5.1 Describe class A amplification with reference to the biasing of a transistor - NSC Electrical Technology Electronics - Question 5 - 2018 - Paper 1

1. Biasing ensures that the transistor operates in the active region, allowing for linear amplification.
2. It stabilizes the operating point of the transistor, preventing distortion and ensuring consistent performance over varying temperatures and signal levels.

The Q-point (quiescent point) on a DC load line represents the DC voltage and current conditions of the transistor in its no-signal state. It determines the operating point where the transistor is biased for optimal linear amplification, allowing it to respond to AC signals without distortion.

For class A, the Q-point is at the midpoint of the load line. In class B, the Q-point is at the edge of the active region (cut-off biasing), allowing operation between the cut-off and saturation points. Class C has the Q-point set below the cut-off point, allowing for less than 180° conduction angle.

1. Capacitor C2 allows AC signals to pass while blocking DC components, facilitating AC coupling between stages.
2. It acts as a coupling capacitor, ensuring that DC biasing conditions of preceding stages do not affect the next stage.

An RC-coupled amplifier operates by using resistors and capacitors to connect multiple amplifier stages. The input AC signal is fed to the base of the first transistor, causing it to amplify the signal. The output is then coupled through a capacitor to the next stage, where further amplification occurs. This setup provides good frequency response and impedance matching between stages.

1. The coupling must allow AC signals to pass while blocking DC voltage levels to maintain biasing conditions.
2. Impedance matching is necessary to maximize power transfer and minimize signal loss between consecutive stages.

Frequency response refers to how an amplifier's output gain varies with different input signal frequencies. It characterizes the behavior of the amplifier across a range of frequencies, reflecting its ability to maintain consistent amplification without distortion.

Half-power points, often marked as -3 dB points on a frequency response curve, denote the frequencies at which the output power drops to half of its peak value. These points signify the bandwidth of the amplifier, defining the limits of effective amplification.

At low frequencies, the reactance of coupling capacitors increases, leading to a reduction in amplifier voltage gain. As the frequency decreases, the capacitors block more of the input signal, which diminishes the overall gain of the amplifier.

Impedance matching can be achieved by selecting resistors and capacitors that align the output impedance of the first stage with the input impedance of the second stage. This maximizes power transfer and minimizes signal reflection between stages.

A transformer is used to ensure impedance matching between the amplifier output and the load, such as a speaker. It allows for efficient transfer of power while also providing isolation between stages, minimizing the risk of feedback issues.

Please refer to the provided ANSWER SHEET 5.5.3 for the required output frequency response curve, indicating the -3 dB points, mid-band frequencies, and general shape of the response.

An oscillator is a device that generates an AC output signal without any externally applied input signal. It produces continuous waveforms, often used for clock signals in circuits.

The type of waveform generally generated by an oscillator is a sine wave, which is a smooth periodic oscillation.

Resistors R1 and R2 form a voltage divider network, which provides the necessary biasing voltage at the base of the transistor. This ensures the transistor operates in the active region, allowing it to amplify signals effectively.

The Hartley oscillator uses two inductors and one capacitor in its tank circuit, facilitating oscillation through voltage feedback. In contrast, the Colpitts oscillator employs two capacitors and one inductor, providing a different feedback mechanism that stabilizes oscillations.

1. The transistor amplifies the damped oscillating signal, boosting its amplitude.
2. It provides a phase shift of 180°, which is essential for sustaining oscillations within the system.

The total phase shift of the oscillating circuit must be zero to ensure sustained oscillation. This is achieved by having the amplifier provide a 180° phase shift and the feedback network contributing an additional 180°, resulting in a total of 360° or 0° cumulative phase shift.

Please refer to the provided ANSWER SHEET 5.7.3 for the required output waveform of the RC-oscillator, which should illustrate the typical sine wave output with relevant amplitude and time markings.

Oscillator circuits do not require external input signals to operate as they are self-sustaining, generating their own signals. In contrast, transistor amplifier circuits require input signals to amplify, as their operation is dependent on external input for performance.

1. RF-oscillators can be used in local oscillators for radio receivers to tune to specific frequencies.
2. They are also utilized in radio circuits for signal generation and modulation applications.

Please refer to the provided ANSWER SHEET 5.9 for the graphical representation of TWO cycles of damped oscillation, illustrating the gradual decrease in amplitude over time.