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 are designed to allow output collector current flow for less than 180° of the input cycle. This configuration is useful for high-frequency applications and typically yields high efficiency.

Negative feedback in amplifier circuits refers to a process where a portion of the output signal is fed back to the input in a way that reduces the overall gain of the system. This helps in stabilizing the gain, enhancing bandwidth, and improving linearity, thereby reducing distortion.

The collector resistor (Rc) limits the maximum current flowing through the collector, thereby protecting the transistor from potential damage due to excessive current.

The coupling capacitors are selected around 10 μF to ensure that they can pass a wide range of frequencies between the stages of the amplifier without introducing significant attenuation, which allows for efficient signal transfer.

The waveform at point B is a copy of the input signal with a DC offset, while the waveform at point C closely follows point B but includes additional amplification, showing a clearer signature of the input signal's characteristics.

As the collector current (Ic) increases due to a positive input signal, the voltage across the collector-emitter (Vce) resistor decreases, illustrating an inverse relationship where increased current results in reduced voltage.

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

$Ap = 10 imes ext{log}_{10} \left( \frac{P_{out}}{P_{in}} \right)$

Substituting the values:

$Ap = 10 imes \text{log}_{10} \left( \frac{18}{3} \right) = 10 imes \text{log}_{10} (6) \approx 7.78 dB$.

The circuit diagram in FIGURE 6.4 is identified as a Hartley oscillator.

The components that determine the oscillation frequency are the inductors (L1 and L2) and the capacitor (C3).

In this circuit, when the transistor is turned on, the collector voltage rises, causing capacitor C1 to charge. As the voltage changes, it drives the transistor's base potential to turn off, leading to oscillation as the tank circuit alternates between charging and discharging.

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

$f_o = \frac{1}{2 \pi \sqrt{L T C}}$

where total inductance is 300 mH and capacitance C3 is 250 μF:

$f_o = \frac{1}{2 \pi \sqrt{300 \times 10^{-3} H \times 250 \times 10^{-6} F}} \approx 18.38 Hz$.

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 of the amplifier, or it acts as an impedance matching component to optimize power transfer.

When the output impedance of transformer T2 is not matched with the speaker's impedance, maximum power transfer will not occur, resulting in a lower output signal level.

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

The two requirements for oscillation are that the total loop gain must be equal to or slightly greater than unity, and the phase difference around the loop must be 360°.

Positive feedback is necessary to overcome any losses in the circuit, ensuring that the output signal is sufficient to sustain oscillations without requiring an input signal.

The RC-network is responsible for setting the frequency of oscillation and controlling the phase shift necessary for feedback within the circuit.

Attenuation refers to the reduction in amplitude or strength of a signal as it passes through a circuit, often occurring when the output voltage becomes smaller than the input voltage due to losses.