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 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 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 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 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:
\[ I_C = \frac{U_{tot} – U_{CE}}{R_A} \]
Worked Example: Setting the Operating Point
The following example shows how to determine the operating point of a transistor circuit using the characteristic field — and how to verify each step in the animation.
Given: Supply voltage Utot = 9 V, desired collector current IC = 4 mA, current gain \( \beta \) = 200.
Step 1 — Determine RA: The transistor should operate in the active region. For an operating point near the middle of the load line, approximately half the supply voltage drops across the transistor: UCE ≈ 4.5 V. From the load line equation:
\[ R_A = \frac{U_{tot} – U_{CE}}{I_C} = \frac{9\,\text{V} – 4.5\,\text{V}}{4\,\text{mA}} = 1125\,\Omega \]
Step 2 — Determine IB: The required base current follows from the current gain:
\[ I_B = \frac{I_C}{\beta} = \frac{4\,\text{mA}}{200} = 20\,\mu\text{A} \]
Step 3 — Find the operating point: In the output characteristic field, the load line runs from UCE = 9 V (at IC = 0) to IC = 8 mA (at UCE = 0 V). The intersection of this load line with the IB = 20 μA curve yields the operating point.
Try it in the animation: Set Utot to 9 V and RA to approximately 1125 Ω. Then adjust UBE until IB ≈ 20 μA — you can read this from the input characteristic in Quadrant III. Observe how the operating point settles near the center of the active region.
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).
Try It Yourself: Guided Experiments
The following experiments help you build an intuitive understanding of the transistor’s characteristic field. Work through each one step by step using the animation above.
Experiment 1: From Cut-Off to Saturation
- Set Utot = 8 V and RA = 1000 Ω.
- Start with UBE = 0 V. Notice: no current flows — the transistor is in the cutoff region. The operating point sits at the right end of the load line.
- Slowly increase UBE to 0.6 V. A base current starts to flow through the input characteristic (Quadrant III), and the collector current rises — the operating point moves leftward along the load line into the active region.
- Continue to UBE = 0.8 V. The collector current reaches its maximum and is no longer proportional to IB — the transistor has entered the saturation region. Nearly all of Utot drops across RA.
Key insight: A small change in UBE (a few tenths of a volt) causes a large change in IC — this is the amplifying action of the transistor.
Experiment 2: The Effect of the Load Resistor
- Set Utot = 8 V and UBE = 0.65 V.
- Set RA = 500 Ω. Observe the steep load line and the resulting operating point.
- Increase RA to 1500 Ω. The load line becomes flatter — for the same base current, the collector current decreases and UCE drops.
- Enable the URA and UTra checkboxes. Watch how the voltage distribution between the resistor and the transistor shifts as RA changes.
Key insight: The load resistor determines how much of the supply voltage is “used up” by the resistor versus the transistor — a crucial factor when designing amplifier circuits.
Experiment 3: Voltage Amplification
- Set Utot = 10 V, RA = 1000 Ω, and UBE = 0.65 V.
- Enable the UTra checkbox to display the transistor voltage.
- Slowly vary UBE between 0.6 V and 0.7 V — a change of only 0.1 V.
- Observe how UCE changes by several volts in response. The ratio of the output voltage change to the input voltage change is the voltage gain of the circuit.
Key insight: The transistor does not “create” energy. The small input signal (UBE) controls the flow of energy from the supply voltage (Utot) to the output — like a valve controlling water flow.
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:
- Cutoff 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 visible: as UBE is gradually increased, the operating point moves along the load line from the cutoff through the active and into the saturation region, while the voltage distribution between the collector resistor and the transistor changes simultaneously.
Common Misconceptions
Working with transistor circuits leads to misunderstandings. The following points clarify the most common ones:
- “The transistor amplifies voltage on its own.” — Not quite. The transistor controls a large collector current using a small base current. Voltage amplification only occurs because this controlled current flows through an external resistor. Without RA, there is no voltage gain. Try it in the animation: set RA to a very low value and observe that UCE barely changes, regardless of UBE.
- “UCE = 0 V means the transistor is off.” — The opposite is true. When UCE approaches 0 V, the transistor is fully conducting (saturated). In the animation, you can see this at the left end of the load line, where nearly all of Utot drops across RA.
- “The current gain β is a fixed constant.” — In practice, \( \beta \) varies with temperature, collector current, and manufacturing tolerances. Two transistors of the same type can have different \( \beta \) values. This is why practical amplifier circuits use negative feedback rather than relying on a specific \( \beta \) value.
- “The characteristic curves are exact.” — The curves shown are idealized. Real transistors exhibit effects such as the Early effect (output characteristics are not horizontal) and temperature dependence (the input characteristic shifts with temperature). The animation uses simplified curves to keep the focus on the fundamental relationships.
Practical Applications
The transistor characteristic field is not only a theoretical diagram — it is an essential tool for dimensioning real circuits. The following examples show how the characteristic field is used in practice.
Switching an LED with a Transistor
A typical LED requires approximately 20 mA and should not be powered directly from a microcontroller output (which can only supply a few milliamps). A transistor acts as an electronic switch: the microcontroller provides a small base current, and the transistor switches the much larger LED current.
In the characteristic field, the operating point for a switch application lies either in the cutoff region (LED off) or in the saturation region (LED on, UCE very small). The active region is avoided — unlike in amplifier circuits.
Simple Audio Amplifier
In an audio amplifier, the transistor must reproduce the input signal as faithfully as possible. For this, the operating point is placed in the center of the active region, so that the signal can swing symmetrically in both directions without being clipped by saturation or cutoff.
In the animation you can experiment with this: set the operating point to the middle of the load line (approximately UCE ≈ Utot/2). Then imagine a small AC signal on UBE — the operating point moves up and down along the load line, and the resulting change in UCE is the amplified output signal.
Motor Control
Small DC motors typically draw currents of several hundred milliamps — far too much for a microcontroller pin. A transistor in the collector branch switches the motor current on and off. For reliable motor control, the transistor must be driven deep into saturation, so that UCE remains as low as possible and little power is dissipated in the transistor itself.
BJT vs. MOSFET: A Comparison
The bipolar junction transistor (BJT) shown in this animation is one of two major transistor families. The other is the MOSFET (Metal Oxide Semiconductor Field Effect Transistor). Both can amplify and switch, but they differ in several ways:
- Control mechanism: The BJT is current controlled — a base current IB controls the collector current IC. The MOSFET is voltage controlled — a gate source voltage UGS controls the drain current ID, with no current flowing into the gate.
- Characteristic field: While the BJT output characteristics show IC as a function of UCE for different IB values, the MOSFET output characteristics show ID as a function of UDS for different UGS values. The input characteristic of the MOSFET is a transfer curve (ID vs. UGS) rather than a diode like IV curve.
- Typical applications: MOSFETs dominate in digital circuits (every processor contains billions of them) and in power electronics due to their high efficiency and fast switching. BJTs remain relevant in analog circuits, audio amplifiers, and educational settings because their behavior is more directly tied to physical principles.
- Why learn the BJT first? The BJT’s characteristic field makes the relationship between input and output quantities particularly visible. Understanding current control, the load line, and operating point regions with a BJT provides a solid foundation for later working with MOSFETs and other semiconductor devices.
A Brief History of the Transistor
Before the transistor, electronic circuits relied on vacuum tubes — bulky, power hungry, and fragile glass devices. In 1947, John Bardeen, Walter Brattain, and William Shockley at Bell Labs demonstrated the first working transistor, a point contact device made from germanium. This breakthrough earned them the Nobel Prize in Physics in 1956.
The invention of the silicon transistor in the 1950s and the integrated circuit in 1958 set the stage for modern electronics. What once filled entire rooms — the ENIAC computer with its 18,000 vacuum tubes — could now be miniaturized to a single chip. Today, a modern processor contains over 10 billion transistors, each switching billions of times per second.
Despite this integration, the fundamental operating principle remains the same: a small signal controls a larger one. The characteristic field shown in this animation captures this principle — the same physics that Shockley described in his early publications still governs every transistor in your smartphone.
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Overview
| Title | Transistor Characteristic Curves |
| Target Audience | Teachers and Lecturers |
| Features | Full screen mode Lossless scaling Large screens and projectors supported |
| License | MIT |
Contributors
C. Hein, S. Rikowski