BJT explorer.

A toy for the bipolar junction transistor. The component in introductory analog electronics that most people I know found genuinely confusing — three terminals, two PN junctions, four operating regions, and the disorienting fact that a tiny base current controls a much larger collector current. This widget lays out a single common-emitter NPN circuit, and as you slide the biasing components it traces a live operating point across the family of characteristic curves and the DC load line. The curve family, the saturation knee, the active flat top — they all become moving parts you can poke at.

circuit

I_C vs V_CE

V_BB 2.00 V R_B 100 kΩ V_CC 12.0 V R_C 2.2 kΩ β150

What you're looking at.

A single NPN in common-emitter configuration. The base is biased by V_BB through R_B; the collector is loaded by R_C from V_CC; the emitter sits on ground. KVL around the input loop gives I_B = (V_BB − V_BE) / R_B, and around the output loop V_CE = V_CC − I_CR_C — the equation of the DC load line drawn on the right.

Each gray curve on the right is one value of I_B; the cobalt curve is your actual I_B. The operating point is the dot where the active I_B curve crosses the load line — that's where the circuit settles. The three regions tell you what the transistor is doing:

cutoff — V_BB < V_BE,on; the base-emitter diode isn't forward biased, no I_B, no I_C, and V_CE = V_CC. The transistor is "off."
active — base diode on, collector hungry for current, I_C ≈ β · I_B, with a slight upward slope from the Early effect. This is amplifier territory: a small change in I_B makes a much larger change in I_C.
saturation — V_CE drops to about 0.2 V and stops; I_C is now pinned at (V_CC − 0.2) / R_C by the load line, and adding more I_B does nothing. This is switch territory: V_CE ≈ 0 means the "switch is closed."

Try dragging V_BB from 0 to 5 V slowly and watch the operating point walk: cutoff at the bottom-right of the load line, climbing through active up to the saturation knee at the top-left. That walk is what a BJT does. Everything else — biasing schemes, small-signal models, AC analysis — is layered on top.

The dots flowing on the wires are conventional current — speed is proportional to magnitude, so the collector path moves much faster than the base path even when both look like "current." That ratio is β. Hover any component or wire to read its current value; the schematic stays clean otherwise.