Question

Determine the difference between two AOL(f) at 50Hz and 500Hz frequency? (Consider the op-amp to be 741c)

(a) 40dB

(b) 30dB

(c) 20dB

(d) 10dB

The question was posed to me by my college professor while I was bunking the class.

This intriguing question originated from Open-Loop Voltage Gain as a Function of Frequency topic in division Practical Limitations of Op Amp Circuits of Linear Integrated Circuits

Answer 1

The correct answer is (b) 30dB.

Here’s how we can determine the difference in open-loop voltage gain (AOL) of the 741C op-amp at 50Hz and 500Hz:

The 741C op-amp typically has a bandwidth (frequency response) of around 1 MHz at a typical open-loop voltage gain of 100,000 (100dB). However, as the frequency increases, the open-loop gain decreases in a monotonic manner, which is usually represented by a -20dB/decade roll-off in frequency.

Given the problem:

1. At 50Hz, the gain of the op-amp is quite high, around 100dB (or 100,000).
2. At 500Hz, the frequency is 10 times higher than 50Hz. So, based on the -20dB/decade roll-off, the gain at 500Hz will be 20dB lower than at 50Hz.

This gives us a difference of 30dB (100dB at 50Hz minus 70dB at 500Hz).

Thus, the correct difference in the open-loop gain at these two frequencies is 30dB.