Lesson Notes By Weeks and Term v3 - Senior Secondary 3
Amplifier
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Amplifier
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Subject: Basic Electronics
Class: Senior Secondary 3
Term: 3rd Term
Week: 1
Theme: Electronic Components And Circuits
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Define amplifier List the classes of amplifier. State the applications of amplifier
The fundamental principle of an amplifier involves converting a small input signal into a larger output signal.
This process requires: An Active Device: Typically a transistor (BJT or FET) or an Operational Amplifier (Op-Amp) is used. These devices have the ability to control a large output current or voltage with a small input signal.
A DC Power Supply: Amplifiers are active circuits and require an external DC power source to operate and to provide the energy needed to increase the signal power. The amplifier does not create energy; it converts energy from the DC power supply into signal power.
Gain: This is the ratio of the output signal to the input signal. It quantifies how much the amplifier strengthens the signal. Voltage Gain ($A_V$): Ratio of output voltage ($V_{out}$) to input voltage ($V_{in}$). $A_V = V_{out} / V_{in}$ Current Gain ($A_I$): Ratio of output current ($I_{out}$) to input current ($I_{in}$). $A_I = I_{out} / I_{in}$ Power Gain ($A_P$): Ratio of output power ($P_{out}$) to input power ($P_{in}$). $A_P = P_{out} / P_{in}$ Since $P = V \times I$, Power Gain can also be expressed as $A_P = A_V \times A_I$. Power gain is often expressed in decibels (dB): $A_{P(dB)} = 10 \log_{10}(P_{out} / P_{in})$ or $A_{V(dB)} = 20 \log_{10}(V_{out} / V_{in})$.
Worked Example 1: Calculating Voltage Gain A microphone produces an audio signal of 50 mV (peak-to-peak). This signal is fed into an amplifier, which outputs a 5 V (peak-to-peak) signal to drive a loudspeaker. Calculate the voltage gain of the amplifier.
Solution: Given: Input voltage ($V_{in}$) = 50 mV = 0.05 V Output voltage ($V_{out}$) = 5 V Voltage Gain ($A_V$) = $V_{out} / V_{in}$ $A_V = 5 \text{ V} / 0.05 \text{ V}$ $A_V = 100$ The amplifier has a voltage gain of
1
0
0. This means the output voltage is 100 times larger than the input voltage. Amplifiers are categorised into classes based on the amount of time the active device (e.g., transistor) conducts current during one complete cycle of the input signal. This affects their efficiency and linearity.
Class A Amplifier: Operation: The active device conducts current for the entire 360 degrees of the input signal cycle. The transistor is always biased in the active region.
Characteristics: High Linearity: Produces very little distortion, making it suitable for high-fidelity audio applications.
Low Efficiency: Typically 20-25% (theoretical maximum 50%). A significant amount of power is dissipated as heat, even when there's no input signal.
Applications: Preamplifiers, small signal amplifiers, driver stages, and high-fidelity audio systems (often for headphones or small speakers).
Class B Amplifier: Operation: The active device conducts current for only 180 degrees (half) of the input signal cycle. Two transistors are typically used in a "push-pull" configuration, with each handling one half of the waveform.
Characteristics: Higher Efficiency: Up to 78.5% theoretical maximum. Less power is dissipated as heat compared to Class
A. Crossover Distortion: A major drawback. Due to the turn-on voltage of the transistors, there's a small dead band around the zero crossing of the signal, causing distortion.
Applications: Power amplifiers, battery-operated devices where efficiency is critical (e.g., portable radios, hearing aids).
Class AB Amplifier: Operation: A compromise between Class A and Class B. The active device conducts current for slightly more than 180 degrees (e.g., 200-220 degrees) of the input signal cycle. This is achieved by slightly biasing the transistors into conduction, overcoming the crossover distortion of Class
B. Characteristics: Good Linearity: Significantly reduces crossover distortion compared to Class
B. Good Efficiency: Better than Class A, typically 50-70%.
Applications: Widely used in audio power amplifiers (e.g., home stereo systems, car audio systems, guitar amplifiers) due to its good balance of linearity and efficiency.
Class C Amplifier: Operation: The active device conducts current for significantly less than 180 degrees (e.g., 90-150 degrees) of the input signal cycle. It is heavily biased into cutoff.
Characteristics: Very High Efficiency: Up to 90% theoretical maximum.
High Distortion: Produces significant distortion as it only amplifies a small portion of the input signal.
Tuned Circuit Required: Typically used with a tuned (resonant) circuit at the output to reconstruct the full waveform by filtering out unwanted harmonics, often amplifying only a specific frequency.
Applications: Radio Frequency (RF) power amplifiers in transmitters, where the high efficiency is crucial and the output is filtered by a resonant circuit (e.g., broadcast radio transmitters, mobile phone base stations). Amplifiers can also be categorised by their primary function, frequency range, or coupling method: Voltage Amplifier: Designed to increase the voltage of a signal. Typically has high input impedance and low output impedance.
Current Amplifier: Designed to increase the current of a signal.
Power Amplifier: Designed to increase the power of a signal, often to drive a load such as a loudspeaker or antenna.
Audio Amplifier: Amplifies signals within the human hearing range (typically 20 Hz to 20 kHz). Used in radios, TVs, sound systems.
Radio Frequency (RF)
Amplifier: Amplifies signals at radio frequencies (e.g., MHz to GHz). Used in radio and TV receivers, transmitters.
Intermediate Frequency (IF)
Amplifier: Amplifies signals at a fixed intermediate frequency within a superheterodyne receiver (e.g., 455 kHz or 10.7 MHz).
Operational Amplifier (Op-Amp): A versatile, high-gain, direct-coupled voltage amplifier with differential inputs. It's a fundamental building block in many electronic circuits, used for amplification, filtering, and mathematical operations. Amplifiers are indispensable in almost all electronic systems. Their applications are widespread in Nigeria and globally: Public Address (PA)
Systems: Used in churches, mosques, schools, town halls, markets, and political rallies to amplify voices and music, ensuring they are heard by a large audience.
Radio and Television Receivers: Amplifiers boost the weak radio frequency (RF) signals picked up by antennas, making them strong enough to be processed and converted into sound and images.
Mobile Phones: Multiple amplifiers are used in mobile phones for both transmitting (boosting the signal to the base station) and receiving (amplifying weak signals from the base station).
Audio Systems: Found in home theatre systems, car stereos, musical instrument amplifiers (for guitars, keyboards), and disc jockey (DJ) equipment to enhance sound output.
Communication Systems: Essential in radio transmitters, receivers, repeaters, and satellite communication systems to boost signals over long distances.
Medical Equipment: Used to amplify very weak biosignals from the body, such as in Electrocardiograms (ECG) for heart activity, Electroencephalograms (EEG) for brain activity, and hearing aids.
Instrumentation and Control Systems: Amplifiers are used to amplify signals from sensors (e.g., temperature sensors, pressure sensors) so they can be processed by control systems or display devices.
Industrial Equipment: Utilized in machinery control, process automation, and test equipment.
Community Development and Communication: In many Nigerian communities, especially in rural areas, weak signals from distant radio stations can make communication difficult. Understanding amplifiers helps students appreciate how simple radio receivers, often powered by batteries or solar energy, amplify these weak signals to provide access to news, educational programs, and entertainment. This knowledge is crucial for technicians involved in setting up or maintaining community radio stations or public address systems for local gatherings, traditional festivals, or political campaigns, where clear sound projection is vital. Economic Opportunity (Repair and Maintenance): The ubiquity of electronic devices like radios, televisions, mobile phones, and sound systems in Nigeria creates a significant demand for their repair and maintenance. A thorough understanding of amplifiers is fundamental for local technicians diagnosing and fixing faulty audio systems, public address equipment, or even car stereos. This knowledge can lead to self-employment opportunities or employment in electronics repair shops, contributing directly to the local economy.
Healthcare Technology: Amplifiers play a critical role in essential medical diagnostic equipment. For instance, in Nigerian hospitals and clinics, devices like Electrocardiogram (ECG) machines use sensitive amplifiers to detect and magnify the extremely weak electrical signals from the heart. Without amplification, these vital signals would be too small to be recorded and analysed, making diagnosis of heart conditions challenging. This illustrates the life-saving application of amplifiers in the health sector.