Concept Notes (Deep Explanation + Examples)
A Power Amplifier (PA) is one of the most vital blocks in any communication system, audio amplifier, or RF transmitter.
Its main purpose is to amplify the power level of a weak input signal so that it can drive a load such as a loudspeaker or antenna.
In ECET ECE, Power Amplifiers are a repeated concept — appearing under Electronic Devices and Circuits (EDC).
🔹 What is a Power Amplifier?
A Power Amplifier increases the power of a signal rather than just its voltage or current.
It is generally the last stage of an amplifier chain.
👉 Example:
In a Bluetooth speaker, the signal from your mobile is first processed (low power) and then amplified by a power amplifier to drive the speaker.
🔹 Difference between Voltage Amplifier and Power Amplifier
| Parameter | Voltage Amplifier | Power Amplifier |
|---|---|---|
| Purpose | Increase voltage level | Increase power level |
| Load | High resistance | Low resistance (like speakers) |
| Output Power | Few mW | Few watts or more |
| Current Handling | Low | High |
| Output Stage | Early stages | Final stage |
🔹 Classification of Power Amplifiers
Power amplifiers are classified based on conduction angle — the portion of the input cycle during which the transistor conducts.
| Class | Conduction Angle | Efficiency | Linearity | Common Use |
|---|---|---|---|---|
| A | 360° | ~25–30% | Excellent | Audio low power |
| B | 180° | ~78.5% | Moderate | Push-pull audio amps |
| AB | 180–360° | 50–70% | Good | Hi-Fi systems |
| C | <180° | Up to 90% | Poor | RF transmitters |
| D | Switching mode | 90%+ | Moderate | Digital audio amps |
🔹 Class A Power Amplifier
- Conduction: Full 360° of the input cycle
- Biasing: Transistor conducts for entire waveform
- Advantage: Low distortion, excellent fidelity
- Disadvantage: Low efficiency (max 25–30%)
- Application: Small audio amplifiers, preamps
Example:
If a sine wave input of 0.1 V is amplified to 10 V to drive a speaker, the transistor continuously conducts current, wasting power even during no signal.
🔹 Class B Power Amplifier
- Conduction: 180° (half cycle)
- Circuit: Two transistors (push-pull configuration) — one conducts for positive half, other for negative half
- Efficiency: Theoretical max ≈
- Drawback: Crossover distortion at zero voltage region
- Application: Audio output stages
🧠 Diagram (imagine):
Two complementary transistors connected to a transformer. The first transistor amplifies the positive half of the waveform, and the second amplifies the negative half. Their combined output gives the full waveform at the load.
🔹 Class AB Power Amplifier
- A compromise between A and B
- Conducts slightly more than 180°
- Eliminates crossover distortion
- Efficiency between 50–70%
- Used in high-quality audio systems
🔹 Class C Power Amplifier
- Conduction < 180°
- Extremely high efficiency (up to 90%)
- Not suitable for audio (high distortion)
- Used in RF transmitters, oscillators, and microwave systems
🔹 Class D Power Amplifier
- Switching mode amplifier using MOSFETs
- Input signal converted to pulse-width modulated (PWM) signal
- High efficiency (90–95%)
- Common in modern digital audio and Bluetooth speakers
🔹 Efficiency Formula (General)
Where:
= AC output power
= DC power supplied
🔹 Power Dissipation and Heat Sinks
Power amplifiers dissipate large amounts of heat.
Hence, heat sinks are attached to the transistor to avoid thermal damage.
🧩 Example:
In your home amplifier IC (like TDA2030), the metal fin on the back is the heat sink — it keeps the IC cool during loud sound playback.
3️⃣ ⚙️ Formulas (Plain LaTeX, NO boxes)
4️⃣ 🔟 10 MCQs (GATE-level + ECET Mix)
1️⃣ The main function of a power amplifier is to increase the:
A) Voltage only
B) Power level
C) Frequency
D) Phase shift
2️⃣ Class A amplifier conduction angle is:
A) 180°
B) 270°
C) 360°
D) <180°
3️⃣ The maximum efficiency of a Class A amplifier is approximately:
A) 25%
B) 50%
C) 75%
D) 90%
4️⃣ Class B amplifier efficiency (theoretical) is:
A) 25%
B) 50%
C) 78.5%
D) 100%
5️⃣ Class C amplifier is mainly used for:
A) Audio frequency amplifiers
B) RF oscillators and transmitters
C) Voltage regulation
D) Pre-amplifiers
6️⃣ Crossover distortion is observed in:
A) Class A
B) Class B
C) Class AB
D) Class D
7️⃣ In push-pull amplifier, each transistor conducts for:
A) 90°
B) 120°
C) 180°
D) 360°
8️⃣ The device commonly used in Class D amplifiers is:
A) BJT
B) MOSFET
C) SCR
D) UJT
9️⃣ Heat sinks are required in power amplifiers to:
A) Increase output power
B) Reduce distortion
C) Dissipate heat
D) Reduce input signal
10️⃣ For maximum output power in a Class B amplifier, load resistance should be:
A) Very low
B) Matched to output impedance
C) Very high
D) Inductive
5️⃣ ✅ Answer Key (WordPress Table Format — NO HTML)
Q.No Answer
1 B
2 C
3 A
4 C
5 B
6 B
7 C
8 B
9 C
10 B
6️⃣ 🧠 Detailed Explanations
1️⃣ B — Power level
Power amplifiers boost the overall power of a signal to drive heavy loads like speakers or antennas.
2️⃣ C — 360°
Class A amplifiers conduct for the entire input cycle, providing high linearity but low efficiency.
3️⃣ A — 25%
The maximum theoretical efficiency for Class A is only 25% because the transistor always conducts current.
4️⃣ C — 78.5%
In Class B push-pull amplifiers, each transistor conducts for half a cycle, improving efficiency to 78.5%.
5️⃣ B — RF oscillators and transmitters
Class C amplifiers are efficient and used where linearity is not required, like RF transmitters.
6️⃣ B — Class B
Crossover distortion occurs at the point where one transistor turns off and the other begins conduction.
7️⃣ C — 180°
Each transistor in a push-pull amplifier conducts for half the signal cycle.
8️⃣ B — MOSFET
Class D amplifiers operate in switching mode, where MOSFETs are preferred for fast switching and low losses.
9️⃣ C — Dissipate heat
Heat sinks prevent overheating of transistors by conducting heat away from the device.
10️⃣ B — Matched to output impedance
Maximum power transfer occurs when load resistance equals the amplifier’s output impedance.
7️⃣ 🎯 Motivation / Why This Topic Matters (ECET 2026)
Power amplifiers are a core ECE exam favorite — they connect theory with real-world electronics like speakers, RF circuits, and transmitters.
Mastering this topic strengthens your understanding of biasing, efficiency, and wave conduction, which directly appears in ECET, GATE, and diploma exams.
Practice circuit questions and efficiency calculations — they boost both marks and confidence.
8️⃣ 📲 CTA (Fixed)
Join our ECET 2026 ECE WhatsApp Group for daily quizzes & study notes:
👉 https://chat.whatsapp.com/GniYuv3CYVDKjPWEN086X9
