Unit 3 Part – C

Q1. Class C Power Amplifier

1️⃣ Biasing:

• Base biased below cutoff so transistor conducts <180°.

2️⃣ Operation:

• Conducts only small input portion → pulses.
• Tuned LC circuit filters harmonics → pure sine wave output.

3️⃣ Efficiency:

• Very high (~80%).

4️⃣ Application:

• Used in RF transmitters, oscillators, tuned amplifiers.

Q2. Transistor as Amplifier & Switch

Amplifier Mode:

1. Biased in active region.
2. Small base input → large collector output.
3. Output 180° out of phase with input.

Switch Mode:

1. Cutoff → Transistor OFF (open switch).
2. Saturation → Transistor ON (closed switch).
3. Used in digital circuits and microcontrollers.

Q3. Frequency Response of RC Coupled Amplifier

1️⃣ Circuit:

• Two stages with RC coupling capacitors and load resistor.

2️⃣ Operation:

• Amplifies AC signal, blocks DC.

3️⃣ Frequency Response:

• Low frequency: Gain ↓ (due to C reactance).
• Mid frequency: Gain constant (flat region).
• High frequency: Gain ↓ (due to stray capacitance).

4️⃣ Application:

• Used in audio, radio, and TV amplifiers.

Q4. Common Emitter Amplifier

1️⃣ Bias: Active region.
2️⃣ Working:

• Input → base; Output → collector.
• Small base signal → large voltage at collector.
• 180° phase inversion.
  3️⃣ Components:
• Coupling capacitor: passes AC, blocks DC.
• Bypass capacitor: stabilizes gain.
  4️⃣ Output: Amplified, inverted waveform.

Part – C Tamil

Q1. Class C Power Amplifier – Working, Conduction Angle & Applications

Definition:
Class C power amplifier ஒரு power amplifier type ஆகும், இது input signal < 180° (half cycle க்கு கீழ்) மட்டும் conduct பண்ணும்.
Transistor biasing cut-off region கு கீழே set பண்ணப்படும்.

1️⃣ Operation:

1. Base bias voltage very low → transistor conduct only for small portion of input signal.
2. Collector current comes as pulses (full of harmonics).
3. LC tuned tank circuit output-இல் connect பண்ணப்படும்.
4. Tank circuit harmonics filter பண்ணி pure sinusoidal wave produce பண்ணும்.

2️⃣ Efficiency:

• Efficiency ≈ 80% (max) → very high compared to Class A & B.

3️⃣ Applications:

• RF transmitters.
• Tuned oscillators.
• High-frequency amplifiers.

Q2. Transistor as Amplifier and as Switch

(a) Transistor as Amplifier:

1. Transistor active region-இல் bias பண்ணப்படும்.
2. Small AC signal base-இல் apply பண்ணும்போது collector-இல் large amplified output வரும்.
3. CE configuration-இல் output 180° out of phase with input.
4. Output signal strength increase ஆகும் without changing frequency or shape.

(b) Transistor as Switch:

1. Cut-off mode: IB = 0 → transistor OFF (open switch).
2. Saturation mode: IB large → transistor ON (closed switch).
3. Used in digital logic circuits, microcontrollers, and timing applications.

Q3. Frequency Response of RC Coupled Amplifier

1️⃣ Circuit Details:

• Two stages of CE amplifiers connected by coupling capacitor (CC).
• Each stage has collector resistor RC, emitter resistor RE, and bypass capacitor CE.

2️⃣ Operation:

• Coupling capacitor passes AC signal from first stage to next, blocks DC.
• Provides good voltage gain for mid-frequency range.

3️⃣ Frequency Response Curve:

• X-axis = frequency, Y-axis = gain (dB).
• Low frequency: Capacitor reactance ↑ → gain ↓.
• Mid frequency: Gain constant (flat region).
• High frequency: Parasitic capacitance ↑ → gain ↓.

4️⃣ Applications:

• Audio amplifiers, radio receivers, public address systems.

Q4. Common Emitter (CE) Amplifier – Operation & Waveforms

1️⃣ Circuit Description:

• Input → Base, Output → Collector.
• Biasing active region இல் set பண்ணப்படும்.

2️⃣ Working:

1. Input AC signal base-இல் apply பண்ணும்.
2. Base current இல் சிறிய மாற்றம் collector current-இல் பெரிய மாற்றம் உண்டாக்கும்.
3. Output voltage collector-இல் develop ஆகும்.
4. Output = amplified & 180° out of phase with input.

3️⃣ Capacitors Role:

• Coupling capacitor: Next stage-க்கு AC pass பண்ணும், DC block பண்ணும்.
• Bypass capacitor: RE-இல் AC stabilize பண்ணும் → gain increase ஆகும்.

4️⃣ Advantages:

• High voltage gain.
• Moderate input & output impedance.
• Easy biasing & stability.

Summary:
CE amplifier → input signal amplify பண்ணும் main stage, phase inversion 180°, audio applications-இல் widely used.