I want to design a MOSFET amplifier circuit with pulse input. The current and frequency of the pulse is 2.5A and 1kHz respectively. How do I select the input gate capacitor for the MOSFET and what is the purpose of adding capacitor across gate? The input voltage for drain is 5V . How the voltage and frequency of the output is varied if the input capacitor and output capacitor value is varied?
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You don't add a capacitor across the input. Any reference to capacitance in the MOSFET data sheet is the effective capacitance that it presents to the driving circuit.
You might want to select a series input resistor for the gate of a few ohms to a few 10s of ohms to control the transient gate input current and the risetime/falltime of the drain current
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\$\begingroup\$ actually sir pulse input is given to gate so I think adding a capacitor smooth/filter the wave. but I don't know it's necessary or not \$\endgroup\$– sivaAug 22, 2016 at 10:25
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\$\begingroup\$ 'Smoothing the wave', if required, is done by adding a series resistor. Adding a capacitor is never necessary, and would make it more difficult to drive the FET. \$\endgroup\$– Neil_UKAug 22, 2016 at 11:23
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\$\begingroup\$ In this case it is not needed but in different applications it is sometimes needed to add an external CGS capacitor. In high power applications it is sometimes used to mitigate Crss feedback to gate voltage. Doing so improves type-II short circuit behaviour effectively reducing short circuit current. In normal operation extra capacitance is easily ruled out by stiffer gate driver. \$\endgroup\$– carlocAug 22, 2016 at 11:56