Bias Circuit

Applied voltage Vo sums up to
Equation C.46
The approximation is based on ro >> RE and is consistent with the fact that the signal voltage
drop across the output of the transistor is much greater than across the emitter resistor.
Eliminating Ib in (C.46) using (C.45) results in the solution for Ro, which is
Equation C.47
The result has two limiting forms based on the relative value of RB: When RB >> rπ + RE,
Equation C.48
The approximate form uses IC
Intuitively, for RB
For RB
0,
Equation C.49
IE.
, the feedback current, gmIbrπ, goes to zero.
The alternative forms on the right in (C.48) and (C.49) use (C.15), βac = gmrπ. Again, the
approximate form uses IC IE. Note that the solution corresponds to the maximum fraction of
Io that can flow through rπ(RB = 0), to induce a feedback current.
When applied specifically to the npn – pnp circuit of Fig. C.10, the limiting case is, for RBp zero,
Equation C.50