# MOSFET amplifier biasing question?

#### noob_engineer

Joined Mar 11, 2010
3
hi all....
i want to design a MOSFET amplifier to drive 4 or 8 ohm speakers, but i cant find MOSFET biasing equations .
i would appreciate if someone explained them to me or pointed me to a website where i can find them .

#### noob_engineer

Joined Mar 11, 2010
3
i couldn't clearly understand this article and how to choose resistors values and it looks complicated .am just a beginner ,is there any other simpler circuits like this one , but with smaller supply voltage.

#### Audioguru

Joined Dec 20, 2007
11,249

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#### noob_engineer

Joined Mar 11, 2010
3
the power supply is too high 40v!!! i was looking for maybe 12 or 15 ,20v max.

#### retched

Joined Dec 5, 2009
5,208
You may want to start off with the e-book at the top of every page on this site.

Your voltage is not high for an audio amp. And you will not get "high" volume levels with lower voltages. If you are a beginner, you will want to read through the basics of op-amps and audio amplification in addition to basic electronics and safety. You will be dealing with capacitors that can bite you. And some can bite you REAL hard.

You can use 5 9v batteries as a power source for the circuit. In parallel, that will give you 45v. Use a voltage regulator to 40v. Depending on your amp draw, you can even step one 9v battery to 40v. Tasers step one 9v battery to 10,000v at extremely low amperages.

#### Audioguru

Joined Dec 20, 2007
11,249
the power supply is too high 40v!!! i was looking for maybe 12 or 15 ,20v max.
The Mosfets have a high voltage loss. 18W into 8 ohms is a signal swing of 34V peak-to-peak. The loss in the Mosfets is 40V - 34V= 6V and could be higher.

If the supply is only 12V then the voltage swing across 8 ohms is only 6V peak-to-peak which produces only 0.6W like a cheap clock radio.

A car radio amplifier IC like a TDA7240A has two amplifiers that drive both wires of a speaker. With a 12V supply the power in a 4 ohm speaker is 11W.