approximation that I_C = I_E to calculate the collector voltage.

V_C = V_CC – I_CR_C

The approximation that V_E = -1v is useful for troubleshooting because you won’t need to perform any detailed calculations. As in the case of voltage-divider bias, there is a more rigorous calculation for cases where you need a more exact result.

The approximation that V_E = -1v and the neglect of β_DC may not be accurate enough for design work or detailed analysis. In this case, Kirchhoff’s voltage law can be applied as follows to develop a more detailed formula for I_E. Kirchhoff’s voltage law applied around the base-emitter circuit in Figure (a), which has been redrawn in part (b) for analysis, gives the following equation:

V_EE + V_RB + V_BE + V_RE = 0

Substituting, using Ohm’s law,

V_EE + I_BR_B + V_BE + I_ER_E = 0