6.1 Describe the term stabilization with reference to amplifiers - NSC Electrical Technology Electronics - Question 6 - 2024 - Paper 1

One advantage of class AB push-pull amplifiers is that they effectively reduce cross-over distortion, enabling better linearity and improved sound quality in audio applications.

The quiescent collector current ( IC) can be determined using the transistor's current gain ( β). Assuming a common β value, the collector current can be calculated as:

I_C = eta imes I_B
where I_B = 20 µA. If we take β = 200, then:

$I_C = 200 imes 20 imes 10^{-6} = 4 ext{ mA}$

The quiescent voltage ( V_CE) can be calculated by analyzing the load line in the transistor's output characteristic curves. Assuming a V_CC of 20 V and considering that the collector current is 4 mA:

$V_{CE} = V_{CC} - (I_C imes R_C)$
Assuming R_C to be 2.5 kΩ, we have:

$V_{CE} = 20V - (4 imes 10^{-3} imes 2500) = 10V$

The quiescent point should be marked on the graph with coordinates reflecting IC = 4 mA and V_CE = 10 V, representing the operating point where the transistor is biased for optimal operation.

Two undesirable effects of transistor biasing include:

1. Thermal runaway, which can occur if the biasing conditions lead to an increase in collector current, resulting in overheating.
2. Distortion of the output waveform, which can arise from improper biasing and allow for nonlinear operation.

One disadvantage of the RC-coupled amplifier is its unsuitability for low-frequency amplification, as the coupling capacitors block DC signals and reduce gain at lower frequencies.

When the temperature of a transistor increases beyond its normal operating points, it leads to an increase in leakage current. This can cause the transistor to enter thermal runaway, potentially damaging it and resulting in failure.

The RC-coupled amplifier acts as a low-frequency filter because the coupling capacitors block DC and low-frequency signals while allowing higher frequencies to pass. This characteristic allows it to effectively reduce noise and unwanted low-frequency interference.

The response curve should depict the gain in dB versus frequency, highlighting the cutoff frequencies at f1 and f2 where the gain starts to roll off, along with the flat mid-frequency region where the amplifier performs optimally.

One disadvantage of the amplifier is that it suffers from poor frequency response, affecting the overall quality, especially at high audio frequencies.

The impedance matching transformer is crucial for maximizing power transfer to the speaker by ensuring that the output impedance of the amplifier matches the impedance of the speaker.

Capacitor C1 serves two key functions:

1. It allows AC signals to pass through while blocking DC signals, ensuring the proper functioning of the amplifier.
2. It helps to manage the frequency response of the amplifier by providing coupling between stages.

The output waveform is inverted because of the nature of the transistor's operation in the common emitter configuration, where an increase in input voltage leads to a decrease in output voltage. The amplification is achieved due to the transistor's ability to control a large collector current based on a small base current.

One advantage of the amplifier is that it produces a much larger output signal than a single Class A biased transistor amplifier, improving efficiency and performance.

Cross-over distortion can be eliminated by biasing the two push-pull transistors into class AB mode, where both transistors are slightly on during the transition, minimizing the gap and improving linear amplification.

During the negative half cycle, transistor Q1 is turned off while Q2 turns on, providing a discharge path for the output. This results in the flow of current through the speaker, producing the negative half of the output waveform.

The frequency response curve in FIGURE 6.7 is derived from a radio-frequency amplifier that shows bandwidth and gain characteristics.

Bandwidth refers to the range of frequencies over which the amplifier maintains an acceptable level of gain. It is defined as the difference between the upper frequency (f2) and lower frequency (f1) at which the gain falls to 70.7% of its maximum value.

The resonant frequency can be altered by varying the values of either the capacitor or inductor in the tank circuit, which affects the overall resonant condition according to the formula:

$f_0 = \frac{1}{2\pi\sqrt{LC}}$

One application of the Colpitts oscillator is in receivers of radio and television, where it generates stable oscillations for signal processing.

The tank circuit is essential for determining the frequency of oscillation in the oscillator circuit, acting as a resonant circuit that selectively allows oscillation at a specific frequency.

The oscillating frequency can be calculated using the formula:

$f_0 = \frac{1}{2\pi\sqrt{LC}}$
Substituting:

$f_0 = \frac{1}{2\pi\sqrt{200 \times 10^{-3} \times 150 \times 10^{-6}}} \approx 29.06 ext{ kHz}$

The type of feedback used in the circuit is positive feedback, which enhances the signal and aids in oscillation.

Each RC combination produces a phase shift of 60 degrees, contributing to the overall feedback mechanism in the oscillator arrangement.

The frequency of oscillation can be calculated using the formula:

$f_0 = \frac{1}{2\pi R_{total} C_{total}}$
Where:

R_{total} = 3 \times 10kΩ = 30 kΩ

C_{total} = 3 \times 0.001 \mu F = 0.003 \mu F Substituting the values:

$f_0 = \frac{1}{2\pi\times 30 \times 10^{3}\times 0.003 \times 10^{-6}} \approx 20.54 Hz$

Attenuation refers to the reduction in amplitude of the output voltage compared to the input voltage. It occurs when a circuit causes a loss of signal power being transmitted, leading to a weaker output.