Why do we limit FET operation to small signals?
The materials used in the construction of the transistor layers are not designed for high amperage, voltages or frequencies. line currents would degrade or blow beyond the semiconductor. signals from a transistor can drive relays and contactors to relieve heavy loads.
Motors are not limited to a weak signal, but are widely used in analog and digital power switching circuits. Fet technology is cheaper and consumes less energy, making it better for manufacturing. Compared to bjt, their limit is usually related to their low analog and radio frequency (RF) gain and frequency responses, but this has also improved over time by reducing the length of the grid to deep sub-microns.
Many fets are limited to small signals when used as linear (analog) amplifiers or as logic gates in a computer. but there are also mosfets of power.
Speed is the essence of any power switch. The control circuit of an electric mosfet must therefore be carefully designed. you want to go as fast as possible between on and off, as an ideal switch does not dissipate any power in either state. but if the switch (any switch) remains in an intermediate state for a given period of time, its losses can quickly destroy it.
the main competitor of the power mosfet is the igbt, the insulated gate bipolar transistor. it is actually a hybrid: a fet drives a bipolar transistor.
you limit fet operation to small signals only when you use fets to build amplifiers or amp blocks. in this case, the fonts are biased in the saturation operating region where they behave approximately as linear amplifiers. the greater the amplitude (of voltage or current) of the amplified signals, the less the amplified signals are deformed. usually, a kind of feedback architecture is also used in the amplifiers to improve the linearity.
On the other hand, if the fets are used to build logical or digital circuits – for example, gates, flip-flops, counters, microprocessors implemented with mosfets, usually using the cmos circuit family – then the fets work also in the break and in the resistive operating regions, where the fet behavior is highly nonlinear.