GopalKrishna Advanced Rural Research Foundation

GARRF Virtual Engineering & Electronics Laboratory

KALAM ZERO RESEARCH FUNDING LAB

EXPERIMENT 20 BJT TRANSISTOR COMMON EMITTER AMPLIFIER

πŸ”¬ Experiment 20 β€” BJT Common-Emitter Amplifier

A transistor can do something remarkable: a small electrical signal applied at its input can control a much larger signal at its output. This fundamental property makes the transistor one of the most important building blocks of modern electronics.

🎯 Experiment Objectives
  • Understand the operation of a bipolar junction transistor.
  • Understand the common-emitter configuration.
  • Study transistor biasing.
  • Observe amplification of a small AC signal.
  • Investigate voltage gain.
  • Understand the effect of collector resistance.
  • Observe phase inversion between input and output.
πŸ‘¨β€πŸŽ“ Think Before You Start
  • Why can a transistor amplify a signal?
  • Why is biasing necessary?
  • Why does a common-emitter amplifier invert the signal?
  • What happens if the input signal becomes too large?

πŸ“š 1. Transistor Fundamentals

A bipolar junction transistor, or BJT, has three terminals:

In an NPN transistor, a relatively small base current controls a larger collector current.

IC β‰ˆ Ξ² IB

where Ξ² is the transistor's current gain.

Key idea:

The transistor does not create energy from nothing. The additional output power comes from the DC power supply, while the input signal controls how that power is delivered to the load.

βš™οΈ 2. Common-Emitter Configuration

In a common-emitter amplifier, the emitter is the common reference terminal for the input and output circuits.

The input is applied between the base and emitter, while the output is taken from the collector and emitter.

Av = Vout / Vin

For a typical common-emitter amplifier, the output is approximately 180Β° out of phase with the input over its linear operating region.

🧩 3. Virtual Experiment Controls

🟒 Amplifier ready.

πŸ”Œ 4. Virtual Common-Emitter Circuit

πŸ“Š 5. Live Results

VCC 12.00 V
Input 20.00 mV
Output 0.44 V
Voltage Gain 22.0Γ—
Phase Shift 180Β°
IC 2.00 mA
VCE 7.60 V
Region ACTIVE

πŸ“ˆ 6. Input vs Amplified Output

The input waveform is shown together with the amplified collector output. In a common-emitter amplifier, the output waveform is inverted relative to the input.

πŸ”¬ 7. Engineering Analysis

πŸ“ 8. Important Relationships

IC β‰ˆ Ξ² IB
# VC VCC βˆ’ ICRC
VCE β‰ˆ VC βˆ’ IERE
Av β‰ˆ βˆ’RC/RE

The last expression is a simplified approximation when the emitter resistor is not bypassed and transistor parameters are neglected.

Important:

A practical transistor amplifier is more complicated than these simplified equations. The virtual laboratory intentionally uses approximations so that students can understand the governing relationships before progressing to detailed transistor models.

πŸ§ͺ 9. Student Observation Table

Trial VCC Vin f Vout Gain VCE Region

πŸŽ“ 10. Experiment Procedure

  1. Set the supply voltage to 12 V.
  2. Set the input signal to approximately 20 mV.
  3. Set the frequency to 1 kHz.
  4. Use a collector resistance of approximately 2.2 kΞ©.
  5. Use a 1 kΞ© emitter resistor.
  6. Keep the transistor in the active region.
  7. Observe the input and output waveforms.
  8. Record the voltage gain.
  9. Enable the emitter bypass capacitor.
  10. Compare the gain with and without the bypass capacitor.
  11. Increase the input signal and observe the onset of distortion.
  12. Change RC and investigate its effect on gain.
⭐ Engineering Challenge

Can you obtain a large voltage gain while keeping the transistor in its active region and avoiding severe clipping?

πŸŽ›οΈ 11. What Does the Emitter Bypass Capacitor Do?

The emitter resistor provides useful DC stabilization, but it also introduces AC negative feedback that reduces voltage gain.

At sufficiently high frequencies, an emitter bypass capacitor provides a lower-impedance path for the AC component around the emitter resistor.

More AC bypass β†’ Less emitter degeneration β†’ Higher AC voltage gain

This demonstrates an important engineering trade-off between stability, gain and frequency response.

⚠️ 12. Understanding Saturation & Cutoff

A transistor amplifier must normally operate in the active region for approximately linear amplification.

Observe carefully:

If the input signal is made too large, the output can no longer follow the input proportionally. The waveform begins to clip.

This is called non-linear distortion.

🌍 13. Real-World Applications

πŸ”¬ 14. Connection to Sensors, PZT & SHM

Sensors frequently generate signals that are too small to be directly processed by measurement electronics. Amplification can therefore be an essential stage in a sensor interface.

πŸ’‘ Research Connection

A piezoelectric or PZT sensor can produce relatively small electrical signals in response to mechanical vibration or structural excitation. A transistor amplifier represents one of the historical foundations of analog signal conditioning used to increase signal amplitude before measurement or further processing.

Modern SHM systems may use integrated instrumentation amplifiers, op-amps or digital acquisition systems, but the underlying concept of signal amplification remains fundamental.

❓ 15. Student Quiz

Q1. A BJT has which three terminals?
Q2. In a common-emitter amplifier, the output is normally:
Q3. The transistor's current gain is commonly represented by:
Q4. For linear amplification, the transistor should normally operate in:
Q5. Excessively large input signals can cause:

πŸ€– CHITTI

GARRF Robotics & Engineering AI Mentor

Explore transistor electronics, amplifiers, sensors, signal conditioning and robotics applications with CHITTI.

πŸ’‘ Try asking:

β€œWhy does a common-emitter amplifier invert the signal?”

β€œWhat is transistor biasing?”

β€œWhat happens when a transistor enters saturation?”

β€œWhy does an emitter bypass capacitor increase gain?”

β€œHow can transistor amplification be used with a PZT sensor?”

⚠️ 16. Physical Laboratory Safety

Important:

This page is an educational virtual simulation. When constructing a physical transistor amplifier, use a current-limited DC laboratory supply and verify transistor pin configuration before connecting the device.

Do not connect experimental circuits directly to mains voltage.

πŸŽ“ 17. Experiment Conclusion

The BJT common-emitter amplifier demonstrates one of the fundamental ideas of electronics: a small input signal can control a much larger signal using energy supplied by a DC power source.

Students can observe the importance of biasing, voltage gain, emitter degeneration, phase inversion and transistor operating regions.

From a simple semiconductor device to the foundation of analog amplification β€” the transistor changed electronics forever.