This tutorial visualizes an NPN Bipolar Junction Transistor (BJT): a thin Base (P) sandwiched between Emitter (N) and Collector (N). In active mode, the Emitter–Base junction is forward-biased and the Base–Collector junction is reverse-biased. A small base current I_b controls a large collector current I_c; most electrons injected from the Emitter are swept across the thin Base into the Collector.

 

Mathematical foundation

1. Structure

Emitter (N): High concentration of electrons. Base (P): Very thin (~10% width), moderate holes. Collector (N): Large region to collect electrons.

2. Active mode

V_BE > 0.7 V forward-biases the Emitter–Base junction: electrons enter the Base. V_CE > 0 reverse-biases the Base–Collector junction: the field pulls electrons into the Collector. Because the Base is thin, most electrons drift through (collector current I_c); a small fraction recombine (base current I_b).

3. Current gain

The three terminal currents obey Kirchhoff’s law, and in active mode the collector current is set by the base current through the gain β:

4. Energy bands

Two junctions: a "hill" at Emitter–Base (lowered by V_BE) and a steep "cliff" at Base–Collector (driven by V_CE) that sweeps electrons into the Collector.

Simulation

The interactive simulator is below. Use the controls to explore the concepts described above.

 

Usage

Use the sliders and Source to explore the NPN BJT:

1. Source: Choose DC (constant voltages), Sinusoidal, Pulse, or Triangle. The V_be and V_ce sliders set the magnitude (amplitude) for non-DC sources. The readouts show instantaneous voltages.
2. Wave speed (Hz): For Sinusoidal, Pulse, and Triangle, this sets the frequency. Use a low value (e.g. 0.1–0.4 Hz) to see the BJT respond in slow motion.
3. V_be (0–1.2 V): Base–Emitter voltage. Above ~0.7 V the EB junction opens: electrons leave the Emitter and enter the Base. Below 0.5 V little current flows.
4. V_ce (0–10 V): Collector–Emitter voltage. In active mode (V_ce > ~0.2 V) the BC junction pulls electrons into the Collector. Arrows show I_e, I_b, I_c (I_e = I_b + I_c).
5. β (gain): Current gain I_c/I_b. Higher β gives more collector current for the same base current.
6. Top canvas: Emitter (N, blue), thin Base (P, red), Collector (N, blue). Blue dots = electrons; red circles = holes in Base. Depletion regions at EB and BC. Energy band below: hill at EB, cliff at BC.
7. Bottom canvas: I_c vs V_ce for several I_b. Red dot = operating point. Saturation when V_ce is small.
8. Oscilloscope: Time-domain dual trace. Upper (yellow): Input V_be. Lower (cyan): Output I_c. With Sinusoidal/Pulse/Triangle sources you see how the BJT amplifies or clips the signal; in Cutoff the output trace flattens.

Comparison: Diode = one PN junction; BJT = two junctions (NP and PN). Small base current controls large collector current; the thin Base lets most electrons cross to the Collector instead of recombining.

Parameters

• V_be: Forward bias for Emitter–Base. ~0.7 V to turn on.
• V_ce: Collector–Emitter voltage. Active when V_ce > ~0.2 V.
• β: Current gain; I_c = β I_b in active mode.
• Energy band: EB hill (lowered by V_be), BC cliff (V_ce) sweeps electrons to Collector.

Lab trials

To help users bridge the gap between the physics animation and real-world electronics, here are four structured lab trials. These are designed to guide a beginner from basic switching logic to high-fidelity amplification.

Lab 1: The "Turn-On" Threshold (DC Analysis)

Objective: Observe the exponential nature of the BJT and find the "knee" of the curve where conduction begins.

Setup:

• Source: DC
• V_ce: 5.0 V (Fixed)
• V_be: Start at 0.0 V and slowly increase.

What to Observe:

• 0.0 V to 0.6 V: Notice the Energy Band "hill" is very high. No particles move to the Collector. I_c remains at 0.000 mA.
• 0.65 V to 0.72 V: The blue Energy Band hill drops below the Conduction Threshold. Suddenly, particles start "spilling" into the Base.
• Physics Insight: A tiny increase in V_be (e.g., from 0.70 to 0.75) causes a massive explosion in particle flow. This is the Exponential Region.

Lab 2: Phase Inversion & Voltage Gain (Sinusoidal)

Objective: Visualize how a BJT acts as an amplifier and why the output "flips" upside down.

Setup:

• Source: Sinusoidal
• V_be: 0.72 V (The "Bias" point)
• AC Amplitude: 0.02 V (Small Signal)
• V_ce: 5.0 V

What to Observe:

• Scope: Look at the relationship between the Yellow (Input) and Cyan (Output) waves.
• Phase Shift: When the Yellow wave peaks UP, the Cyan wave peaks DOWN. This is the 180° phase inversion of the Common Emitter amplifier.
• The LED: Notice the LED glows brightest when the Cyan V_out is at its lowest point (because low V_out means maximum I_c current is flowing through the LED/Load).

Lab 3: Finding the "Q-Point" (Biasing Lab)

Objective: Learn how to center a signal to prevent distortion (clipping).

Setup:

• Source: Sinusoidal
• AC Amplitude: 0.05 V (Medium Signal)
• V_be: Start at 0.60 V.

The Challenge:

• At 0.60 V, you will see CUTOFF CLIPPING (the bottom of the wave is missing).
• Slowly increase V_be. Watch the white Q-Point dot on the oscilloscope.
• Goal: Adjust V_be until the Cyan wave is perfectly centered between the "Cutoff Ceiling" and "Saturation Floor."
• Observation: This "centered" state is called Class A Biasing, providing the cleanest sound/signal.

Lab 4: The Saturation Wall (Power Limit)

Objective: Observe what happens when the collector doesn't have enough "suction" (Voltage) to handle the base current.

Setup:

• Source: DC or Sinusoidal
• V_be: 0.85 V (High drive)
• V_ce: Start at 5.0 V and slowly decrease toward 0.1 V.

What to Observe:

• Energy Bands: As V_ce drops, the "cliff" on the right side of the Energy Band diagram flattens out.
• Particle Flow: Even though V_be is high, the particles stop "shooting" into the Collector and start lingering in the Base.
• Scope: You will see SATURATION CLIPPING. The output wave hits a floor and can't go any lower because the transistor is "fully wide open" but out of supply voltage.

Lab 5: BJT as Switch (Pulse Train)

Objective: See the BJT act as a logic inverter: input high → output low; input low → output high.

Setup (default):

• Source: Pulse Train
• AC Amplitude: 0.50 V (large signal)
• DC Offset: 0.50 V
• V_be: 0.00 V (wave provides 0 V low, ~1 V high)
• V_ce: 5.0 V

What to Observe:

• Scope: Severe clipping on both top and bottom. The wave looks like a square wave, not a sine.
• Logic: When input is High (1), output is Low (0). When input is Low (0), output is High (1). This is the Logic Inverter behavior.

Summary Table for Students

Vbe Setting	Vce Setting	Operating Region	Use Case
< 0.6 V	Any	Cutoff	Digital "OFF" (Switch)
0.7 – 0.8 V	> 1.0 V	Active	Analog Music/Signal Amp
> 0.8 V	< 0.3 V	Saturation	Digital "ON" (Switch)

Limitations

• Qualitative model: particle animation and currents illustrate behavior; they are not a SPICE-accurate Ebers–Moll or Gummel–Poon solution.
• Constant β: real β varies with collector current and temperature, and the Early effect (output-conductance slope in the active region) is not modelled.
• Ideal turn-on: the ~0.7 V threshold is treated as fixed; the true exponential I_c–V_be relation and leakage are simplified.
• NPN, active-focused: only an NPN device is shown; reverse-active and breakdown regions are out of scope.