Minimal transistor implementation: A single bipolar junction transistor (BJT) stage can be wired to implement which basic digital logic gate?

Introduction / Context:
At the lowest level, logic gates are built from transistors and resistors. Understanding what a single transistor can realize helps in appreciating how complex logic functions are composed and why certain gates are considered universal only when more devices are used.

Given Data / Assumptions:

• Single active device (one BJT) with resistive biasing.
• Digital operation (saturation/cutoff for switching or small-signal inversion for logic-level inversion).
• Standard positive logic conventions.

Concept / Approach:
A common-emitter BJT stage provides inversion: a HIGH input drives the transistor into conduction and pulls the output LOW; a LOW input turns it off, allowing the pull-up to produce a HIGH output. Thus, a single BJT can act as a NOT gate (inverter). Creating AND, OR, NAND typically requires multiple transistors or diode-resistor networks with transistor inversion.

Step-by-Step Solution:

Verification / Alternative check:
Many TTL inverters (e.g., one section of a 7404 die) boil down to a single transistor stage performing inversion, often augmented by additional devices for noise margins and current drive. The core function, however, is inversion using one active device.

Why Other Options Are Wrong:

• AND / OR: Require combining multiple inputs; at least two active paths are needed.
• NAND: Although NAND can be made from series/parallel transistor networks, it still needs more than one active device to handle multiple inputs.

Common Pitfalls:
Assuming a diode-resistor network alone can implement robust logic levels with good noise margins—it typically needs a transistor stage for restoration and inversion.

Final Answer:
NOT gates