Past Questions

Transformer changes voltage level via electromagnetic induction, frequency unchanged.

Ideal voltage source has zero internal resistance, terminal V independent of load.

CRO uses electrostatic focusing (and electrostatic deflection) for electron beam.

ARM = Advanced RISC Machines (originally Acorn RISC Machine).

FM broadcast channel bandwidth is ~200 kHz (150 kHz deviation + guard).

Weak battery reduces driving current, causing ohm-meter to read high/more than actual; generally inaccurate high reading.

Current through coil produces magnetic deflection torque moving pointer.

TRIAC is bidirectional switch = two SCRs in antiparallel/inverse parallel.

In saturation (on) both BE and BC junctions are forward biased.

VHF propagates as space wave/line-of-sight in troposphere, not reflected by ionosphere.

RC circuit with output across C is low-pass filter (passes low frequencies).

RAM = Random Access Memory.

RCL series contains resistance, inductance and capacitance together.

A chopper is a DC-DC converter used to obtain a variable DC output from a fixed DC input, so its main use is to change the voltage level of DC. Option (b) is correct. Option (c) is not the standard primary use because choppers are mainly classified by voltage conversion, though current changes may occur indirectly. Option (d) is incorrect because decreasing ripple in rectifiers is the job of filters, not choppers. Option (a) is incorrect because rectification means converting AC to DC, which is done by a rectifier, not a chopper.

In a buck converter, the inductor’s main role is to store energy when the switch is ON and release it when the switch is OFF, helping maintain continuous current to the load. This energy storage action is fundamental to step-down conversion. Option b is related but secondary: the inductor also reduces current ripple, but that is a result of its energy-storage behavior, not its main purpose. Option a is incorrect because rectification is done by a diode or synchronous switch, not the inductor. Option d is incorrect because an inductor is not used simply for making an electromagnet in a buck converter.

In a single-phase half-wave inverter, only one SCR is required to control the output during each conduction interval, so one SCR is gated at a time. The other options are incorrect because two, three, or four SCRs are associated with more complex inverter topologies such as full-bridge or multi-device configurations, not a single-phase half-wave inverter.

A parallel-to-serial converter is a circuit that takes multiple input lines (parallel data) and sends them sequentially over a single output line (serial data). 1. **Multiplexer (MUX):** A multiplexer is a combinational circuit that selects one of several input signals and forwards the selected input into a single output line. By using a counter to cycle through the select lines of a MUX, the parallel inputs are sampled one by one and sent out serially. Therefore, a MUX is the fundamental building block for a parallel-to-serial converter. 2. **De-multiplexer (DEMUX):** This is the exact opposite of a MUX. It takes one input and distributes it to one of several outputs. It acts as a serial-to-parallel converter. 3. **Decoder:** A decoder converts a binary code into a specific output signal (e.g., 3-to-8 line decoder). It does not perform data conversion from parallel to serial. 4. **Digital Counter:** While a counter is often used to provide the selection signals for a MUX in a parallel-to-serial converter, the counter itself does not perform the data routing; it only provides the timing/addressing logic.

In a semiconductor, charge carriers are divided into majority and minority carriers based on the type of doping. 1. **N-type material**: Doped with pentavalent impurities. Electrons are the majority carriers, and holes are the minority carriers. 2. **P-type material**: Doped with trivalent impurities. Holes are the majority carriers, and electrons are the minority carriers. An **npn transistor** consists of a thin layer of p-type semiconductor (the base) sandwiched between two layers of n-type semiconductor (the emitter and collector). When discussing the 'minority carriers' of the transistor in a general context or specifically regarding the dominant regions (the n-type emitter and collector), the minority carriers are the holes. Furthermore, the operation of an npn transistor relies on the injection of electrons (majority carriers of the emitter) into the base (p-type), where they become minority carriers. However, in the context of the n-type materials that define the 'npn' structure, holes are the minority carriers. - Option (a) is incorrect because free electrons are the majority carriers in n-type material. - Option (c) and (d) are incorrect because donor and acceptor ions are fixed in the crystal lattice and are not mobile charge carriers.

With negative feedback Af = A/(1+βA) =20/(1+0.1*20)=20/3≈6.67.

A Phase-Locked Loop (PLL) consists of three main components: a Phase Detector (PD), a Low Pass Filter (LPF), and a Voltage Controlled Oscillator (VCO). 1. The Phase Detector compares the input signal with the feedback signal from the VCO and produces an error signal. This error signal contains both a DC component (representing the phase difference) and high-frequency components (sum and difference frequencies resulting from the mixing process in the PD). 2. The function of the Low Pass Filter (LPF) is to remove these high-frequency components (noise and ripples) and allow only the slow-varying DC voltage to pass through. 3. This filtered DC voltage then acts as the control voltage for the VCO, ensuring that the VCO frequency adjusts smoothly to track the input frequency without jitter or instability caused by high-frequency noise. Analysis of options: - Option (a) is incorrect because an LPF removes high frequencies, not low frequency noise. - Option (b) is correct as the primary role of the LPF is to suppress high-frequency noise and ripples from the phase detector output. - Option (c) is incorrect; while the filter helps in creating the control voltage, 'tracking' is the overall function of the loop, not specifically the filter. - Option (d) is incorrect; the VCO changes the frequency, not the filter.

In the 8085 microprocessor instruction set, instructions are categorized by their size (number of bytes) based on whether they require immediate data or memory addresses. 1. **MOV A,B (Option d):** This is a register-to-register data transfer instruction. Both the source (B) and destination (A) are encoded within the opcode itself. Therefore, it requires only one byte of memory. This is a 1-byte instruction. 2. **MVI B, 05 (Option a):** This is a 'Move Immediate' instruction. It requires one byte for the opcode and a second byte for the immediate data (05). This is a 2-byte instruction. 3. **LDA 2500H (Option b):** This is a 'Load Accumulator Direct' instruction. It requires one byte for the opcode and two additional bytes to specify the 16-bit memory address (2500H). This is a 3-byte instruction. 4. **IN 01 (Option c):** This is an 'Input from Port' instruction. It requires one byte for the opcode and one byte to specify the 8-bit port address (01). This is a 2-byte instruction.

Cascading refers to the process of connecting multiple amplifier stages in series, where the output of one stage becomes the input to the next. 1. **Overall Gain Calculation**: In a cascaded system, the total voltage gain (Av_total) is the product of the individual gains of each stage. For example, if two stages have gains of A1 and A2, the total gain is Av_total = A1 × A2. Since gains are typically greater than 1, the product results in a significantly higher overall gain than any single stage could provide alone. 2. **Analysis of other options**: - **Option b**: The gain of an individual amplifier is determined by its internal components (resistors, transistors) and biasing; cascading them does not decrease the inherent gain of each stage. - **Option c**: Each amplifier does not necessarily 'work less'; in fact, later stages must handle larger signal swings, and the first stage must maintain high sensitivity. - **Option d**: Cascading usually requires the same or even more stable power supply rails for each stage to prevent clipping and maintain linearity; it does not lower the required supply voltage. Therefore, the primary purpose and result of cascading amplifiers is to increase the overall gain of the system.

By Kirchhoff’s current law for a transistor, the emitter current is the sum of the base and collector currents: I_E = I_B + I_C. Hence option (a) is correct. Option (d) states the same equation, so it is also mathematically correct; however, if a single answer is expected, (a) is the intended choice as written. Option (b), I_B = I_E - I_C, is just an algebraic rearrangement of the same relation and is also equivalent. Option (c), I_C = I_E + I_B, is incorrect because collector current cannot exceed the emitter current by adding base current; instead, I_C = I_E - I_B.

A transducer is defined as a device that converts energy from one form to another. In the context of instrumentation, it typically converts a physical quantity (such as pressure, temperature, or light) into an electrical signal (voltage or current), or vice versa. For example, a thermocouple converts thermal energy into electrical energy. Analysis of other options: - a. Transformer: A transformer changes the voltage and current levels of alternating current (AC) electricity, but it does not change the form of energy; it remains electrical energy throughout. - b. Transponder: A transponder is a device that receives a signal and automatically transmits a response; it is a communication device, not an energy converter. - d. Transcontinental: This is an adjective referring to something that crosses a continent (e.g., a transcontinental railroad) and has no relevance to electrical instrumentation.

A crowbar circuit is a protection circuit designed to prevent an overvoltage condition from damaging a load or a power electronic device (like an SCR). It works by intentionally creating a short circuit (a 'crowbar') across the power supply rails when a fault is detected. This triggers a fast-acting fuse or circuit breaker to blow, thereby disconnecting the power source and protecting the sensitive components from high voltage spikes. - Option (a) is incorrect because over-current protection is typically handled by fuses or circuit breakers. - Option (c) and (d) are incorrect as 'over rectification' and 'unrectification' are not standard electrical fault conditions that require a crowbar circuit.