N-Channel Enhancement MOSFET as amplifier: How can I calculate the voltage after amplified?

Thread Starter

Walt Peter

Joined Mar 18, 2016
Hello. I wish to create an amplifier. The concept is as below.


I heard that Transistor NPNs are useful in amplify, but I find out that it cannot be used in this project as the Transistors will damage. So, I found out another type of Transistor - the N-Channel Enhancement MOSFET. We know that MOSFET can act as an amplifier, but how? Does it more likely to be a "resistant controller" that limit the flow of current in another circuit? Does it really increase the voltage? If it does, how can I protect MOSFET in a circuit? (Sorry for so many questions. I just getting confusing.)

I designed a circuit as picture below:


In the picture, I decided to use the TO-220 AB MOSFET. Does it suitable? If I run this circuit:

How much the voltage is received by the DC Motor ?(Or flowing in the circuit something like that)
How can I calculate the amplified voltage? (Can I simply sum up both voltage at Gate and Drain to get the amplified voltage?)

Thank you.


Joined Jan 18, 2008
That circuit will not work.

With an N-channel mosfet, the simplest design has the source connected to ground (i.e., negative in your drawing) and the load (in your case the motor) goes between the drain and the positive supply.

The reason is that as the mosfet conducts, the voltage drop across is is very low. Thus, when conductiong, the source voltage is approximately the same as the drain voltage. BUT, you need the gate to be grater than the source, which in your drawing is the same or less than the drain voltage.



Joined Sep 17, 2013
If your power supply is 9V you can't boost that to 12V using just a transistor. You would need to make/buy a DC-to-DC boost converter, which is much more complex.


Joined Nov 23, 2012
If you are starting with a 9V battery that is intended to supply about 100 mA or less. Then that battery can supply 0.9 watts. If you use a DC/DC converter, then you can boost to 12 V and current will be less to make sure you do not break any laws of physics and exceed your input power of 0.9w. Also, the DC/DC converter will only be 80 to 90% efficient and that brings you down (seemingly in several ways) to 0.75 to 0.8 watts max (about 65 mA). Not much for a motor - hopefully it still works out for you.