Bias Circuit

In electronic amplifiers, all three terminal configurations are possible: common base, common
emitter, and common collector (emitter follower). Themost frequently employed amplifier
stage is based on the common-emitter mode, but the other two serve important roles in
electronic amplifiers. In all three, the emitter – base voltage is the input control-terminal
voltage. In the common-emitter and common-base, the output current is the collector current
and in the common collector, it is the emitter current. The common-collector configuration is
usually referred to as the emitter follower.
In the subunits that follow, the detailed SPICE relations that generally relate the branch
currents to the terminal voltages are developed. This includes the addition of many aspects of
the real transistor which were not included in the discussion above of the highly idealized
transistor.
Initially, the forward-active mode is explored, and this is followed by a discussion of the
reverse-active mode. The reverse-active mode is where the situation in Fig. B.3 is reversed.
Thus, VCB is positive (base – collector junction forward biased), while VEB is zero or negative. In
the reverse-active mode, VCB becomes the input voltage, and the output current is the emitter
current, IE.
Although the transistor is not operated in the reverse-active mode, the relationships developed
for this case can be combined with those from the forward-active mode. The combination
produces the general equations for relating currents and terminal voltages in transistors for all
bias possibilities (i.e., where the transistor is biased out of the active mode).