interfacing between stages

Two railway lines only carry a through train if their gauges match and the platforms line up. So it is when one circuit stage hands its signal to the next: the handoff only works if their electrical edges fit. Interfacing between stages is the deliberate design of that handoff — making sure the impedance, the voltage level, the speed, and the power one block offers actually match what the next block can accept, so the signal passes cleanly instead of being weakened, distorted, or lost at the join.

The most common interface question is impedance. A stage with a high output resistance feeding a stage with a low input resistance suffers the loading effect: the second stage drags down the first, like a thin hose feeding a wide pipe. The usual cure is a buffer (a voltage follower) between them — high input impedance so it does not load the source, low output impedance so it can drive the next stage stiffly. For voltage signals you want the source impedance low and the load impedance high (voltage bridging). The other interface questions are just as real: levels (a 5 V logic output into a 3.3 V input needs level shifting or it damages the part), and coupling (a DC-blocking capacitor passes the wanted AC signal while keeping one stage's bias voltage out of the next).

Honestly, more real-world failures hide in the gaps between blocks than inside any block. Each block, tested alone with a friendly lab source and load, behaves; chained to its real neighbours it may not, because the neighbour is not the friendly lab load. Note one important exception to the impedance habit: for power transfer or high-frequency signals on a transmission line you instead match impedances (equal source and load) to avoid reflections — but matching delivers only 50 percent efficiency, so it is for signals and RF, not for delivering bulk power. Knowing which rule applies where is a hallmark of the designer.