amplify 40kHz audio signal using TL082: first two stages are fine, but high noise from the third sta

[Ken Smith]

It depends on the amount of information you need to process. I was
thinking more along the lines of ultrasound as used in the marine or
medical world. There you often have to log a whole enchilada of echoes
in short sequence. The usual scenario is to use a burst of 2-3 cycles
followed by a long enough listening phase. The receiver bandwidth is
then matched to that burst.

Yes, depending on other factors, you need a wide bandwidth if you need to
resolve multiple targets.

In the marine case, some side scan sonars effectively end up with a narrow
band width. The data from several pulses are averaged together to improve
the SNR.

It also depends on how you define "wide band". The signal is all within
the same decade, in most such systems. There is usually as you said 2-3
cycles at least. In a few systems, the bandwidth really is wide and the
signal can't really be said to have cycles. There is more than one zero
crossing but the time between them increases as you go along.