Bias Circuit

Equation C.67
The result is significantly lower than f2 in (C.61) because Rop(f) is in parallel with ron, which is
much smaller than the high-frequency value of Rop(f). The parameter βacp appears from the
association βacp = gmrπp [(C.15)].
Mathcad-generated plots of (C.65) for the exact and approximate values for fp and fz are
shown in Fig. C.14. The parameter and component values are from the plots of Fig. C.13 with
the addition of VAFn = 250 V, βacn = βacp = 50, and RBn = RBp = 330 kΩ. With capacitor Cb = 2
µF, f3dB = 3.8 H (exact fp and fz) and f3dBapprox = 4.5 Hz [(C.67)].
Figure C.14. Plots of (C.65) with approximate and exact values for fz,
(C.63), and fp, (C.66). The approximate form is sufficiently close for
selecting Cb.