We're analysing in class common source, common gate and common drain amplifiers and the analysis of resistive load and current load is being done but I don't get the purpose of doing this... how does it help us to get the behaviour of the amplifier.
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\$\begingroup\$ I'm not sure that there is a better single way to understand the behaviour of a FET. Maybe try and state what you hoped might be achieved from this analysis and maybe someone can help? \$\endgroup\$– Andy akaCommented Oct 17, 2015 at 21:23
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The amplifier's performance - specifically gain - is a function of the load impedance, in different ways for each configuration, and is always a maximum into an infinite load impedance (i.e. a current source). Gain will be lower with a load resistance, depending on that load.
Seeing how each topology differs in this respect will let you understand practical circuits, and choose the best configuration for any purpose, such as to drive a specific load.
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\$\begingroup\$ The above explanation is true because transistors act as CURRENT SOURCES and, thus, develop an output voltage across (and proportional to) the load resistance. \$\endgroup\$– LvWCommented Oct 18, 2015 at 8:28