MOSFET as amplifier: Can voltage after amplified can be calculated?

[Walt Peter]

Hello. I am going to start my first project. I now wants to use a N Channel Enhancement MOSFET, named TO-220 AB to as an amplifier. The bellow is a circuit I created a moment ago.

The main voltage input is 9 Volts and there is another input I used as a controller of voltage in the circuit, which is 1~9 Volts. If, I run this circuit:

How can I know the voltage received by the DC motor (after the original voltage is amplified) ?
OR
How can I calculate the voltage after amplified? Can I simply sum up the voltage at Gate and Drain and I will get it?

Thank you.

[Zero999]

You have a source follower there which has no voltage gain. If anything it has voltage loss. When the input is 9V, the voltage across the motor will be the MOSFET's threshold voltage less.

[alsetalokin4017]

@Walt Peter:

The "TO220AB" refers to the mosfet's package style, it is NOT a mosfet part number. There are many many different mosfets available in the TO220AB package, and they will all have different relationships between the applied Gate voltage and the Drain-Source resistance. If you know the _part number_ of the actual mosfet you want to use, you can then look at the Data Sheet for that part number and you will usually find typical graphs which will show you the relationship for your particular part.

However, mosfets are not usually used in the way you seem to be intending. Usually we would expect a mosfet to be fully OFF or fully ON, not in-between, because mosfets operated in the "linear region" (partially ON, determined by Gate voltage)  will waste a lot of power by heating up. Also the "linear region" is typically a very small range of Gate voltage. Mosfets function differently than bipolar junction transistors in this way.

If you are trying to control a motor's speed with a mosfet, you should use a Pulse-Width Modulation scheme, which turns the mosfet fully ON and OFF but in brief pulses. The width of the pulses are varied by the circuit in order to control the motor's speed.