Bias Circuit

The minus sign comes from having assigned the current direction of the emitter current out of
the transistor (Fig. B.7). The parameter IS is common to (B.7) and (B.13). Equations (B.7) and
(B.13) are combined to obtain the general case of VBE and VBC both positive (forward bias) in a
following unit.
The reverse-operation base current is [the reverse-operation equivalent of (B.12)]
Equation B.14
where βR is the dc current ratio (reverse-active mode) for the case of no leakage component of
base current, and is given by βR = IE/IB. In practice, the reverse ideal current gain is 0.1 < βR
< 10, and therefore, the reverse-operated transistor is not a useful configuration in, for
example, analog amplifiers. However, the reverse-active mode equations are essential, as is
shown below, for developing the general equations for the forward mode. The general form
includes the possibility that the transistor will be out of the active mode.
Although VAR (reverse Early voltage), ISC (reverse leakage saturation current) and nc (base –
collector leakage emission coefficient) are officially SPICE parameters, we will not be
concerned with these as they would rarely be a factor in the study of analog circuits.