Question

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

I got this question in my homework.

This interesting question is from Open-Loop Voltage Gain as a Function of Frequency in section Practical Limitations of Op Amp Circuits of Linear Integrated Circuits

Answer 1

The false statement is (d) At unity gain crossover frequency, the open loop gain AOL(f) dB is zero.

This is NOT true. At unity gain crossover frequency (often called gain-bandwidth product frequency), the loop gain is actually equal to 1 (or 0 dB), but this does not mean the open-loop gain is zero. The open-loop gain at the unity gain crossover frequency is approximately equal to the closed-loop gain of the configuration being used.

The correct statements are:

(a) True - The open loop gain is approximately constant from 0Hz to break frequency fo

(b) True - -3dB frequency is the break frequency

(c) True - Open loop gain increases 20dB per decade after break frequency

(d) False - The open loop gain at unity crossover is NOT zero