6. Consider an op-amp where the inverting input voltage =3.7mv, non-inverting input voltage=6.25mv and open-loop voltage gain =142dB. Find the output voltage.
A. 0.21v
B. 0.45v
C. 0.78v
D. 0.36v
Answer: D
Open loop voltage gain, AoL(f) = Vo/Vid
VO = AOL(f) × (Vin1-Vin2)
= 142 dB×(6.25-3.7)
= 142×2.55 = 0.36v.
7. Express the open-loop gain of the op-amp in complex form?
A. A/√ [1+(f/fo)2
B. 20log{A/√[1+(f/fo)2}
C. A/[1+j(f/fo)].
D. None of the mentioned
Answer: C
The open-loop gain of the op-amp AOL(f) is a complex quantity and is expressed as
AOL(f) = A/[1+ j(f/fo)] .
The remaining equations are expressed in polar form.
8. Determine the difference between two AOL(f) at 50Hz and 500Hz frequencies? (Consider the op-amp to be 741C.
A. 40dB
B. 30dB
C. 20dB
D. 10dB
Answer: C
AOL(f) dB= 20log[√ [1+ (f/fo)2]
At f= 50 Hz,
AOL(f) dB = 20log(200000)- 20log(√(1+(50/5)2) = 106.02-20.04 ≅ 86dB
At f= 500Hz
AOL(f) dB =20log(200000)-20log(√(1+(500/5)2) = 106.02-40 ≅ 66dB
Therefore, the difference between AOL(f)dB = 86-66 = 20dB.
9. 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/fo)
= -tan-1(0/5) = 0o
10. At what frequency AOL(f)=A?
A. 50Hz
B. 10Hz
C. 5Hz
D. 0Hz
Answer: D
For any frequency less than break frequency (fo =5Hz) the gain is approximately constant and is equal to A.
