# MOSFET Power Amplifier Coursework help

#### Ryth

Joined Mar 28, 2018
4
Hey, I am an A2 Electronics student and need to prove that my MOSFET Power Amplifier is functional, image is attached.
I was told to use the P=V^2/R formula to prove that the power at the input is greater than that of the output.
I currently have no clue on how to use this formula to prove this, any help would be greatly appreciated.
I can give additional information of needed.

Cheers,
Ryan

#### MrAl

Joined Jun 17, 2014
11,489
Hey, I am an A2 Electronics student and need to prove that my MOSFET Power Amplifier is functional, image is attached.
I was told to use the P=V^2/R formula to prove that the power at the input is greater than that of the output.
I currently have no clue on how to use this formula to prove this, any help would be greatly appreciated.
I can give additional information of needed.

Cheers,
Ryan
Hi,

What are you using to estimate the Ron of the MOSFET?

If you just assume Ron, then you can come up with a formula for the power in the solenoid vs the power in the mosfet and go from there for the proof. Make sense?

#### Ryth

Joined Mar 28, 2018
4
Hi,

What are you using to estimate the Ron of the MOSFET?

If you just assume Ron, then you can come up with a formula for the power in the solenoid vs the power in the mosfet and go from there for the proof. Make sense?
Could you possibly show me this? Sorry, I'm just slightly confused.

#### crutschow

Joined Mar 14, 2008
34,457
When the MOSFET is ON, some of the power is dissipated in the ON resistance of the MOSFET and some is dissipated in the solenoid resistance.
Using Ohm's law you should be able to determine the power dissipations for those two resistors in series.
I assume you do understand Ohm's law(?).

#### ebp

Joined Feb 8, 2018
2,332
"...prove that the power at the input is greater than that of the output."

I'm assuming that is backwards, but it depends on what you mean by "input" - the input to the FET gate or the +5 volt supply.

What do you know about the solenoid?
What do you know about how the gate of a FET works (in general terms, not in great detail)?

#### Ryth

Joined Mar 28, 2018
4
"...prove that the power at the input is greater than that of the output."

I'm assuming that is backwards, but it depends on what you mean by "input" - the input to the FET gate or the +5 volt supply.

What do you know about the solenoid?
What do you know about how the gate of a FET works (in general terms, not in great detail)?
I know that the solenoid runs off a 5V supply and has a resistance of 5.1 ohms.

I wish to know how to calculate the power at the input (far left box) and at the output (between the drain of the MOSFET and Solenoid) in order to prove that the Power Amplifier works.

#### ebp

Joined Feb 8, 2018
2,332
What must you do with the gate to make the FET turn on?

How would you calculate the power in solenoid if the FET acted like a perfect switch when ON? If it acted like a fixed voltage drop? If it acted like some small resistance? You already have been given some clues about how to approach this. Are any of those three things what you would expect? Can you say why?

General hint:
In electrical circuits in general, you expect voltages and currents.

#### MrAl

Joined Jun 17, 2014
11,489
Could you possibly show me this? Sorry, I'm just slightly confused.
Hi,

When the mosfet is on, the resistance is Ron. The resistance of the coil is Rcoil. These two are in series, so the total resistance is R=Ron+Rcoil.

When current flows (mosfet turned on) only part of the power shows up in Rcoil because some also shows up in Ron. Your job is to calculate the power and then show how there is more total input power (5v line) than there is in Rcoil.

#### Dodgydave

Joined Jun 22, 2012
11,304
Could you possibly show me this? Sorry, I'm just slightly confused.
What is the number of the fet you're using, like IRF540 , irf630,, etc....