How is FET a voltage controlled device?
the voltage applied between the gate and the source controls the drain current (id). which means you use the voltage to control the output current. therefore, fet is a voltage controlled device.
just explained because it consumes very little power to operate. it relies on the voltage differential to create a field effect necessary for its operation (hence the field effect transistor name). the amplitude of the voltage differential dictates the operating speed of the device
a fet is controlled by a voltage at the terminal of the gate and, because of the very high grid impedance, flows in or out of the gate. compare to a bipolar transistor in the active state which is controlled by the current flowing from the base to the emitter, which allows to obtain a collector-emitter current proportional to the base-emitter current.
a fet is a voltage-controlled device because the current between the drain and the source is controlled by the voltage at the gate with reference to the source (vgs).
This contrasts with the current controlled bjt because the current flow between the collector and the emitter is controlled by the current flowing through the base.
why is a FET voltage controlled device?
The most common type of fet is a type of insulated door. since the door is isolated from the drain and the source, no current can cross it (or very little, in practice). the fet has no choice: it can not be controlled by the current, it must be controlled by the electrostatic field created by the voltage on its grid. (hence its name: field effect transistor.)
or something like that!
To be short and simple:
You vary the voltage at the terminal, you get a constant current at the output. this is because the gate voltage is used to control channel resistance or pinch condition.
While a transistor is a current-controlled device because the base current is used to control the output collector current (taking into account the configuration of the common emitter).
in practical cases, the mosfet is better than the fet (the effects of ionization in the fet make it unfit for use in practical circuits). that’s why we’ve installed a layer of metal oxide on the fet to improve its applications.
For more detailed explanations, study the output response graphs of fet and compare them with those of mosfet.
jfet is a voltage-controlled device because the input signal is applied to the reverse-biased pn junction (source gate), which allows only a leakage current (in nano amps). the input bias current is negligible, the voltage having a dominant effect (very high impedance of 100 megohms). the output current flows from the source to the drain; the variation of the gate voltage controls the drain current. Instrumentation amplifiers
jfet are used to amplify thermocouple signals in the microvolt range since virtually no bias current is drawn from the sensor.
by comparing bjt, it uses a live-biased pn junction inrush current in a range from micro to milliamps, resulting in low input impedance. Here, the change of the base current controls the greatest variation of the current of the collector. therefore, bjt is a current controlled apparatus.
it can be noted that in the two amplifiers bjt jfet, the voltage signals are given as input.