The diagram above shows a very simple MOSFET amplifier. T1 is an depletion N-channel MOSFET. Assume its datasheet tells us that the V_GS threshold voltage is -2V, and that y_FS=10mA/V at V_GS=0V. If R1=470Ω, the gain will be 10mA/V∙470Ω = 4.7.