4.1 Refer to FIGURE 4.1 and answer the questions that follow - NSC Electrical Technology Electronics - Question 4 - 2022 - Paper 1

Increasing the gate-source voltage $V_{gs}$ enhances the conductivity of the channel between the drain and source. Once $V_{gs}$ surpasses a certain threshold, the MOSFET transitions into saturation, allowing maximum current flow from drain to source.

The characteristic curve in FIGURE 4.2 is the input/output characteristic of a Field Effect Transistor (FET). It describes how the output current varies with the input voltage.

The output waveform should be sketched in accordance with the transfer characteristics of the FET. The waveform typically exhibits a non-linear relationship between the input voltage and output current with specific regions for cutoff and saturation.

One application of the UJT (Uni-Junction Transistor) is as a triggering device to switch on SCRs and TRIACs in power control circuits.

When $V_{BB}$ is applied, it creates a potential difference across the two bases, forming a voltage $V_{x}$ at the point where the P-N junction is forward-biased. This condition turns the UJT on, allowing current to flow.

In the negative resistance region, the UJT exhibits a decline in current with an increase in voltage. This is due to the feedback mechanism where the emitter current decreases as the voltage rises, ultimately leading to a regenerative cycle.

The circuit diagram in FIGURE 4.4 depicts a UJT being used in an oscillator configuration. This involves a capacitor that charges and discharges through resistors.

In this circuit, when DC voltage is applied, the capacitor charges exponentially. Upon reaching the peak voltage, the UJT turns on, allowing the capacitor to discharge quickly, creating oscillation due to the RC time constant between the components.

Upon connecting a supply, the circuit provides voltage to the input of a relay. When an adequate voltage level is reached, the relay is energized, causing the normally open (NO) contact to close.

The voltage at point A in the inverting operational amplifier circuit is 1.4 V, as established from the reference voltage across the resistors.

The gain of the operational amplifier is calculated using the formula: $A_v = -\frac{R_f}{R_{in}}$. Given $R_f = 14 k\Omega$ and $R_{in} = 1 k\Omega$, the gain is $A_v = -\frac{14 k\Omega}{1 k\Omega} = -14.$

When an AC signal is applied, the output signal will be inverted and have a 180° phase shift relative to the input signal.

Increasing the value of $R_f$ results in:

1. Amplified signal output, enhancing amplification properties.
2. Improved signal-to-noise ratio.
3. Reduced negative feedback, allowing for a greater response to input changes.

Pin 7 provides the discharge path for the timing capacitor and timing resistor in the 555 timer IC configuration.

The 555 IC is triggered when the voltage at pin 2 falls below one-third of the supply voltage. This action activates the timing circuit, resulting in the output toggling from low to high, which initiates its functionality.