6.1 Explain the following classes of amplifiers: 6.1.1 AB 6.1.2 C 6.2 Define the term negative feedback with reference to amplifier circuits - NSC Electrical Technology Electronics - Question 6 - 2021 - Paper 1

Class C amplifiers operate with the output collector current flowing for less than 180° of the input cycle. This configuration is typically used in high-frequency applications, as it offers a high efficiency at the cost of increased distortion.

Negative feedback in amplifier circuits occurs when a portion of the output signal is fed back to the input in a manner that opposes the input signal. This mechanism reduces the overall gain and improves stability and bandwidth of the amplifier.

The collector resistor (Rc) sets a limit to the maximum current flowing in the collector circuit, thereby protecting the transistor from damage due to excessive current.

The coupling capacitors are selected to pass the desired frequency range of signals while blocking DC components. A capacitance value around 10 µF allows them to efficiently couple AC signals while preventing low-frequency signals that could interfere with operation.

The waveforms at point B should show an amplified version of the input signal with appropriate DC offset. At point C, the waveform will depict the output signal after accounting for the phase shift introduced by the circuit, maintaining similar characteristics as the input but at increased amplitude.

As the input signal goes positive, the collector current (Ic) through the transistor increases, resulting in a corresponding drop in collector-emitter voltage (Vce). This relationship illustrates the active operation of the transistor, where increased input leads to enhanced output current and variation in voltage levels.

The power gain in decibels (dB) can be calculated using the formula:

ext{Ap} = 10 imes ext{log}_{10} rac{P_{out}}{P_{in}}

Here, $P_{out} = 18$ watts and $P_{in} = 3$ watts. Thus:

ext{Ap} = 10 imes ext{log}_{10} rac{18}{3} = 10 imes ext{log}_{10} 6 = 7.78 ext{ dB}

The circuit diagram in FIGURE 6.4 is identified as a Hartley oscillator, which is used for generating sinusoidal oscillations.

The components that determine the oscillation frequency are the inductors L1 and L2 along with the capacitor C3.

When the circuit is powered, the transistors' collector voltage rises, causing capacitor C1 to charge. The inductor influences the voltage across it to create oscillation. It causes the transistor to alternate between states, allowing continual recharging of the tank circuit at a stable frequency.

The oscillation frequency ($f_0$) can be calculated using the formula:

f_0 = rac{1}{2 imes ext{π} imes ext{√(LC)}}

Substituting the values:

The circuit diagram in FIGURE 6.5 is identified as a two-stage transformer-coupled amplifier.

Transformer T1 serves as a coupling component between the two stages or as an impedance matching component to ensure efficient signal transfer.

If the output impedance of transformer T2 is not matched with the speaker impedance, maximum power transfer will not occur, resulting in lower output levels and inefficiencies.

The circuit in FIGURE 6.6 is an RC phase-shift oscillator.

1. There must be positive feedback that results in a combined 360° voltage phase shift around the loop.
2. The gain of the amplifier must be equal to or slightly greater than unity.

Positive feedback is essential as it compensates for any attenuation in the feedback circuit and ensures that the circuit maintains its oscillatory behavior by reinforcing the input signal.

1. The RC network determines the frequency of oscillation.
2. It acts as an integrator or differentiator, shaping the voltage waveforms in the circuit.

Attenuation refers to the reduction of signal strength as it passes through a circuit element. It occurs when the output voltage becomes smaller than the input voltage, leading to a loss of signal power.