Bipolar Ecl 1960s Advance to Mecl Ii

# Bipolar ECL Logic (1960s): Step 16 — Advancing to MECL II

Motorola's second-generation refinement, and what actually changed. MECL II, introduced in the mid-1960s, roughly doubled switching speed over MECL I without changing the fundamental current-switch topology established in Steps 2-3. This step covers what process and circuit refinements MECL II made, since the topology itself was already settled by this point in the series.

## 1. What Got Faster, and Why
MECL II's speed gain came primarily from two sources that do not require any new circuit topology: faster bipolar transistors (improved epitaxial planar processing reduced base width and junction capacitance, raising f_T and directly shortening the transit-time term identified in Step 9's delay breakdown), and tighter internal biasing that reduced the voltage swing and node capacitance the current switch had to charge on each transition, within the noise-margin budget worked out in Step 13. Typical MECL II gate propagation delays reached roughly 1-2 nanoseconds, a meaningful step down from MECL I's 2-8 nanosecond range, achieved by optimizing the same circuit this series has already fully derived rather than inventing a new one.

## 2. Real Diagram: MECL I vs. MECL II Delay and Process Comparison

## 3. The Tradeoff MECL II Did Not Solve
MECL II's speed gain did not come with a power reduction — if anything, the faster transistors and tighter biasing kept per-gate power draw in roughly the same 1.5-3 milliwatt range as MECL I, consistent with the state-independent power mechanism from Step 11: making the current switch faster generally means either more current or less capacitance, and MECL II mostly used the 'more current' lever. This left the power/speed tradeoff unresolved going into the next generation, MECL 10K, which took a different approach by prioritizing manufacturability, interfacing ease, and moderate power over raw speed — the subject of Step 17.

## 4. Companion Diagram: Stage Sequence and Trend Summary

## Advancing to MECL II's Place in the Process Lineage
This is step 16 of 20 in the Bipolar ECL Logic (1960s) series, covering MECL II's process- and biasing-driven speed improvement over MECL I without a change to the underlying current-switch topology.