GopalKrishna Advanced Rural Research Foundation

GARRF Virtual Engineering & Electronics Laboratory

KALAM ZERO RESEARCH FUNDING LAB

EXPERIMENT 08 FET / MOSFET CHARACTERISTICS COMMON SOURCE

🔬 Experiment 08 — FET Characteristics & Common-Source Amplifier

A Field Effect Transistor, or FET, is a semiconductor device in which the electric field at the gate controls the current flowing between the source and drain.

🎯 Experiment Objective

To study the drain characteristics of a MOSFET, observe how gate voltage controls drain current, identify the operating regions, and understand the basic operation of a common-source amplifier.

👨‍🎓 Think Before You Start
  • Why does a MOSFET require very little gate current?
  • How does gate voltage control drain current?
  • What is the difference between a BJT and a MOSFET?
  • Why is the common-source amplifier useful?

📚 1. Theory

MOSFET Terminals

A MOSFET has three primary terminals used in this experiment: Gate (G), Drain (D), and Source (S). The gate voltage controls the conductive channel between the drain and source.

Voltage-Controlled Device

Unlike a BJT, where base current plays the controlling role, a MOSFET is primarily controlled by its gate-to-source voltage. Ideally, the steady-state gate current is approximately zero.

ID = K(VGS − VTH)²

For a simplified enhancement-mode MOSFET in saturation, where K is a device constant and VTH is the threshold voltage.

Operating Regions

Common-Source Amplifier

The common-source configuration is analogous in many ways to the common-emitter BJT amplifier. A small change in gate voltage can produce a larger change in drain voltage. The output is normally phase-inverted.

Av = Vout / Vin

🧩 2. Select Experiment

🟢 Ready. Select an experiment and begin.

🔌 3. Virtual MOSFET Circuit

📊 4. Live Measurements

VGS 3.00 V
VTH 2.00 V
VDS 6.00 V
ID 0.00 mA
K 2.00
Region CUT-OFF

📡 5. Common-Source Amplifier

Input Amplitude 0.10 V
Output Amplitude 0.00 V
Voltage Gain 0.00
Phase 180°

Input and Output Waveforms

The simplified common-source amplifier produces an amplified output with approximately 180° phase inversion.

📈 6. FET Characteristic Curves

The output-characteristic graph shows drain current ID as a function of drain-source voltage VDS for different gate-source voltages.

🔬 7. Engineering Analysis

Adjust VGS and VDS to observe how the MOSFET operating region changes.

📝 8. Student Observation Table

Set a measurement point and click ADD OBSERVATION.

Trial VGS VDS ID Region

🎯 9. Identify the MOSFET Operating Region

Move VGS and VDS to explore cut-off, linear and saturation operation.

🎓 10. Experiment Conclusion

Complete the experiment to observe how gate voltage controls drain current and how a MOSFET can be used as an amplifier.

❓ 11. Student Quiz

Q1. The three primary MOSFET terminals used here are:
Q2. The MOSFET is primarily controlled by:
Q3. If VGS is below the threshold voltage in this enhancement-mode model, the MOSFET is approximately:
Q4. A common-source amplifier normally produces:
Q5. Compared with an ideal BJT base, the MOSFET gate ideally draws:

🤖 CHITTI

GARRF Robotics & Engineering AI Mentor

Need help understanding MOSFET operation? Ask CHITTI for guidance.

💡 Try asking:

"What is a MOSFET?"

"What is threshold voltage?"

"Explain cut-off, linear and saturation regions."

"Why does a MOSFET have high input impedance?"

"What is the difference between BJT and MOSFET?"

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

👨‍🎓 12. How to Perform Experiment 08

  1. Select Output Characteristics.
  2. Set a suitable threshold voltage.
  3. Choose a gate-source voltage VGS.
  4. Vary VDS gradually.
  5. Observe the drain current ID.
  6. Record several measurement points.
  7. Repeat the experiment with different VGS values.
  8. Observe the family of drain characteristic curves.
  9. Select Transfer Characteristics and vary VGS.
  10. Observe how the drain current changes.
  11. Select Common-Source Amplifier.
  12. Adjust the input signal and observe the amplified output.
⭐ Engineering Thinking Challenge

If VGS is increased while the MOSFET remains in its saturation region, what should happen to ID?

⚠️ Real Laboratory Safety

Important:

MOSFETs can be damaged by excessive voltage, current, temperature and electrostatic discharge. In a physical laboratory, use appropriate current limiting and ESD precautions.