Analog Electronics – Problems Page 27
Ultra Hard Multi-Concept Problems (GATE Level)
This section contains multi-concept problems combining diode circuits, transistor amplifiers, op-amp circuits and oscillators.
Problem 1
A silicon diode conducts 2 mA current when forward voltage is 0.7 V. Find the dynamic resistance at this operating point.
Solution
rd = VT / I
VT = 25 mV
rd = 0.025 / 0.002
rd = 12.5 Ω
Problem 2
A BJT transistor has β = 100 and collector current IC = 2 mA. Find the transconductance gm.
Solution
gm = IC / VT
gm = 0.002 / 0.025
gm = 0.08 S
Problem 3
An op-amp inverting amplifier has Rin = 2 kΩ and Rf = 20 kΩ. If Vin = 0.2 V, calculate output voltage.
Solution
Av = −Rf / Rin
Av = −20k / 2k = −10
Vo = Av × Vin
Vo = −10 × 0.2 = −2 V
Problem 4
A Hartley oscillator uses L1 = 3 mH, L2 = 7 mH and C = 150 pF. Find oscillation frequency.
Solution
Total inductance
L = L1 + L2 = 10 mH
f = 1 / (2π√LC)
f ≈ 130 kHz
Problem 5
An op-amp differential amplifier has Ad = 3000 and Ac = 0.3. Find CMRR in dB.
Solution
CMRR = Ad / Ac
CMRR = 3000 / 0.3 = 10000
CMRR(dB) = 20 log (10000)
CMRR = 80 dB
Problem 6
An RC oscillator uses R = 8 kΩ and C = 0.02 μF. Find oscillation frequency.
Solution
f = 1 / (2πRC)
f ≈ 995 Hz
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