41. Express the open-loop gain of the op-amp in complex form?

A. A/√ [1+(f/f_{o})^{2}
B. 20log{A/√[1+(f/f_{o})^{2}}
C. A/[1+j(f/f_{o})].
D. None of the mentioned

Answer: C

The open-loop gain of the op-amp A_{OL}(f) is a complex quantity and is expressed as

A_{OL}(f) = A/[1+ j(f/f_{o})] .

The remaining equations are expressed in polar form.

42. Determine the difference between two A_{OL}(f) at 50Hz and 500Hz frequencies? (Consider the op-amp to be 741C.

A. 40dB
B. 30dB
C. 20dB
D. 10dB

Answer: C

A_{OL}(f) dB= 20log[√ [1+ (f/f_{o})^{2}]

At f= 50 Hz,

A_{OL}(f) dB = 20log(200000)- 20log(√(1+(50/5)^{2}) = 106.02-20.04 ≅ 86dB

At f= 500Hz

A_{OL}(f) dB =20log(200000)-20log(√(1+(500/5)^{2}) = 106.02-40 ≅ 66dB

Therefore, the difference between A_{OL}(f)dB = 86-66 = 20dB.

43. At what frequency, the phase shift between input &output voltage will be zero?

A. -40Hz
B. 0Hz
C. -22Hz
D. 20Hz

Answer: B

At 0Hz the phase shift between input and output voltage is zero.

At f=0Hz

φ(f) = – tan^{-1} (f/f_{o})

= -tan^{-1}(0/5) = 0^{o}

44. At what frequency A_{OL}(f)=A?

A. 50Hz
B. 10Hz
C. 5Hz
D. 0Hz

Answer: D

For any frequency less than break frequency (f_{o} =5Hz) the gain is approximately constant and is equal to A.

For example, f_{o} =0Hz,

Then A_{OL}(f) dB= 20log(200000-20log[√1+(0/5)^{2})]

= 106dB.

Where A =20,000 ≅ 106dB.

45. What happens when the frequency increases?

A. A_{OL}(f) continues to drop
B. A increases
C. f_{o} –> 0Hz
D. None of the mentioned

Answer: A

The open-loop voltage gain as a function of frequency is given as

A_{OL}(f) = A/ [√ 1+ (f/f_{o})] A

t a frequency above f_{o}, the denominator value increases, causing the gain, A_{OL}(f) to decrease. Thus, as frequency increases, the gain A_{OL}(f) continues to drop.

46. What will be the absolute value of phase shift, if the frequency keeps increasing?

A. Increase towards 45^{o}
B. Decrease towards 45^{o}
C. Increase towards 90^{o}
D. Decrease towards 90^{o}

Answer: C

For any frequency above break frequency, the absolute value of phase shift increases towards 90^{o} with an increase in frequency.

47. Which of these statements is false?

A. The open-loop gain A_{OL}(f) dB is approximately constant from 0Hz to f_{o}

B. When input signal frequency and f is equal to break frequency f_{o}, the gain frequency is called -3dB frequency

C. The open-loop gain A_{OL}(f) dB is approximately constant upto break frequency f_{o}, but thereafter it increases 20dB each time there is a tenfold increase in frequency.

D. At unity gain crossover frequency, the open-loop gain A_{OL}(f) dB is zero

Answer: C

When A_{OL}(f) is approximately constant up to break frequency, there will be a 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.

48. What is the maximum phase shift that can occur in an op-amp with a single capacitor?

A. 180^{o}
B. 60^{o}
C. 270^{o}
D. 90^{o}

Answer: D

At corner frequency, the phase angle is -45^{o} (lagging) and at the infinite frequency, the phase angle is -90^{o}. Therefore, a maximum of 90 ^{o} phase change can occur in an op-amp with a single capacitor.

49. How can the gain roll-off be represented in dB/octave?

A. 12 dB/octave
B. 6 dB/octave
C. 10 dB/octave
D. 8 dB/octave

Answer: B

Octave represents a two-fold increase in frequency. Therefore, 20 gain roll-off at the rate of 20 dB/decade is equivalent to 6 dB/octave.

50. Select the correct magnitude and phase for the frequency range.

List-I

List-II

1. f1

i. Gain is 3dB down from the value of A_{OL} in dB

2. f=f_{1}

ii. Gain roll-off at the rate of 20 dB/decade

3. f>>f_{1}

iii. Magnitude of the gain is 20logxA_{OL} 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

Answer: A

Properties of magnitude and phase angle characteristics equations.