raja.cedt
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hello haykp,
Let me explain what is small signal to understand this bit clearly.
Small signal means how your ckt parameters are changing with applied signal(don't confuse with the ac or dc, it's just change in the bias forget about the frequency for now ). So for example take a transistor with 100uA bias current for .5v vgs, now if you change vgs by 1mv then your current changes by 10uA, so in small signal model we only represent changes (not fixed quantities), now if you draw small signal model vgs and ids would be 1mV and 10uA. Think about an ideal voltage source, no change in the voltage hence short ckt, if it has some internal resistance, instead of short ckt, add resister from that node to gnd and same applicable to current source.
NOW Coming to your last post, we generally keep big decap between supply and gnd to support large amounts of switching currents. This acts as ideal voltage source, so you can represent it by short ckt.
SORRY FOR THE BIG POST.
Thanks, raj.
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