Kerala Plus Two Electronics (DHSE) flashcards

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

Mass-action law

In a semiconductor at thermal equilibrium the product of electron and hole concentrations is constant: n·p = n_i². For silicon near 300 K, n_i is about 1.5 × 10^10 cm^-3.

Barrier potential of silicon

The forward voltage at which a silicon p-n junction begins conducting appreciably, about 0.7 V. Germanium junctions start near 0.3 V, so they conduct at lower supply voltages.

Transistor beta relation

Beta is the DC current gain I_C/I_B, related to alpha by β = α/(1 − α). Alpha is I_C/I_E and is always slightly less than one in a BJT.

Zener diode operation

A heavily doped diode designed to operate in controlled reverse breakdown. Biased in reverse, it holds a nearly constant voltage V_Z across its terminals, so it can regulate a supply.

Half-wave rectifier output

A single diode passes one half cycle, giving DC output V_m/π (about 0.318 V_m) and ripple at the supply frequency. Ripple factor is about 1.21, the poorest of the rectifiers.

Bridge rectifier peak inverse voltage

In a bridge rectifier each diode blocks a peak inverse voltage of about V_m, half that of a centre-tap full-wave rectifier. Two diodes conduct in series per half cycle.

Capacitor filter ripple formula

Ripple voltage is approximately I_dc/(f_r·C), where f_r is the ripple frequency. Larger capacitance or a higher ripple frequency (full-wave) reduces the ripple amplitude.

Three-terminal regulator 7805

A fixed positive voltage regulator IC giving +5 V output. The 78xx family provides positive outputs and the 79xx family negative ones; both need input and output decoupling capacitors.

Common-emitter voltage gain

Gain magnitude is (R_C ∥ R_L)/r_e, where r_e ≈ 26 mV/I_E. The output is 180° out of phase with the input, so the gain is written with a negative sign.

Barkhausen criterion

An oscillator needs loop gain Aβ equal to one and a total loop phase shift of 0° or 360°. The amplifier and feedback network together supply the required phase.

RC phase-shift oscillator frequency

For three identical RC sections, oscillation occurs at f = 1/(2πRC√6). Each section contributes 60°, giving 180° from the network plus 180° from the CE amplifier.

Class B efficiency

A push-pull Class B stage conducts for 180° per device and can reach about 78.5% maximum theoretical efficiency. Class A is far lower, about 25% with a resistive load.

Negative feedback effects

Feedback factor (1 + Aβ) reduces gain but widens bandwidth, lowers distortion and noise, raises input impedance and lowers output impedance. The trade-off is reduced overall gain.

De Morgan's theorems

The complement of a product equals the sum of the complements, and the complement of a sum equals the product of the complements: (A·B)′ = A′ + B′ and (A + B)′ = A′·B′.

Universal gates

NAND and NOR are called universal because either gate alone can realise every Boolean function, including NOT, AND and OR. This simplifies IC inventory in digital design.

Full adder outputs

A full adder adds three bits A, B and C_in. Sum = A ⊕ B ⊕ C_in and carry out = AB + C_in(A ⊕ B); it is built from two half adders and an OR gate.

JK flip-flop toggle

When both J and K inputs are high, the JK flip-flop toggles its output on each active clock edge. This removes the invalid state that affects the simple SR flip-flop.

AM modulation index

Defined as m = (V_max − V_min)/(V_max + V_min) for the envelope. Values above one cause overmodulation, producing envelope distortion and splatter in adjacent channels.

Carson's rule

The bandwidth of an FM signal is approximately 2(Δf + f_m), where Δf is peak deviation and f_m is the highest modulating frequency. Wideband FM therefore needs more bandwidth than AM.

Nyquist sampling theorem

A band-limited signal with highest frequency f_max must be sampled at f_s ≥ 2f_max. Sampling below this rate causes aliasing, where high frequencies appear as false low frequencies.

AM total power

Total transmitted AM power is P_c(1 + m²/2). At full modulation (m = 1) the two sidebands together carry only one-third of the total power.

8051 machine cycle

One machine cycle equals 12 oscillator periods. With a 12 MHz crystal, one machine cycle lasts 1 µs, which sets timer counts and instruction timing calculations.

8051 interrupt vectors

Reset starts at 0000H. External interrupt 0 uses 0003H, timer 0 uses 000BH, external interrupt 1 uses 0013H, timer 1 uses 001BH and the serial port uses 0023H.

Microprocessor versus microcontroller

A microprocessor is only a CPU and needs external memory and peripherals. A microcontroller integrates CPU, RAM, ROM, timers and I/O ports on one chip for dedicated control.

8051 register banks

The 8051 has 128 bytes of internal RAM, the first 32 of which form four register banks of eight registers. Bank selection is set by two bits in the PSW.

Shunt for ammeter

A low resistance placed in parallel with a galvanometer converts it to an ammeter. Its value is R_sh = I_g·R_g/(I − I_g), where I is the required full-scale current.

CRO frequency measurement

Read the period as horizontal divisions multiplied by the time/div setting, then take the reciprocal. Frequency in hertz equals one divided by the period in seconds.

Lissajous circle figure

A circle on a CRO with signals on X and Y plates indicates equal frequencies with a 90° phase difference. A straight diagonal line indicates equal frequencies and zero phase shift.

Voltmeter loading effect

A voltmeter of finite resistance draws current and lowers the voltage it measures in high-resistance circuits. High-impedance meters reduce this error to negligible levels.

Resistor colour code

Bands read black 0 through white 9, with the third band as a multiplier. Gold and silver indicate tolerances of ±5% and ±10% respectively.

Capacitor code 104

The first two digits give the value and the third the number of zeros in picofarads, so 104 means 10 × 10^4 pF, equal to 100 nF or 0.1 µF.

Diode test with a DMM

In diode-test mode a good silicon diode shows about 0.5 to 0.7 V forward with the red probe on the anode, and reads open with the probes reversed. A short reads near zero both ways.