3.1 Name TWO types of junction field effect transistors (JFETs) - NSC Electrical Technology Electronics - Question 3 - 2018 - Paper 1

To overcome the leakage current, the construction of the JFET was modified by electrically isolating the gate terminal from the drain-source channel through the implementation of an extremely narrow layer of metal-oxide-silicon (MOS).

The type of MOSFET used in this circuit is an Enhancement-mode N-channel MOSFET.

The lamp will switch ON as soon as the gate voltage ( $V_{GS}$) is raised to a sufficient level, which allows the internal current of the MOSFET to flow.

If $R_{GS}$ is short-circuited, it will cause the internal conductive channel of the MOSFET to disperse, effectively cutting the current flow and switching the lamp OFF.

Region E is the region of negative resistance in the UJT characteristic curve.

Between points C and D, the UJT triggers ON as the internal resistance decreases, allowing a larger current to flow, which further decreases the voltage across the emitter.

The transistors are connected in a Darlington pair configuration.

Pin 3 is the non-inverting input of the 741 op amp.

The type of package in which the integrated circuit is constructed is a Dual-in-line Package (DIP).

The output signal of the op amp will be a sine wave with a zero crossing at 0 V and peak values at +V and -V.

Open-loop gain is the gain of an Op-Amp without any feedback applied from the output to the input. In contrast, closed-loop gain is the gain of an Op-Amp with feedback applied, resulting in a stable output voltage that depends on the feedback network.

To calculate the output voltage ($V_{OUT}$), the formula used is:

$V_{OUT} = V_{IN} \left( 1 + \frac{R_F}{R_{IN}} \right)$

Given: $V_{IN} = 5 \text{ mV}$, $R_F = 220 \text{ k}\Omega$, $R_{IN} = 440 \Omega$,

So, $V_{OUT} = 5 \times 10^{-3} \left( 1 + \frac{220000}{440} \right)$

Calculating this gives: $V_{OUT} = 5 \times 10^{-3} \left( 1 + 500 \right) = 5 \times 10^{-3} \times 501 = 2.51 \text{ V}$

When the voltage on pin 2 of the 555 timer is less than 1/3 of the supply voltage, the timer goes high. Conversely, when the voltage is higher than 2/3 of the supply voltage, the timer resets. This forms a flip-flop operation that provides timing control for various applications.