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

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