Why is MOSFET Voltage and BJT Current controlled?
In the bjt, the output collector current of the transistor is proportional to the input current entering the base terminal of the device, making it a current-controlled device.
The fet uses the voltage applied to its input terminal (gate) to control the current flowing through it, the output current being proportional to the input voltage. the control of the current flowing in this channel is obtained by varying the voltage applied to the gate. That’s why it’s called voltage controlled device.
in a mosfet, the door is isolated from the source and the drain by a very great resistance. therefore, there is no power in the door. therefore, it must be a voltage controlled device. in particular, the resistance of the channel is a function of the gate-to-source voltage.
Incidentally, this strong resistance inside the door makes the loading of an unpleasant capacitor, hence the occasional need for mosfet pilots. However, you also essentially get an open circuit for a load for the source driving the mosfets gate.
in a bjt, the base is connected to the transmitter via an internal resistor. the resistance is in the low ohms. therefore, there is a way of circulating the current. therefore, the bjt is a current controlled apparatus.
specifically, the current flowing through the device (collector to emitter – assuming an npn) is a function of the current in the base. and, since the current flows through the apparatus, and some things, like the mosfets, flow at the source, you have a resistor, a collector at the transmitter.
Since there is a low-value resistor that looks into the base of a transistor, you end up with a rather low input impedance, which can be increased by increasing the resistance of the transmitter.
Another thing i think neato about mosfets is, since it’s a real resistance between the drain and the source and not just a current through a voltage drop, you can literally measure a resistance to a mosfets via an ohmmeter connected ds or sd.
simply apply a voltage on the conductors g-s then measure the resistance between d and s. therefore, analog switches are more feasible with a mosfet.
