Input impedance of a CE amplifier: By definition, the small-signal input impedance is the ratio of which two AC quantities at the input port?

Introduction / Context:
Input impedance is a fundamental parameter that dictates how strongly a stage loads its source. In a common-emitter (CE) BJT amplifier, we measure it at the base port.

Given Data / Assumptions:

• Small-signal linear operation.
• Port of interest: base node relative to signal ground.

Concept / Approach:

By definition, input impedance Z_in equals the applied small-signal voltage divided by the small-signal current entering the same port. For a CE amplifier, those quantities are the base voltage v_b and base current i_b.

Step-by-Step Solution:

Verification / Alternative check:

Differential hybrid-π model directly shows Z_in ≈ r_π || biasing network at midband, which derives from v_b/i_b.

Why Other Options Are Wrong:

• Options (b)–(d) use currents or voltages at other terminals and thus do not represent the input port definition.

Common Pitfalls:

• Confusing input transresistance (v/i at different ports) with input impedance (same port).

Final Answer:

AC base voltage to AC base current.