When is a MOSFET considered a voltage controlled resistor?
All transistors are voltage-controlled resistors. Trans istor is a kind of compilatio, transient resistance I think they were originally called, and the phenomenon they display is called transistance.
the mosfet is a three-terminal device. The basic idea behind using the MOSFET as a voltage-controlled resistor is to control the width of the conductive channel (resistor) between two terminals by modulating the voltage at the third terminal.
most often, the resistance of the drain to the source is modulated by the gate voltage, vgs.
when it works in the active part of its operating range. for example, when it is used in the most linear part of this range for the amplification of analog signals. The disadvantage of this type of operation is that it can become very hot – it must absolutely be mounted correctly on a heat sink of the appropriate size.
A mosfet will behave much more like a switch if it is operated only at the ends of its operating region: cutoff or saturation. at break, it looks a lot like an open switch; at saturation, it looks a lot like a closed switch. By switching the power supplies (and certain types of amplifiers), the mosfet will have to cut or saturate most of the time, spending as little time as possible in the active region. (It will still spend a little time there, because it has a non-zero rise time and a non-zero fall time for saturation and saturation transitions), the unit will work much colder and can thus control much more power going to the load if it is used more like a switch than a resistance. the switching frequencies are generally quite high (to reduce the size of the inductors and transformers elsewhere in the circuit), but the higher the frequency, the higher the percentage of time spent by the device in its active region (when it behaves as a resistance) is high. go up and down transitions.