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Hi I make this class A amplifier but how to replace R5 (10 ohm/10 W) with transistor because its getting really hot and how to achieve up to 10 Watt with +/- supply. I have +/-25v with 4A please help

Class A Amp

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  • \$\begingroup\$ If it's getting hot that might be ok. If you have a dual power supply you should look into AB class amplifiers. \$\endgroup\$ – Vladimir Cravero Oct 31 '15 at 11:46
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I'm not going to try and explain how to fix your amplifier (because you are starting from a weak position with what you currently have), I'm going to suggest you look at slightly different approaches.

Here's a class A amplifier circuit that looks interesting: -

enter image description here

And this is the link to the page.

I'm showing this because it is a class A amplifier with some benefits compared to your original circuit. Firstly it replaces the bias resistor in the output with a transistor (In this circuit Q5 is the active transistor and Q4 acts as a bias resistor). Secondly, it will be able to swing its output voltage a bit closer to the power rails. You have to remember that this is a class A amplifier and although the output stage bias resistor has been replaced with a transistor, the same amount of power will be dissipated in that transistor. However, with a little bit of tweaking/adds this power can be reduced by feeding a small antiphase signal to Q4's base. It can be fiddly but this is why I'm suggesting a better circuit starting point. If you decide to build this, you might want to follow up with a question onhow to tweak it to make it a little bit more efficient.

A different approach is the conventional class AB push pull: -

enter image description here

If you want more ideas the follow this link to google images.

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