ive designed a LNA for 0.1G to 3G, i havent added input couplign capacitor and the simulations results are just fine, the problem is when i add input coupling cap, the gain becomes negative and all the results deteriote. i wanted to know in this case can i use an external coupling capacitor which will not be considered in simulations? and also is it necessary to add coupling capacitor inthe schematic and layout. can we just ignore the coupling capacitor?

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Just use a perfect capacitor of say 100uF or larger, with no parasitic inductance included. This should be good down to around 100MHz (6 Ohms reactance in series).
If the original value of capacitor you used was too small then that might explain the negative gain you were seeing.
What simulator are you using?