PHY067 · Electronics · Class 12 + College
Transistor current gain
Objective: To measure the current gain of an NPN transistor in common-emitter mode.
🧰 Apparatus
- NPN transistor (BC547)
- Breadboard
- Two low-voltage supplies (3 V, 9 V)
- Base resistor (100 kΩ) and potentiometer
- Microammeter and milliammeter
📐 Formulas and SI units
I_C = β I_B
I_E = I_B + I_C
💡 Theory
In an NPN transistor a small base current controls a much larger collector current: I_C = βI_B, where β (or h_FE) is typically 100–300. The emitter current is the sum, I_E = I_B + I_C. This is how transistors amplify and switch.
🪜 Procedure
- Build the common-emitter circuit on the breadboard.
- Set a small base current with the potentiometer.
- Read I_B and I_C.
- Increase I_B in steps and record I_C.
- Plot I_C against I_B; the slope is β.
📋 Observations Simulated values
✅ Result and conclusion
20 µA into the base of a β = 100 transistor gives 2 mA in the collector and 2.02 mA in the emitter.
🔬 In a real lab
β varies widely between transistors of the same type and with temperature; at large I_B the transistor saturates and I_C stops rising.
⚠️ Safety
Low-voltage DC only; check pin order (E-B-C) before switching on.
❓ Quick check
1. I_B = 50 µA, β = 200. I_C?
2. The largest current flows in the…
3. A transistor can act as a…
🏋️ Practice
EasyI_C = 3 mA, I_B = 15 µA. β?
Answer: 200
MediumI_E for that case?
Answer: 3.015 mA
ChallengeName the three terminals.
Answer: Emitter, base, collector