Maharashtra HSC Electronics Technology (Vocational) flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Depletion region

Region around a PN junction depleted of free carriers, formed when electrons and holes diffuse and recombine; the fixed ions create a potential barrier that must be overcome for forward conduction.

Zener diode regulation

Operated in reverse breakdown, the diode holds a nearly constant voltage; a series resistor drops the surplus voltage and limits current as input or load varies.

Transistor beta (hFE)

DC ratio of collector current to base current in a BJT, typically 50 to 300; it determines the base drive required to switch or amplify a given collector current.

LED forward voltage

Typically 1.8–2.2 V for red and 3.0–3.4 V for blue or white; a series resistor R = (Vsupply − Vf)/If sets the operating current safely.

Resistor colour code

First two bands give digits, third gives the multiplier, fourth gives tolerance; brown-black-red-gold reads 1 kΩ ±5 %, with a gold multiplier meaning 0.1.

RC time constant

τ = RC seconds; a capacitor charges to 63.2 % of the supply in one τ and is treated as fully charged after five τ, when it reaches 99.3 %.

Series resonance

At f = 1/(2π√(LC)) the reactances XL and XC cancel, impedance falls to R and current peaks; used in tuning circuits and band-pass filters.

Capacitor impedance

XC = 1/(2πfC) ohms; reactance falls as frequency rises, so a capacitor blocks DC, passes high frequencies and serves as a coupling or bypass element.

Ripple factor

RMS ripple voltage divided by the DC output voltage; about 1.21 for half-wave and 0.48 for full-wave rectifiers, further reduced by filter capacitors.

Bridge rectifier PIV

Peak inverse voltage across each diode equals the peak secondary voltage; four diodes give full-wave output without needing a centre-tapped transformer.

7805 regulator

Three-terminal fixed positive linear regulator delivering +5 V; input must be at least 2 V higher, with 0.33 µF input and 0.1 µF output bypass capacitors.

Transformer turns ratio

Vs/Vp = Ns/Np; a step-down transformer lowers mains voltage before rectification and isolates the low-voltage circuit from the supply.

Barkhausen criterion

For sustained sine oscillation, loop gain must be at least unity and the total phase shift around the feedback loop must be zero or 360 degrees.

Class B amplifier

Each output transistor conducts for 180 degrees, giving a maximum theoretical efficiency of 78.5 %; crossover distortion appears unless the stage is biased into class AB.

Negative feedback

Feeding a fraction of the output back out of phase lowers gain but widens bandwidth, reduces distortion, raises input impedance and stabilises the operating point.

Op-amp virtual ground

With negative feedback and very high open-loop gain, the inverting input sits at almost the same potential as the non-inverting input, behaving as a virtual ground.

De Morgan's theorems

A NAND equals a bubbled OR and a NOR equals a bubbled AND: (A·B)' = A' + B' and (A + B)' = A'·B'.

JK flip-flop toggle

When J and K are both high the output changes state on every active clock edge, which is why JK stages form ripple and synchronous counters.

Multiplexer

Combinational circuit that selects one of 2^n data inputs under the control of n select lines and routes the chosen signal to a single output.

Half adder

Adds two single bits: sum = A XOR B and carry = A AND B; two half adders plus an OR gate form a full adder.

Nyquist sampling theorem

A band-limited signal must be sampled at more than twice its highest frequency to be reconstructed without aliasing; practical systems sample higher to relax filter requirements.

AM modulation index

m = Vm/Vc for amplitude modulation; values above 1 mean overmodulation with sideband splatter, and percentage modulation equals m × 100.

Shannon capacity

C = B log2(1 + S/N) bits per second; capacity grows linearly with bandwidth but only logarithmically with the signal-to-noise ratio.

8051 program counter

16-bit register holding the address of the next instruction; it can address 64 KB of code memory and increments automatically after each instruction fetch.

Watchdog timer

Counter that resets the microcontroller if firmware fails to clear it within the set period, recovering the system from hangs and infinite loops.

UART baud rate

Bits per second on the serial line; transmitter and receiver must match, and each frame carries a start bit, data bits, optional parity and a stop bit.

Cathode ray oscilloscope

Displays voltage against time; the vertical amplifier drives deflection, the time base sweeps horizontally, and frequency is read as f = 1/T from the graticule.

Loading effect

A meter of low impedance draws current from the circuit under test and reads low; voltmeters need input impedance far higher than the resistance being measured.

Accuracy versus precision

Accuracy is closeness to the true value; precision is repeatability of readings. A meter can be precise yet consistently offset and therefore inaccurate.

Cold solder joint

Dull, grainy, convex joint caused by insufficient heat or contaminated surfaces; it creates an intermittent high-resistance connection and must be reworked.

PCB solder mask

Polymer layer covering the copper except at pads; it prevents solder bridging between traces and protects the copper from oxidation and corrosion.

Trace width and current

Wider copper traces carry more current and drop less voltage; a common outer-layer rule of thumb is about 1 mm of width for every 1 A.