semiconductor

State the purpose of the index dot on the body of a 741 IC package. [1 Mark]

The index dot indicates the starting sequence for pin numbering on the 741 IC package (Pin 1 is closest to / directly to the left of the dot).

Name the type of package in which the 741 IC is constructed and state how to identify Pin 1. [2 Marks]

Package: Dual In-Line Package (DIP or DIP-8). Pin 1 Identification: Identified as the first pin to the left of and below the notch or indentation, or closest to the dot on the body.

Describe the effect the op-amp will have on an input sine wave applied to Pin 2 (Inverting Input). [2 Marks]

The output sine wave will appear amplified and inverted ($180^\circ$ phase shift relative to the input).

Explain why a 741 operational amplifier requires a dual rail power supply ($+V<em>{CC} / -V</em>{CC}$). [2 Marks]

A dual power supply allows the output voltage waveform to swing both above $0\,\text{V}$ ($+V<em>{CC}$) and below $0\,\text{V}$ ($-V</em>{CC}$) without clipping.

List FOUR main characteristics of an ideal operational amplifier. [4 Marks]

1. Infinite voltage gain ($A<em>V = \infty$); 2. Infinite input impedance ($Z</em>{IN} = \infty$); 3. Zero output impedance ($Z_{OUT} = 0\,\Omega$); 4. Infinite bandwidth; 5. Infinite Common Mode Rejection Ratio (CMRR); 6. Unconditional stability.

State TWO advantages of an operational amplifier. [2 Marks]

1. Small size; 2. Cheap; 3. Low power consumption; 4. Highly stable; 5. Highly reliable.

Explain the difference between open-loop gain and closed-loop gain with reference to op-amps. [2 Marks]

Open-Loop Gain: The maximum gain of an op-amp when there is no feedback resistor connected between output and input terminals. Closed-Loop Gain: The gain of an op-amp when a feedback component (resistor) is connected between output and input to control and stabilize the gain.

Explain why operational amplifiers are seldom used in open-loop mode. [2 Marks]

Operational amplifiers have an extremely high open-loop gain ($\approx 100\,000$), which creates poor stability of the output voltage and causes the output to saturate very easily.

Give TWO reasons why negative feedback is important when an op-amp is used as a linear amplifier. [2 Marks]

1. It controls and stabilizes the amplifier gain. 2. Output signal distortion is reduced. 3. Circuit bandwidth is increased.

State the purpose of the feedback resistor ($R_F$) in an operational amplifier circuit. [1 Mark]

The feedback resistor provides negative feedback to reduce and control the voltage gain of the amplifier to a required level.

Explain the term bandwidth as a characteristic of an operational amplifier. [2 Marks]

Bandwidth is the range of input signal frequencies that the operational amplifier can process and amplify (from $0\,\text{Hz}$ DC up to radio frequencies) without output waveform distortion or loss of gain.

Describe what is meant by infinite bandwidth of an op-amp. [2 Marks]

It means that the operational amplifier can operate within an infinite range of frequencies with the exact same amount of amplification or gain.

Explain the term Common Mode Rejection Ratio (CMRR) with reference to operational amplifier characteristics. [1 to 2 Marks]

Common Mode Rejection Ratio is the ability of an op-amp to suppress common-mode noise signals present simultaneously on both inputs while amplifying differential signals (or the ratio of output voltage to common input voltage).

Explain why operational amplifiers are known as differential voltage amplifiers. [1 Mark]

Because an operational amplifier amplifies the voltage difference between its two input terminals ($V<em>1 - V</em>2$).

List the three internal stages of a 741 operational amplifier. [3 Marks]

1. Input stage: Differential amplifier; 2. Intermediate stage: High-gain differential amplifier; 3. Output stage: Common collector (emitter follower output driver).

Explain the function of the intermediate stage in the internal circuit of a 741 op-amp. [2 Marks]

The intermediate stage compares the differential output from the first stage and amplifies this difference, resulting in the very high overall voltage gain possessed by an op-amp.

Name the 741 op-amp circuit that uses 100% feedback. [1 Mark]

Voltage follower / Buffer

State the typical operating (supply) voltages of a 741 operational amplifier. [1 to 2 Marks]

$\pm 5\,\text{V}$ to $\pm 15\,\text{V}$ (or $\pm 5\,\text{V}$ to $\pm 18\,\text{V}$)

State the typical operating supply voltage range for a 555 IC. [1 to 2 Marks]

$+5\,\text{V}$ to $+18\,\text{V}$ (or $+4.5\,\text{V}$ to $+16\,\text{V}$)

Explain the function of the three $5\,\text{k}\Omega$ internal resistors inside a 555 IC. [2 Marks]

They form a voltage divider network that divides the supply voltage ($V<em>{CC}$) into three equal values ($\frac{1}{3}\,V</em>{CC}$ and $\frac{2}{3}\,V_{CC}$) to set reference threshold levels for the internal comparators.

Name the primary building blocks inside a 555 IC. [3 Marks]

1. Two comparators; 2. One RS flip-flop; 3. Three $5\,\text{k}\Omega$ resistors (voltage divider network); 4. NPN discharge transistor and output driver.

Explain the function of the RS flip-flop inside a 555 IC / why it is known as a memory cell. [2 Marks]

The RS flip-flop has two stable states that temporarily store/maintain the output state (HIGH or LOW) it last received until new information is received from the comparators or reset pin.

Describe the function of Pin 4 (Reset Pin) on a 555 IC and explain why it is held at $+V_{CC}$ during normal operation. [3 Marks]

Function: Pin 4 resets the internal flip-flop. Connecting Pin 4 to $0\,\text{V}$ (ground) forces Output Pin 3 and Discharge Pin 7 to $0\,\text{V}$. $+V<em>{CC}$ Connection: It is held at $+V</em>{CC}$ to prevent accidental resetting of the IC during normal operation.

Explain the function of Pin 6 (Threshold Pin) on a 555 IC. [2 Marks]

Pin 6 monitors the voltage across the timing capacitor. When the voltage exceeds $\frac{2}{3}\,V_{CC}$, Comparator 1 resets the internal flip-flop, pulling Output Pin 3 LOW and switching ON the discharge transistor.

Explain how the 555 IC is triggered with reference to Pin 2 (Trigger Pin). [3 to 4 Marks]

When the voltage on Pin 2 falls below $\frac{1}{3}\,V_{CC}$, Comparator 2 sets the RS flip-flop, causing Output Pin 3 to go HIGH and turning OFF the discharge transistor so the timing capacitor can begin charging.

State the function of Pin 7 (Discharge Pin) on a 555 IC. [1 to 2 Marks]

Pin 7 provides a low-resistance discharge path to ground for the external timing capacitor and timing resistor when activated by the internal discharge transistor.

State the function of the internal NPN discharge transistor ($T_1$) in a 555 IC. [1 Mark]

It provides a low-resistance path for discharge current to flow from Pin 7 to ground only when output $\bar{Q}$ from the internal flip-flop goes HIGH.

Explain how Pin 5 (Control Voltage Pin) should be connected when not used for modulation, and why. [2 Marks]

Pin 5 must be connected to ground via a small-value capacitor ($0.01\,\mu\text{F}$) to filter out unwanted supply noise that could affect circuit timing.

Name the THREE modes of operation of the 555 IC. [3 Marks]

1. Monostable mode; 2. Astable mode; 3. Bistable mode

State TWO industrial / practical applications of the 555 IC. [2 Marks]

1. Timers / Delay circuits; 2. Pulse-width modulation (PWM) / Servo motor positioning control; 3. Temperature measurement and oven control; 4. Free-running oscillators / Tone generators; 5. Digital logic probes / Warning flasher lights.

State TWO disadvantages of the 555 IC. [2 Marks]

1. They cannot cope with high currents, high voltages, or high power requirements (limited to $200\,\text{mA}$ current and $18\,\text{V}$ supply voltage). 2. They can be damaged by overloading or by fluctuating power supplies.