FOCUS ON FIELD EFFECT TRANSISTORS.

Field-effect transistors often outperform bipolar devices, but they usually cost more. In amplifiers, FETs are primarily used when high input impedances are needed (over 10//1//3 ohms is possible) and in applications where input leakage current must be minimized (less than 500 pa is easily attained). Essentially, bipolar transistors are current amplifiers whereas FETs are true transconductance (voltage in, current out) amplifiers. Bipolar transistors have superior offset-voltage drift while FETs are better for leakage-current drift with temperature variations. However FETs can be easily biased to the zero temperature coefficient point, whereas this is much more difficult - and expensive - with bipolar transistors.