why we make gate resiance high when we turn on fet and small when we turn off

 

Who told you the gate resistance was small?

read this and see if some of your questions can be answered

http://www.rason.org/Projects/jfetamp/jfetamp.htm

 

thanks very much

 

If you are referring to a gate resistor with e.g. an anti-parallel diode, this is to change the switching behaviour of the FET, especially important in power MOSFETs. Heat dissipation of the FET, the driver and noise generated by the transistor's switching have to be considered when designing gate drives.

There are many things to consider and I recommend you read http://focus.ti.com/lit/ml/slup169/slup169.pdf
which will give ALL the answers you want (and all you don't want, too)

 

thanks very very much

 

please i need to learn pic microcontroler so what is the easy and fast way to learn it