Define Resistor Transistor Logic as the Incumbent Bipolar Baseline This Series Compares Against

# Define Resistor-Transistor Logic: The Incumbent Bipolar Baseline This Series Compares Against

## 1. Why RTL's Specific Circuit Topology Matters, Not Just Its Name

Resistor-transistor logic implements a NOR gate using input resistors feeding the base of a single bipolar transistor, whose collector output is pulled up by a resistor to the supply — the simplest possible bipolar logic topology, and the one already in volume production across the industry by the early 1960s, which is why this series treats it as the baseline MOS is actually being compared against, not a strawman chosen after the fact. The circuit's own fan-in/fan-out arithmetic sets the comparison's terms directly:

## 2. Real Diagram: RTL's Fan-Out Chain, Where the Design Gets Hard

RTL's defining limitation shows up specifically when one gate's output has to drive several downstream gate inputs: each added fan-out load pulls the output voltage swing down further, and RTL's narrow noise margin — the gap between a valid '1' and a valid '0' — erodes with every additional load, a structural constraint the next step quantifies directly.

## 3. RTL's Fan-Out Chain, Where the Design Gets Hard

This fan-out-dependent noise margin is exactly what DTL, taken up next, was specifically designed to address — not a different switching mechanism, but a circuit-topology fix aimed at this one measured weakness.