Among the three basic BJT configurations (common emitter, common base, common collector), which configuration exhibits the highest input impedance?

Introduction / Context:
Small-signal amplifier design often starts by choosing a BJT configuration based on input/output impedance and gain needs. Recognizing which configuration gives the highest input impedance is vital for buffering and impedance matching, especially when interfacing high-impedance signal sources.

Given Data / Assumptions:

• Compare CE, CB, and CC configurations qualitatively.
• Assume typical transistor parameters (moderate beta, room temperature), linear region.

Concept / Approach:

The common collector (emitter follower) presents a high input impedance because the input sees the base, and the emitter follows with near-unity gain, reducing the effective base current demand. CB has very low input impedance (input at emitter), and CE has moderate input impedance set by r_pi in parallel with biasing networks. Thus, CC > CE >> CB for input impedance, generally speaking.

Step-by-Step Solution:

Verification / Alternative check:

Hybrid-pi or T-model analyses show CC input resistance roughly beta times the emitter resistance seen, plus base bias network, giving large values compared to CE or CB under similar conditions.

Why Other Options Are Wrong:

• CE: moderate input impedance; not the highest.
• CB: very low input impedance; the lowest of the three.
• Push–pull and cascode are multi-transistor topologies, not the basic single-transistor configurations asked.

Common Pitfalls:

Equating voltage gain with impedance; CC has near-unity voltage gain but excels as a buffer due to its high input and low output impedance.

Final Answer:

Common collector (CC, emitter follower)