How does an Amplifier Work? (Class-A)

Summary

This video explains the working of a Class A amplifier, a type of electronic device that amplifies electrical signals. It covers the basic components, such as transistors, resistors, and capacitors, and how they interact to amplify weak input signals. The video explores the phase shift, feedback mechanisms, and biasing needed to ensure proper amplification, including a discussion on DC biasing and capacitor usage. It also touches on the limitations of Class A amplifiers, such as low efficiency and distortion, and introduces improvements like the Darlington transistor and the use of capacitors to enhance performance.

Takeaways

• 😀 Class A amplifiers use a single transistor to amplify electrical signals by increasing the amplitude of the input voltage.
• 😀 The transistor amplifies the base current to the collector current, and the output voltage is directly influenced by the input voltage.
• 😀 The output of a Class A amplifier always reaches the supply voltage (Vcc) and requires a resistor to regulate the voltage drop.
• 😀 The relationship between the base current and the collector current controls the output voltage, causing it to change with the input signal.
• 😀 Class A amplifiers invert the phase of the input signal by 180 degrees, leading to an inverted output signal.
• 😀 High transistor gain can cause distortion, so feedback is added through a resistor between the emitter and the ground to limit the gain.
• 😀 The gain of a Class A amplifier is determined by the ratio of the resistor in the collector (Rc) to the resistor in the emitter (Re).
• 😀 A voltage divider is used to provide the necessary DC bias to keep the input voltage above the 0.7V threshold required to activate the transistor.
• 😀 Capacitors are added in the circuit to block DC components and ensure that only the AC signal is amplified, avoiding interference with the input signal.
• 😀 Using a Darlington transistor (two transistors in one package) increases the gain and current handling capacity of the amplifier, but requires careful management of inductive effects from components like transformers.
• 😀 Class A amplifiers are known for low distortion but suffer from poor efficiency, typically less than 30%, due to the transistor being always on during operation.

Q & A

What is the primary function of a Class A amplifier?

The primary function of a Class A amplifier is to increase the amplitude of electrical signals using a transistor. It amplifies the voltage applied at the base of the transistor to a higher voltage at the collector.

Why is a resistor added between the transistor and the power supply in a Class A amplifier?

A resistor is added to control the voltage drop, which is directly proportional to the current flowing through it. This current is equivalent to the collector current, which is influenced by the input voltage. The resistor helps regulate the output voltage and ensures proper functioning of the amplifier.

What role does feedback play in the operation of a Class A amplifier?

Feedback is used in a Class A amplifier to limit the gain and prevent distortion. By adding a resistor between the emitter and the ground, negative feedback is introduced, which stabilizes the gain of the amplifier and reduces saturation, resulting in a more controlled output.

How is the gain of a Class A amplifier determined?

The gain of a Class A amplifier is determined by the ratio of the collector resistor (Rc) to the emitter resistor (Re). This ratio allows for better control over the total gain, as opposed to relying solely on the transistor's inherent gain.

Why is a DC bias necessary in a Class A amplifier?

A DC bias is necessary because a transistor requires a minimum of 0.7V at the base to turn on and begin amplifying. Without the bias, the input signal may fall below this threshold, preventing the transistor from operating correctly.

What is the purpose of the capacitor in a Class A amplifier circuit?

The capacitor in a Class A amplifier circuit is used to block DC components and only allow the AC signal to pass through. This ensures that the amplified output is an AC signal, while preventing DC bias from affecting the source or output.

How does a Darlington transistor improve a Class A amplifier?

A Darlington transistor, which consists of two transistors in a single package, improves a Class A amplifier by increasing the current handling capacity and gain. This results in higher amplification of weak signals and provides high input impedance and low output impedance.

What are the drawbacks of using inductive components, like a transformer, in a Class A amplifier circuit?

Inductive components like transformers can generate back electromotive forces (EMFs) due to their windings and core, which may damage the transistor. Additionally, transformers can introduce inefficiencies and distortions in switching circuits, making them less ideal for high-performance amplifiers.

Why is the efficiency of a Class A amplifier low, typically under 30%?

The efficiency of a Class A amplifier is low because the transistor is always on (biased), even when no signal is being amplified. This continuous operation causes significant energy loss, leading to low overall efficiency.

What happens to the output signal when the input signal voltage is high in a Class A amplifier?

When the input signal voltage is high, the base current increases, which in turn increases the collector current. This leads to a higher voltage drop across the resistor, resulting in a lower output voltage. The output is inverted relative to the input signal, with a 180-degree phase shift.