vsrpkumar,
Impulse invariant method suffers from aliasing. Frequencies above the Nyquist frequency (1/2 the sampling frequency) get aliased. For exmaple with a Nyquist frequency of 1000Hz, a 1001 Hz signal would appear at the output as a 1 Hz signal.
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With the Bilinear transform, there is no aliasing. However, the equivalent digital frequencies are warped. The warping becomes very apparent at frequencies near the nyquist frequency and above. Filters design using the Bilinear z transform will have amplitude vs frequency characteristics that are superior to the equivalent analog prototype. However, the time and phase responses are significantly different from the corresponding analog prototype.
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Google on these topics and you will get lots of hits.
Regards,
Kral