Concept Notes
1. What is Biasing?
- Biasing is the process of applying DC voltages and currents to a device (like a diode or transistor) so that it works properly in the desired region of operation.
- Without biasing → transistor/diode may not amplify or switch correctly.
2. Need for Biasing
- To keep transistor in active region (for amplification).
- To maintain stability of operating point (Q-point).
- To avoid distortion in output signal.
3. Types of Biasing Techniques
(A) Fixed Bias (Base Resistor Bias)
- Base resistor provides biasing.
- Simple but unstable with temperature changes.
Formula:
Example:
If 
:
(B) Collector-to-Base Bias (Feedback Bias)
- A resistor from collector to base gives feedback.
- Improves stability compared to fixed bias.
Formula:
(C) Voltage Divider Bias (Self Bias)
- Most commonly used biasing in amplifiers.
- Provides good thermal stability.
Formula:
(D) Emitter Bias
- Uses negative supply for stability.
- Better than fixed bias.
Formula:
4. Stability Factor (S)
- Indicates how stable
is with respect to variations in 
or 
. - Smaller S = better stability.
Formulas:
- Fixed bias:
- Collector feedback bias:
- Voltage divider bias:
⚙️ Important Formulas
Base current (fixed bias):
Collector current:
Emitter current:
Voltage divider equivalent:
Stability factor:
🔟 MCQs
Q1. The purpose of biasing in a transistor is:
a) To increase current gain
b) To maintain operating point
c) To reduce resistance
d) To increase power factor
Q2. In fixed bias circuit, base current is:
a) 
b) 
c) 
d) 
Q3. Stability factor for fixed bias is:
a) 1
b) 
c)
d) 0
Q4. Voltage divider bias gives:
a) Poor stability
b) Good stability
c) Zero stability
d) Infinite stability
Q5. In emitter bias, emitter current is:
a)
b) 
c) 
d) 
Q6. Collector-to-base feedback bias helps in:
a) Reducing leakage current
b) Improving stability
c) Reducing collector voltage
d) Increasing frequency response
Q7. For voltage divider bias, Thevenin voltage is:
a) 
b) 
c)
d) 
Q8. Which biasing has stability factor = ?
a) Fixed bias
b) Collector feedback bias
c) Voltage divider bias
d) Emitter bias
Q9. The operating point is also called:
a) Cutoff point
b) Saturation point
c) Q-point
d) Breakdown point
Q10. Best biasing technique for amplifiers:
a) Fixed bias
b) Collector feedback bias
c) Voltage divider bias
d) Emitter bias
✅ Answer Key
| Q No | Answer |
|---|---|
| Q1 | b |
| Q2 | a |
| Q3 | b |
| Q4 | b |
| Q5 | b |
| Q6 | b |
| Q7 | b |
| Q8 | a |
| Q9 | c |
| Q10 | c |
🧠 Explanations
- Q1: Biasing ensures proper Q-point.
- Q2: Fixed bias →
. - Q3: Fixed bias stability factor = .
- Q4: Voltage divider bias gives best thermal stability.
- Q5: In emitter bias, .
- Q6: Feedback improves stability.
- Q7: Thevenin voltage across R2 = .
- Q8: Fixed bias → .
- Q9: Q-point = operating point.
- Q10: Voltage divider bias is most widely used.
🎯 Motivation
- ECET frequently asks biasing numericals.
- If you understand formulas of I_B, I_C, V_TH, stability factor, you can answer quickly.
- Remember: “Right biasing = Right amplification.”
📲 CTA
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