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

1. To set the operating conditions (points) of the transistor, ensuring that it operates in the desired region (active region).
2. To stabilise the operating point of the transistor, preventing distortion and allowing for reliable amplification.

The Q-point, or quiescent point, is the point on the DC load line that represents the DC operating voltage and current of the transistor when no input signal is applied. It indicates the voltages across the transistor and the current through it, thus defining the transistor's status in the load line diagram.

In the load line diagram, the Q-point for:

• Class A is located at the midpoint of the load line.
• Class B is positioned at the crossover point on the load line.
• Class C is typically situated to the right of the load line.

1. Capacitor C2 serves as an AC coupling component, blocking DC components while allowing AC signals to pass through.
2. It helps in stabilizing the signal by isolating stages of the amplifier, preventing feedback of DC voltage.

An RC-coupled amplifier typically consists of multiple amplification stages connected through resistors and capacitors. The AC input signal is applied to the first stage, where it is amplified. The collector resistor develops an alternating voltage, which is transferred via capacitor C2 to the input of the second stage, allowing for further amplification. This process is repeated through the stages of the amplifier.

1. Impedance matching is essential to ensure that maximum power is transferred between stages, minimizing signal loss.
2. DC isolation is required to prevent the DC bias of one stage influencing the operation of the following stage.

Frequency response is the ability of an amplifier circuit to respond to a range of frequencies. It indicates how the gain of the amplifier changes over the frequency spectrum.

The half-power points, also known as -3 dB points, are the frequencies at which the output power of the amplifier has dropped to half of its peak value. This represents a level of -3 dB in the frequency response curve.

At low frequencies, the reactance of the decoupling capacitors increases, causing a reduction in the overall gain of the amplifier. This is due to the capacitors not being able to pass lower frequency signals effectively.

Impedance matching can be achieved by selecting a transformer with appropriate turns ratio or utilizing resistors to adjust the impedance levels between the stages, ensuring optimal signal transfer.

A transformer is used to match the impedance of the amplifier output to the load (such as a speaker), which maximizes power transfer and minimizes signal reflection.

An oscillator is a device that generates an AC output signal without any externally applied input signal. It relies on internal feedback mechanisms to sustain oscillations.

An oscillator typically generates a sine wave output.

Resistors R1 and R2 are used to form a voltage divider that biases the base of the transistor, setting the proper operating point for the amplifier.

The Hartley oscillator utilizes two inductors and one capacitor in its tank circuit, whereas the Colpitts oscillator uses two capacitors and one inductor. This difference affects the frequency determination of the oscillations.

1. The transistor amplifies the damped oscillating signal generated by the circuit.
2. It provides a phase shift of 180°, facilitating sustained oscillation.

The total phase shift is zero because the RC network provides a phase shift of 180°, while the transistor amplifier also contributes a phase shift of 180°. These two shifts cancel each other out, resulting in a total phase shift of zero.

Oscillator circuits do not require an external input signal to function; they are self-sustaining. In contrast, transistor amplifier circuits need an input signal to amplify.

1. In local oscillators, commonly used in radio receivers.
2. In radio circuits for signal generation.

Damped oscillations refer to the gradual decrease in amplitude of oscillations over time due to energy losses. In the first cycle, the signal peaks at maximum voltage and slowly decreases in subsequent cycles, eventually settling towards zero amplitude.