TTL output stage design — purpose of the totem-pole Why does the TTL logic family use a totem-pole output structure on many gates?

Introduction / Context:
Transistor–transistor logic (TTL) commonly employs a totem-pole output. Recognizing its purpose is key to understanding fan-out, rise/fall times, and why wired-OR is not permitted on standard TTL outputs.

Given Data / Assumptions:

• Standard TTL gate with a push-pull (totem-pole) pair at the output.
• Comparison to open-collector style outputs.
• Focus on drive capability and switching speed.

Concept / Approach:

A totem-pole stage stacks two transistors so one sources current (active pull-up) while the other sinks current (active pull-down). Only one conducts at a time, yielding low output impedance in both logic states. This reduces rise and fall times markedly compared with passive pull-ups or a single active device.

Step-by-Step Solution:

Verification / Alternative check:

Contrast with open-collector outputs that require an external resistor for pull-up; these are slower due to RC charging and allow wired-OR. Totem-pole forbids tying outputs together because two active drivers would contend (short conflict).

Why Other Options Are Wrong:

• Active pull-up only / active pull-down only: incomplete; totem-pole provides both.
• Enables wired-OR: incorrect; totem-pole prevents wired-OR due to contention.
• Eliminates output resistors: not the core reason; some series limiting may still exist.

Common Pitfalls:

• Assuming all TTL is totem-pole; open-collector variants exist for wired-OR and level interfacing.
• Forgetting that bus sharing requires tri-state or open-collector, not totem-pole.

Final Answer:

It provides active pull-up and active pull-down.