Transistor-level NOR and XNOR

by ourcircuits

13 parts

Two logic gates built out of transistors and resistors, the way gates were made before gates were parts. On the left, resistor-transistor NOR: two NPNs share one 4.7 kΩ collector resistor with their emitters grounded, so either base going high drags the output down. On the right, XNOR: each base is driven through 27 kΩ from one input while its emitter is tied to the other, so a transistor conducts only when the two inputs disagree. A 1 kHz and a 500 Hz clock walk all four combinations every 2 ms, and the live Ebers-Moll models reproduce both truth tables exactly — NOR measures 19.6 mV with both inputs high, 26.5 mV with one, and 5.0000 V with both low, while XNOR measures 5.0000 V when the inputs agree and 42.2 mV when they differ. Those are real levels off a real rail, produced by a measured 0.670 V of V_BE and a pull-up through Rc, where the palette's behavioural NOR and XNOR parts simply force exactly 0 V and exactly 5 V no matter what hangs off them.