---
title: "Transistor Bias and Output Curves Interactive Lab | Sandbox Physics"
description: "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."
canonical_url: "https://sandboxphysics.com/labs/transistor-output-characteristics/"
language: "en"
page_type: "lab"
alternate_languages:
  zh-CN: "https://sandboxphysics.com/zh/labs/transistor-output-characteristics/"
last_modified: "2026-10-24"
---

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.

> Interactive simulation: open this page in a web browser to change the parameters and run the model.

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.

> Synthetic sweep data. Axis definitions and model assumptions are retained in the text.

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

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

Nobel · Condensed matter

## What did the original experiments establish?

Follow the apparatus, observations and interpretation in the historical evidence. Each article explains where this teaching model fits.

[Read the historical evidence](https://sandboxphysics.com/stories/condensed-matter/transistor-point-contact/) [All six reading paths](https://sandboxphysics.com/stories/condensed-matter/)

Physics tutorial

## Transistor Bias and Output Curves

**Background** An NPN transistor places a thin hole-rich base between two electron-rich regions. The base-emitter junction injects electrons, and a reverse-biased collector junction collects most of them.

**Why it matters** The same base drive can give different collector currents near saturation. Measuring an output sweep reveals where the collector ceases to collect effectively.

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 misconception** A transistor multiplies base current by the same factor at every output voltage.

**Better mental model** The 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

1. 01

   #### Start in the amplification region

   Choose Amplify and change Base current.

   **What to observe:** The electron stream and collector current grow together.

2. 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.

3. 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.

Physical conclusion

A thin base lets most injected electrons reach the collector; forward bias of both junctions spoils the usual current gain.

Fields & matter · Starter path

## From a field to an electron microscope

Map an electric field and steer a charged particle. Then find the electron's wavelength and use electron beams to see a crystal and a surface.

[Start here → **Make an Electric Field Visible** Move the probe around a dipole and check that the field always points downhill on the potential landscape. Observe · 8 min](https://sandboxphysics.com/labs/electric-field/)

1. [1 Make an Electric Field Visible](https://sandboxphysics.com/labs/electric-field/)
2. [2 A Charge Bending through Fields](https://sandboxphysics.com/labs/charged-particle/)
3. [3 Electron Wavelength & Voltage](https://sandboxphysics.com/labs/electron-wavelength/)
4. [4 Electron Diffraction & Reciprocal Space](https://sandboxphysics.com/labs/electron-diffraction/)
5. [5 TEM Bright Field, Dark Field & SAED](https://sandboxphysics.com/labs/tem-mode-selector/)
6. [6 SEM Secondary-Electron Topography](https://sandboxphysics.com/labs/sem-topography/)

[View the full path →](https://sandboxphysics.com/topics/fields-and-matter/#starter-path)