Maharashtra HSC Electronics (Bifocal) flashcards

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

Ohm's law

For a resistive element at constant temperature, V = IR. Rearranged, I = V/R and R = V/I. Power dissipated is P = VI = I²R = V²/R.

Capacitive reactance

Opposition a capacitor offers to AC: X_C = 1/(2πfC), in ohms. It falls as frequency or capacitance rises, so a capacitor passes high frequencies more easily than low ones.

LC resonance frequency

The frequency at which inductive and capacitive reactances cancel: f_r = 1/(2π√(LC)). In a series circuit impedance is minimum; in a parallel tank it is maximum.

Resistor colour code

Bands give digit, digit, multiplier, tolerance: black 0 through white 9; gold ±5%, silver ±10%. Example: brown-black-red-gold is 1 kΩ ±5%.

Depletion region

Region around a PN junction depleted of free carriers by diffusion. Its barrier potential is about 0.7 V for silicon and 0.3 V for germanium, and it widens under reverse bias.

Bridge rectifier PIV

Each of four diodes in a bridge must withstand a peak inverse voltage equal to the peak secondary voltage V_m, half that needed by a centre-tap full-wave rectifier (2V_m).

Zener diode regulation

Operated in reverse breakdown, a Zener holds a nearly constant voltage over a range of currents, so it can shunt-regulate a load when fed through a series resistor.

LED forward bias

A light-emitting diode is a forward-biased junction made from direct-bandgap material such as GaAsP or GaN; recombination of carriers releases photons whose energy sets the colour.

BJT current relations

Emitter current splits into base and collector: I_E = I_B + I_C. Current gains are α = I_C/I_E and β = I_C/I_B, linked by β = α/(1−α).

BJT active region

Operating condition where the emitter-base junction is forward biased and the collector-base junction reverse biased. The transistor then behaves as a current amplifier with I_C = βI_B.

JFET transfer characteristic

Drain current follows I_D = I_DSS(1 − V_GS/V_P)², where I_DSS is saturation current at zero gate bias and V_P is pinch-off voltage. The gate junction is reverse biased.

Voltage-divider bias

Two resistors set base potential and an emitter resistor provides negative feedback, so the Q-point stays nearly fixed despite large β spread and temperature changes.

Common-emitter gain

Voltage gain magnitude is approximately β(R_C ∥ R_L)/r_e with r_e ≈ 26 mV/I_E. The output is inverted, 180° out of phase with the input.

Class B efficiency

A push-pull Class B stage conducts for 180° per device and reaches a theoretical maximum efficiency of 78.5%, but suffers crossover distortion unless biased as Class AB.

Barkhausen criterion

An oscillator sustains oscillation when loop gain Aβ is at least unity and the total phase shift around the feedback loop is 0° or a multiple of 360°.

Negative feedback effects

Returning a fraction β of output to the input reduces gain by (1 + Aβ) but widens bandwidth, lowers distortion and noise, and alters input and output impedance.

De Morgan's theorems

NAND and NOR can be rewritten as inverted inputs: (A·B)‾ = Ā + B̄ and (A + B)‾ = Ā · B̄. These allow conversion between gate types.

Full adder outputs

Adding A, B and carry-in gives Sum = A⊕B⊕C_in and Carry-out = AB + C_in(A⊕B). Two half adders plus an OR gate implement it.

JK flip-flop toggle

When J and K are both high, the JK flip-flop complements its output on each active clock edge. This toggle action makes it the basis of binary counters.

Universal gates

NAND and NOR are called universal because either alone can build NOT, AND, OR and any other logic function, which simplifies IC inventory in digital design.

Ripple factor values

Ratio of AC ripple to DC output: about 1.21 for a half-wave rectifier and 0.48 for full-wave centre-tap or bridge circuits. Smaller values mean smoother DC.

7805 regulator

A three-terminal fixed positive regulator in the 78xx family. With input above about 7.5 V and adequate heatsinking it delivers a regulated +5 V output up to roughly 1 A.

LM317 output formula

Adjustable positive regulator output: V_out = 1.25(1 + R2/R1) + I_adj·R2, where 1.25 V is the internal reference between OUT and ADJ terminals.

Zener series resistor

The resistor feeding a Zener shunt regulator is R_S = (V_in − V_Z)/(I_Z + I_L). I_Z must exceed the knee current at minimum load to keep regulation.

CRO voltage measurement

Peak voltage equals vertical deflection in divisions multiplied by the volts/division setting and the probe attenuation factor. Period uses divisions times time/division.

Lissajous frequency ratio

In a CRO X-Y display, the ratio of horizontal to vertical frequency equals the ratio of the number of tangent points on the horizontal and vertical sides of the pattern.

Ammeter and voltmeter

An ammeter connects in series and needs very low resistance; a voltmeter connects in parallel and needs very high resistance, so each disturbs the circuit minimally.

AM modulation index

m = V_m/V_c, the ratio of modulating to carrier amplitude. Values above 1 cause overmodulation and envelope distortion; bandwidth is twice the modulating frequency.

Superheterodyne IF

Mixing the incoming signal with a local oscillator converts it to a fixed intermediate frequency - 455 kHz for AM and 10.7 MHz for FM - where selective amplification is easier.

Nyquist sampling theorem

To reconstruct a signal, the sampling rate must be at least twice its highest frequency component. PCM then quantises and encodes each sample into binary words.

8051 memory map

The classic 8051 has 4 KB internal program ROM, 128 bytes internal data RAM organised in four register banks, and 32 I/O lines on ports P0 to P3.

8051 machine cycle

On the classic 8051 one machine cycle equals 12 oscillator periods, so a 12 MHz crystal yields 1 µs per machine cycle, setting timer and instruction timing.

Cold solder joint

A dull, rounded joint formed when the iron is too cool or the surface is dirty. It has high resistance and intermittent contact; reheat with fresh rosin-core solder.

Polarised capacitor marking

On an electrolytic capacitor the stripe marks the negative terminal. Reversing polarity causes heating, bulging and possible rupture, so check before applying power.