Amplitude Modulation Carrier and Freq.

Why do we take the carrier frequency greater than modulating signal frequency in amplitude modulation?
Explain it in terms of overlapping of sidebands?

I don't want to do your homework for you since you won't learn anything that way. However, there are two ways to look at this that should show you problems that come up if the AM modulating signal approaches or even exceeds the carrier in frequency.

First, think of the signal in the time domain. In the normal case you have a bunch of carrier cycles that increase and decrease in amplitude. There are many carrier cycles in each rise or dip of the modulating signal. Simple circuitry can get away with looking only at the peaks of successive waves to perform demodulation.

Now think what this looks like when the modulation signal is faster than the carrier. You'd have to look at the carrier at multiple points in the wave. At each point you divide what you get by the instantaneous expected carrier signal at that point. This is not only a lot more complicated since it requires tracking the phase of the carrier and doing a divide, but it also falls apart as the carrier crosses 0. What is the modulation signal level at the carier zero crossing? You can't tell because all possible modulation levels result in the same carrier level at that point. This isn't a formal mathematical proof, but should give you a hint that information is lost twice per carrier cycle, and this means there is going to be a practical upper limit on the modulation frequency.

Another way to look at this is in frequency space. Plot the frequency content of the AM waveform with linear frequency on the X axis and amplitude on the Y. Pure carrier (no modulation) is a spike at the carrier frequency with zero elsewhere. Now think what the sidebands look like up to a maximum modulation bandwidth. Surely there is such a plot already in one of your textbooks. It's a pretty classic plot. Now think what happens as the modulation frequency exceeds the carrier frequency. What happens to the low sideband in particular?

sir plz explain ur 2nd example fully.in that example LSB will be at -ve freq. means it won't exist practically but what does this overlapping of sidebands mean here?how it happens & why it occur?
plz clear me this concept fully...

There is no need to use texting abbreviations here. There is no cost per character, it's annoying to read, and abbreviations in general accross languages often don't work well.

It would also be helpful if you gave some background on yourself. What university are you attending? What is your major? What year are you in? What is your general electronics background? What class is this assignment from?

By my second example, you mean picturing the modulated carrier in frequency space? If so, what did you find when you looked that up? Again, I'm not going to do all your homework for you, but may be willing to help you understand it if you do your part.

For starters, draw a plot showing the spectrum of a 1MHz carrier modulated with a arbitrary signal that can have frequencies from 20-100 kHz. Now draw the same plot but with the modulation frequency ranging from 20 KHz to 1.1 MHz. Show both these plots here so we have something further to talk about.