GopalKrishna Advanced Rural Research Foundation

GARRF Virtual Engineering & Electronics Laboratory

KALAM ZERO RESEARCH FUNDING LAB

EXPERIMENT 11 BJT TRANSISTOR COMMON-EMITTER VIRTUAL LAB

🔬 Experiment 11 — BJT Characteristics & Common-Emitter Amplifier

The bipolar junction transistor (BJT) is one of the most important devices in electronics. It can operate as an amplifier or as a switch. In this experiment students investigate the relationship between base current and collector current and then explore the operation of a common-emitter amplifier.

🎯 Experiment Objectives
  • Understand the basic operation of an NPN transistor.
  • Understand emitter, base and collector terminals.
  • Study collector current as a function of base current.
  • Estimate transistor current gain β.
  • Identify cutoff, active and saturation regions.
  • Understand the common-emitter amplifier.
  • Observe voltage amplification and phase inversion.
👨‍🎓 Think Before You Start
  • Why is the base current much smaller than the collector current?
  • What does transistor current gain β represent?
  • What happens when the base current becomes zero?
  • Why is the common-emitter configuration useful for amplification?

📚 1. Theory

Bipolar Junction Transistor

A BJT contains three semiconductor regions called the emitter, base and collector. For an NPN transistor, a small base current controls a much larger collector current when the device is operating in the active region.

Transistor Current Gain

β = IC / IB

where β is the common-emitter current gain.

Collector Current

IC ≈ β IB

This approximation applies primarily when the transistor is in the forward-active region and is not saturated.

Operating Regions

Common-Emitter Amplifier

The common-emitter configuration provides voltage and current gain. The output voltage is approximately 180° out of phase with the input voltage in the normal common-emitter amplifier.

Av = Vout / Vin

🧩 2. Virtual Experiment Controls

🟢 Virtual transistor experiment ready.

🔌 3. Virtual Circuit

📊 4. Live Measurements

Base Current 20 μA
Collector Current 2.00 mA
β 100
VCE 5.00 V
Vout 10.00 V
Gain —
Operating Region ACTIVE

📈 5. BJT Output Characteristics

The graph shows collector current versus collector-emitter voltage for several base-current levels.

📡 6. Common-Emitter Amplifier Waveform

The amplifier output is shown with phase inversion relative to the input signal.

🔬 7. Engineering Analysis

Select a configuration to begin analysis.

📝 8. Student Observation Table

Trial Mode IB IC VCE β Region

🎓 9. Experiment Conclusion

The BJT demonstrates current amplification because a relatively small base current controls a substantially larger collector current. In the active region the collector current is approximately β times the base current. In a common-emitter configuration this transistor action can be used to amplify an input signal, with the output undergoing phase inversion.

❓ 10. Student Quiz

Q1. A BJT has how many terminals?
Q2. The current gain of a common-emitter transistor is commonly represented by:
Q3. In the active region:
Q4. A transistor used as an amplifier normally operates in:
Q5. The output of a typical common-emitter amplifier is:

🤖 CHITTI

GARRF Robotics & Engineering AI Mentor

CHITTI can help students understand the transistor experiment and reason through the measurements.

💡 Try asking:

"What is an NPN transistor?"

"What does transistor beta mean?"

"Explain cutoff, active and saturation."

"Why does a common-emitter amplifier invert the signal?"

"How can I calculate collector current?"

👨‍🎓 11. How to Perform Experiment 11

  1. Select BJT Output Characteristics.
  2. Choose a base current, starting with a small value.
  3. Change VCE gradually.
  4. Observe the collector current.
  5. Repeat the measurement for different base currents.
  6. Compare the resulting curves.
  7. Estimate β from IC/IB.
  8. Identify cutoff, active and saturation regions.
  9. Select Common-Emitter Amplifier.
  10. Change the input signal and transistor parameters.
  11. Observe the amplified output and phase inversion.
  12. Record your observations and answer the quiz.
⭐ Engineering Challenge

Design a virtual common-emitter amplifier that produces useful voltage gain without driving the transistor into saturation. Experiment with VCC, RC, β and input amplitude.

⚠️ Real Laboratory Safety

Important:

This is a virtual experiment. In a physical laboratory, use current-limited power supplies and correctly rated components. Incorrect transistor biasing can cause excessive collector current and overheating. Never connect an experimental circuit directly to mains voltage.