Analysis of the BJT DC Bias Conditions:

A typical DC bias circuit for biasing the transistor to behaves as an amplifier may be usually simplified by invoking Thevenin's theorem. Therefore, looking out of the 3 terminals of the BJT, the Thevenin's equivalent as illustrated in the circuit in Figure is easily conceivable.

Figure: Basic Transistor Bias Circuit

To analyze such an equivalent circuit, we write a loop equation for the base-emitter loop.

                                                 V_BB  - V_EE  = I _B R_BB  +V_BE  + I _E R_EE

Then by using the relation I_C = β I_B =α I_E, we may express I_B and I_E as functions of the

current I_C , as follows :

V _BB       - V _EE  = I_C  / β  R _BB      +V _BE     + (I_C  / α  )R_EE

We can then solve for IC.

I_C  = V_BB  - V_EE  - V_BE / R_BB (I_C    / β) + ( I_C/  α ) R_EE

An "educated guess" of the value of V_BE should be made to evaluate this. Typical values are 0.7V for I_C transistors, 0.6-0.65 V for low-power discrete transistors, and 0.5-0.6 V for higher-power discrete transistors.

At last, we may check whether or not the transistor is in active mode. For the active mode, we must check that V_CB > 0, which may be done as follows :

V_CB  = V_C  - V_B  = (V_CC  - I_C R_CC ) - (V_BB  - I _B R_BB )

or         = (V_CC  - I_C R_CC ) - (V_EE  + I _E R_EE  + V_BE )