9.42 Figure P9.42 shows a diode-connected transistor with
the bias circuit omitted. Utilizing the BJT high-frequency,
hybrid-π model with rx = 0 and ro = ∞, derive an expression for
Zi(s) as a function of re and Cπ . Find the frequency at which the
impedance has a phase angle of 45° for the case in which the
BJT has fT = 400 MHz and the bias current is relatively high.
What is the frequency when the bias current is reduced so that
Cπ Cμ? Assume α = 1.