Why is it ideal for an amplifier to have large input impedance?Doesn't that imply that very less current is allowed at the input.Lesser the input is lesser the o/p will be.

 

There is no relationship between an amplifier's input impedance and it's output.
High input impedance is desireable so the amplifier does not load the preamplifier's output.

 

Because you're amplifying the voltage of the input. A higher input impedance will reduce the current driving requirements of the circuit doing the driving.

In lower frequency and DC circuits it's to your advantage to have a high input impedance, less strain on the preceeding stage and often higher accuracy however in higher frequency, especially RF circuits, both the input and output impedances should be the same due to the potential creation of standing waves. You'll learn that later on - I hope - we don't have enough truly educated RF engineers at the present and anyone that gets really good at RF is going to be in high demand when it comes to designing high speed computer circuitry - especially when it comes to PC board layouts.