Skip to main content

Single Mosfet Class A Power Amplifier

The post discusses a simple, cheap single mosfet class A power amplifier circuit which can be used for any small scale audio amplifier application.

By: Dhrubajyoti Biswas

Zero Negative Feedback Amplifier

This following data will detail how to build an amplifier having zero negative feedback which implies building a zero component amplifier. The amplifier will be single-ended and Class A.

To begin with, let us first lay down the circuit design of the proposed amplifier, as given under:

Single Mosfet Class A Power Amplifier

How to Build the Circuit

To build the circuit we need: A MOSFET, some capacitors and resistors and a sturdy power supply which has to be properly filtered using large filter capacitors. The amplifier we are building is built by incorporating the device 2SK1058 N-Channel MOSFET from Hitachi. The pin diagram is shown below:

K1058 mosfet image

The capacitors that we used in this experiment is that of Sprague. This is used to couple the input and on the output a large electrolytic along with polyester bypass capacitor of 10µF. To manage the load, we used four non-inductive wire wound resistors of 10W.

However, to achieve total resistance of 15 ohms, the resistors are wired on a series of two, which makes 30 ohms and furthermore setting up the sets in parallel. Please note, the device would be hot and susceptible to burning during idle mode, hence caution is utmost important.

Class-A can never be an ideal option for a high efficiency amplifier design, but as we are applying the idea in this setup, we had to use more than 20 Watts power to generate a modest 4.8 Watts audio. The heat-sink used for the mosfet was of 0.784 °C/W.

Power Supply

The power supply used for this single mosfet class A power amplifier circuit is 18VAC and 160VA EI transformer attached to a bridge rectifier of 25 amp to generate DC power of 24 Volts.

To filter and smooth the power we used 10000µF capacitors and Hammond make 10mH choke of 5 amp over pi filter setting [Cap - Choke - Cap]. The bias was via a pot of 100K along with resistor of 1M.

Care should be taken to adjust the pot only for so long as half of the DC gets over the load resistors and MOSFET.

power supply for class A amplifier

Need Help? Please leave a comment, I'll get back soon with a reply!


  1. hi SWAGATAM i have my subwoofer with two speaker but the inner base went off how can troubleshoot that and make my inner base play again

  2. sorry, I don't know much about speaker construction details, so can't help with this...

  3. I have much more piece frome. This taip transistors
    Can I have more power?

  4. can i modify this circuit to use j162 mosfet? schematic will be a great help..

  5. it may be possible by reversing the polarities of all the relevant parameters...but I don't recommend using a p-channel fet for the config

  6. hi sir.I'm trying this project.I did not understand the input method.please help me.

  7. Hi Ajith,

    the input will be your music input which needs to be amplified, for example from a mobile phone's headphone socket, DVD player's earphone socket or from an ipod etc....

  8. I need a power amplifier circuit which works upto 500KHz square wave. Will you suggest any circuit for that.

  9. Please Google search the following:

    "BTL power amplifier circuit"

    you will be able to find many good options

  10. i need your help,
    i want to make circuit to drive bike horn 12v 3A. (musical like mocc horn).
    a portable mp3 player with 3 watt output is used for input .
    any transistor mosfet circuit can be use ful please

  11. use a 12V 7AH battery, and switch the horn though a 3 amp ON/OFf switch

  12. Hello Sir

    I have a Hybrid audio amplifier B2200 which got burnt one channel,the cause of the problem I don't know but I found the mains fuse bridged probably by the user.I tested the components on the pcb and found several transistors ,fuses and resistors burnt. I replaced all I thought were bad and powered the amp. There is sound coming out but it's low power and very distorted .I did tested the output for dc offset- negative probe to the negative terminal and positive to positive I got -0.151vdc. Now am stuck don't know where to start.

    I know it's very difficult for you to troubleshoot without schematic but I just thought you may have raw idea where the problem might be lying so I can just concentrate on that area.

    Please you may find some photos of it here

  13. Hi Francis, sorry, it will be impossible to find the fault since the design looks too sophisticated and have many complex troubleshooting might require a thorough practical investigation


Post a Comment