This set of Linear Integrated Circuit Questions and Answers for Aptitude test focuses on “Open-Loop Voltage Gain as a Function of Frequency – 2”.
1. Which of these statements is false?
a) The open loop gain AOL(f) dB is approximately constant from 0Hz to fo
b) When input signal frequency and f is equal to break frequency fo, the gain frequency is called -3dB frequency
c) The open loop gain AOL(f) dB is approximately constant upto break frequency fo, but there after it increase 20dB each time there is a tenfold increase in frequency.
d) At unity gain crossover frequency, the open loop gain AOL(f) dB is zero
Explanation: When AOL(f) is approximately constant up to break frequency, there will be 20dB decrease each time there is a tenfold increase in frequency. Therefore it may be considered that the gain roll of at the rate of 20dB/decade.
2. What is the maximum phase shift that can occur in an op-amp with single capacitor?
Explanation: At corner frequency, the phase angle is -45o (lagging) and at infinite frequency the phase angle is -90o. Therefore, a maximum of 90 o phase change can occur in an op-amp with a single capacitor.
3. How can the gain roll off represented in dB/octave?
a) 12 dB/octave
b) 6 dB/octave
c) 10 dB/octave
d) 8 dB/octave
Explanation: Octave represents a two fold increase in frequency. Therefore, 20 gain roll off at the rate 20 dB/decade is equivalent to 6 dB/octave.
4. Select the correct magnitude and phase for the frequency range.
| 1. f<
||i. Gain is 3dB down from the value of AOL in dB|
|2. f=f1||ii. Gain roll off at the rate of 20 dB/decade|
|3. f>>f1||iii. Magnitude of the gain is 20logxAOL in dB|
a) 1-iii, 2-i, 3-ii
b) 1-I, 2-ii, 3-iii
c) 1-iii, 2-ii, 3-i
d) 1-ii, 2-iii, 3-i
Explanation: Properties of magnitude and phase angle characteristics equations.
5. The specific frequency at which AOL (dB) is called
a) Gain bandwidth product
b) Closed loop bandwidth
c) Small signal bandwidth
d) All of the mentioned
Explanation: AOL (dB) is zero at some specific value of input signal frequency called unity gain band width. The mentioned terms are the equivalent terms for unity gain band width.
6. Find out the expression for open loop gain magnitude with three break frequency?
a) A/[1+ j(f/fo1)]x[1+ j(f/fo2)]x[1+ j(f/fo3)].
b) A/[1+ j(f/fo)]3
c) A/A/[1+ j(f/fo1)]+ [1+ j(f/fo2)]+ [1+ j(f/fo3)].
d) All of the mentioned.
Explanation: The gain equation for op-amp is AOL(f) = A/[1+ j(f/fo1)]x[1+ j(f/fo2)]x[1+ j(f/fo3)].
7. Find out the frequencies that are avoided in frequency response plot?
a) None of the mentioned
b) Single break frequency
c) Upper break frequency
d) Lower break frequency
Explanation: Op-amps have more than one break frequency because only a few capacitors are present. Often upper break frequencies are well above the unity gain bandwidth. So, they are avoided in frequency response plot.
8. In a phase response curve of MC1556, the phase shift is -162.5o about 4MHz. If the approximate value of first break frequency is 5.5Hz. Determine the approximate value of the second break frequency?
Explanation: The phase angle equation is φ(f) = – tan-1 (f/fo1)- tan-1(f/fo2)
=> -162.5o = – tan-1(3MHz/5.5) – tan-1(3MHz/fo2)
=> tan-1(3MHz/fo2) = +162.5o-89.99o
=> 3MHz/fo2= tan(72.50o)
=> fo2 = 3MHz/tan(72.50o)=945.89kHz.
9. Consider a practical op-amp having two break frequency due to a number of RC pole pairs. Determine the gain equation using the given specifications:
A ≅ 102.92dB; fo1 =6Hz ; fo2 =2.34MHz .
a) 140000/[1+ j(f/6)]x[1+ j(f/2.34)].
b) 140000/[1+ j(f/6)]x[1+ j(f/234)].
c) 140000/[1+ j(f/6)]x[1+ j(f/23400)].
d) 140000/[1+ j(f/6)]x[1+ j(f/2340000)].
Explanation: Gain of op-amp at 0Hz, A = 10102.92/20
=> A = 105.146 =139958.73 ≅ 140000.
The gain equation with two break frequency value is given as
AOL(f) = A/[1+ j(f/fo1)]x[1+ j(f/fo2)] =140000/[1+ j(f/6)]x[1+ j(f/2.34)].
10. What is the voltage transfer function of op-amp in s-domain.
d) None of the mentioned
Explanation: For the voltage transfer function in s-domain is expressed as AOL(f) = A/[1+ j(f/fo)] =A/[1+j(ω/ωo)]= A×ωo/(jω/jωo) = A×ωo / (s+ωo).
11. Calculate the unity gain bandwidth of an op-amp. Given, A≅ 0.2×106 & fo =5Hz?
Explanation: The break frequency, fo = UGB/A
=> =fo×A = 0.2×106 × 5 = 106.
Sanfoundry Global Education & Learning Series – Linear Integrated Circuits.
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