Flashcards · 7408 · Topic 13

Electronics flashcards

30 cards on Topic 13, subtopics 3.13.1 to 3.13.6. Pick a subtopic or study them all.

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N-channel enhancement MOSFET: Terminals: drain, source, gate. The gate is insulated, so input resistance is very high. Off when V_GS is below the threshold voltage V_th; on above it.
Zener diode: Used reverse biased. Conducts at the breakdown voltage V_Z if the current is above the minimum operating current. With a series resistor it gives a constant reference voltage.
Photodiode: Reverse biased (photoconductive mode): the reverse current rises with light intensity. Its spectral response curve has a peak and a long-wavelength cut-off.
Hall effect sensor: Output depends on magnetic flux density. Uses: attitude monitoring, and as a tachometer (one pulse per turn).
Choosing the series resistor for a zener diode: Resistor current = load current + minimum zener current. Resistor pd = supply − V_Z. R(max) = pd ÷ current; choose the next value below.

Noise: analogue vs digital signals: Analogue noise is amplified along with the signal; digital pulses are regenerated by comparing with a threshold.
Pulse code modulation (PCM): Each sample is sent as a binary code.
Number of levels and step size for an 8-bit ADC with a 0–5.0 V input: Levels = 2⁸ = 256. Step size = 5.0 ÷ 256 ≈ 0.020 V (19.5 mV).
Effect of more bits and a faster sampling rate: Better quality (finer quantisation, follows faster changes) but more data. The highest frequency reproduced is about half the sampling rate.

Resonant frequency of an LC circuit: f₀ = 1/(2π√(LC)).
Q factor of an LC filter: Q = f₀/f_B, where f_B is the bandwidth between the 50% energy points. High Q gives a narrow peak.
Ideal op-amp: Infinite open-loop gain and infinite input resistance. Connections: supply rails, V₊, V₋ and output.
Op-amp as a comparator: V_out = A_OL(V₊ − V₋). Output goes to positive saturation when V₊ > V₋ and negative saturation when V₋ > V₊.

Inverting vs non-inverting amplifier: Inverting: gain −R_f/R_in; low input resistance. Non-inverting: gain 1 + R_f/R₁; very high input resistance.
Derive the gain of an inverting amplifier: Huge gain and finite output make V₋ ≈ V₊ = 0 V (virtual earth). No current enters the op-amp, so V_in/R_in = −V_out/R_f, giving V_out/V_in = −R_f/R_in.
Inverting amplifier with R_f = 100 kΩ and R_in = 10 kΩ; V_in = 0.50 V. Find V_out.: Gain = −100/10 = −10, so V_out = −5.0 V (within the supply rails).
Summing and difference amplifiers: Summing: V_out = −R_f(V₁/R₁ + V₂/R₂ + …). Difference: V_out = (V₊ − V₋)R_f/R₁.
Real op-amp vs ideal: Finite gain that falls with frequency, finite input resistance, output limited by the rails. Gain × bandwidth is constant, so a higher closed-loop gain gives a narrower bandwidth.
Open-loop vs closed-loop gain: Open-loop A_OL: the op-amp alone. Closed-loop (such as R_f/R_in): the circuit with feedback.

When is the output 1 for AND, NAND, OR, NOR and EOR gates?: AND: both inputs 1. NAND: not both 1. OR: at least one 1. NOR: both 0. EOR: exactly one 1.
Binary vs BCD vs Johnson counter: Binary: n outputs, 2ⁿ states. BCD: counts 0 to 9 (10 states). Johnson: n stages, 2n states, one bit changes per pulse.
How do you make a modulo-n counter?: Write n in binary, AND together the outputs that are 1 in n, and connect the AND output to reset. The counter shows 0 to n − 1.
Duty cycle vs mark-to-space ratio: Duty cycle = mark ÷ period. Mark-to-space ratio = mark ÷ space.
Astable: effect of a larger R or C in the timing network: Longer period, so a lower clock frequency.

Wire vs optical fibre vs radio/microwave links: Wire: cheap, low bandwidth, interference. Fibre: high bandwidth, secure, low loss, costly to lay. Radio/microwave: no cable; microwaves are line of sight.
Ground wave vs sky wave: Ground wave: long wavelengths diffract round the Earth. Sky wave: refracted and reflected by the ionosphere.
Why do satellites use different up-link and down-link frequencies?: So the satellite's receiver is not de-sensed by its own powerful transmitter.
Time-division multiplexing (TDM): Channels share one link in turn, in time slots.
AM vs FM bandwidth: AM: 2f_M. FM: 2(Δf + f_M), where f_M is the highest information frequency and Δf the frequency deviation.
Purpose of the modulator, the multiplexer and the tuned receiver in a communication system: Modulator: puts the signal onto a carrier. Multiplexer: combines channels on one link. Tuned receiver: selects the wanted carrier.