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Analog Electronics – Problems Page 25

Very Hard Numerical Problems (GATE Level)

This section contains advanced numerical problems combining diode circuits, BJT amplifiers, operational amplifiers and oscillators.

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Problem 1

A silicon diode has reverse saturation current Is = 10⁻¹² A. Calculate diode current when forward voltage is 0.7 V at room temperature.

Solution

Diode equation: I = Is (e^(V/VT) − 1)

At room temperature:

VT ≈ 25 mV

Substitute values:

I = 10⁻¹² (e^(0.7/0.025) − 1)

I ≈ 1.2 mA

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Problem 2

A BJT amplifier has Rc = 4 kΩ and re = 25 Ω. Find the voltage gain of the CE amplifier.

Solution

Av = −Rc / re

Av = −4000 / 25 Av = −160

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Problem 3

An inverting op-amp has Rin = 5 kΩ and Rf = 50 kΩ. If input voltage is 0.1 V, calculate output voltage.

Solution

Av = −Rf / Rin

Av = −50k / 5k = −10

Vo = Av × Vin Vo = −10 × 0.1 = −1 V

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Problem 4

An LC oscillator uses L = 2 mH and C = 200 pF. Find oscillation frequency.

Solution

f = 1 / (2π√LC)

f ≈ 251 kHz

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Problem 5

An op-amp differential amplifier has Ad = 2000 and Ac = 0.5. Calculate CMRR in dB.

Solution

CMRR = Ad / Ac

CMRR = 2000 / 0.5 = 4000

CMRR(dB) = 20 log(4000)

CMRR ≈ 72 dB

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Problem 6

A Wien bridge oscillator uses R = 10 kΩ and C = 0.01 μF. Find oscillation frequency.

Solution

f = 1 / (2πRC)

f ≈ 1591 Hz