The following animation illustrates the characteristic curve family of a bipolar junction transistor (BJT) in a common-emitter configuration. Input, output, transfer, and current gain characteristics are displayed together in a four-quadrant diagram and interactively linked.
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Description of the Animation
The animation shows a simple transistor circuit (common-emitter configuration) on the left, with a collector resistor RA in the collector branch and the supply voltage Utot. To the right, the complete four-quadrant characteristic curve diagram is shown, in which all four characteristic curves of the transistor are displayed together.
The circuit and the characteristic diagram are interactively linked: changes to the supply voltage, base-emitter voltage, or collector resistor immediately affect the operating point, the voltage distribution in the circuit, and the current flow.
The Four Characteristic Curves
- Output characteristic family (Quadrant I): Collector current IC as a function of UCE for various base currents IB (red curves, 20–100 μA)
- Current gain characteristic (Quadrant II): Relationship between base current IB and collector current IC
- Input characteristic (Quadrant III): Base current IB as a function of the base-emitter voltage UBE
- Transfer characteristic (Quadrant IV): Collector current IC as a function of UBE
The red load line in the output characteristic diagram is determined by the supply voltage Utot and the collector resistor RA. Its intersection with the current IB curve defines the operating point of the transistor.
Fundamental Relationships
In the active region, the collector current is approximately proportional to the base current:
\[ I_C = \beta \cdot I_B \]
with the current gain factor \( \beta \) (also hFE, typically 50–500). For the currents, Kirchhoff’s current law applies:
\[ I_E = I_C + I_B \]
The load line describes the relationship between collector current and collector-emitter voltage in the external circuit:
\[ I_C = \frac{U_{tot} – U_{CE}}{R_A} \]
Interactive Controls
The following quantities can be adjusted using the controls:
- Utot (0–10 V): Supply voltage – shifts the endpoint of the load line along the UCE axis
- UBE (0–0.8 V): Base-emitter voltage – determines the base current via the input characteristic and thus the operating point
- RA (0–2000 Ω): Collector resistor – changes the slope of the load line
Checkboxes allow additional quantities to be shown or hidden: the partial voltages across the collector resistor (URA) and across the transistor (UTra), as well as the total current (Itot) and the base current (IB).
Physical Background
The characteristic curve family summarizes the electrical properties of the transistor in a single diagram and makes the coupling of the four characteristic curves visible. A small base current controls a much larger collector current — this is the basis of the transistor’s amplifying action.
Depending on the position of the operating point, the transistor operates in different regions:
- Cut-off region: No significant current flow — the transistor is switched off.
- Active region (linear region): The collector current is approximately proportional to the base current — essential for amplifier circuits.
- Saturation region: The collector current is limited by the external circuit — important for switching applications.
The transition between these regions is directly visible: as UBE is gradually increased, the operating point moves along the load line from the cut-off through the active and into the saturation region, while the voltage distribution between the collector resistor and the transistor changes simultaneously.
Practical Applications
- Amplifier circuits: Setting the operating point for linear signal amplification
- Switching applications: Using the transistor as an electronic switch between the cut-off and saturation regions
- Circuit design: Graphical determination of currents, voltages, and resistances from the characteristic diagram
- Teaching: Illustrative presentation of the interplay of the four transistor characteristics
Overview
| Title | Transistor Characteristic Curves |
| Target Audience | Teachers and Lecturers |
| Features | Full-screen mode Lossless scaling Large screens and projectors supported |
| License | MIT |