C-X1a · A small current controls a larger one

Transistor Bias and Output Curves

Why does the collector current stop following the base current at low output voltage? At low output voltage both transistor junctions conduct forward, so extra base current no longer produces the usual increase in collector current. Raising the output voltage reverse-biases the collector junction and restores amplification.

Drive the base of a transistor and watch electrons cross its thin base. Record an output sweep to find the change from saturation to amplification.

Collector current $0$

Base-emitter voltage $0$

Emitter current $0$

Device power $0$

The reciprocal Ebers–Moll model describes a synthetic NPN transistor at a fixed temperature of three hundred kelvin. Gains are constant, and leakage follows the same ideal junction law. The model omits Early effect, series resistance, high injection, breakdown, charge storage and self-heating. This junction transistor illustrates carrier control; it does not reproduce the geometry or measurements of the 1947 point-contact transistor. Layer widths, particle numbers and speed are schematic.

Synthetic model acquisition

A transistor output sweep

The stored sweep retains the settings used to generate it.

V_{CE}/\mathrm V,\quad I_C/\mathrm{mA}

The dot marks the operating point stored with this sweep. Reacquire after changing settings.

Start with the essentials

Focus question: Why does the collector current stop following the base current at low output voltage?
One-sentence intuition: A thin base lets most injected electrons reach the collector; forward bias of both junctions spoils the usual current gain.

Core mathematical model

Current continuity

I_E=I_B+I_C

The emitter current leaving the transistor equals the two terminal currents entering it.

Forward-active current gain

I_C\simeq\beta_F I_B

This approximation applies when the collector-base junction is reverse-biased, away from saturation and breakdown.

Common difficulties

A common inference to test

Typical misconceptionA transistor multiplies base current by the same factor at every output voltage.

Better mental modelThe familiar gain law needs reverse bias at the collector junction. In saturation both junctions conduct forward and their coupled currents must be solved together.

Run the experiment

01
Start in the amplification region
Choose Amplify and change Base current.
What to observe: The electron stream and collector current grow together.
02
Reduce the collector voltage
Choose Saturation, then increase Collector voltage slowly.
What to observe: The collector current rises through a knee before approaching a plateau.
03
Record the output
Record sweep and compare the low-voltage knee with the nearly flat high-voltage branch.
What to observe: The stored curve holds the base current and gain fixed while scanning collector voltage.