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I am currently working on a class A audio amplifier designed to work with line in voltage (about -400mv to +400mv). I plan on using an IRFP240 for the mosfet but am undecided on the transistor stage. resistor values will need changing of course, but I would like to think I have the arrangement of this circuit correct? I would love to know any improvements, as I am a beginner at audio amplifier design and the design goals are to be a simple, but high quality audio amplifier.

my circuit wip

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  • \$\begingroup\$ You'll probably want to add an emitter bypass cap in parallel with your 2k emitter resistor. You'll get a lot more AC signal gain while maintaining your DC bias. \$\endgroup\$ – Aaron Jun 19 at 0:47
  • \$\begingroup\$ yes, i have seen this cap there before and know is will be a near short from an AC voltage? yes, i see the purpose of that now. well answered. is that not running the risk of affecting the audio signal in anyway? I know it can not be to low of a value as that will produce a highpass filter? \$\endgroup\$ – G Frank Jun 19 at 0:49
  • \$\begingroup\$ You have positive feedback here. with inversion on each transistor \$\endgroup\$ – Sunnyskyguy EE75 Jun 19 at 2:07
  • \$\begingroup\$ Before you can do any real design I suggest that you educate yourself on the subject. Your design now looks like you "slapped some components together" which is never going to work properly. Circuit design isn't that easy and to master it you must start with proper circuit analysis. Then analyze existing designs, learn how they work and why they certain concepts were chosen. Only then are you ready to make your own variations. \$\endgroup\$ – Bimpelrekkie Jun 19 at 6:09
  • \$\begingroup\$ Also never drive a lower AC coupled R with a higher R on drain. It is very inefficient and cannot go full signal range. \$\endgroup\$ – Sunnyskyguy EE75 Jun 19 at 6:25
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MOSFETs have very large input capacitance (several thousand pF).

To drive this effectively you will need to run the driver stage at higher current with a lower collector resistor.

Although feeding the collector resistor from the drain of the FET gives DC feedback to stabilize the operating point it will have the disadvantage of limiting the output swing as it goes towards ground. A different arrangement with separate DC feedback would be preferable.

Using a resistor feed for the drain does limit the overall efficiency to ~15% (depending upon the actual value). A constant current source can achieve 25%. An active source turns the stage into push-pull with a 50% efficiency limit.

One of the classic class-A audio amplifiers was designed by John Linsey Hood fifty years ago, it is worth reading about.

The Class A Amplifier Site.

Simple Class A Amplifier

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  • \$\begingroup\$ Im interested in the constant current source solution, would it be as simple as replacing the 15ohm load resistor with something like the LM317 voltage regulator? \$\endgroup\$ – G Frank Jun 19 at 13:46
  • \$\begingroup\$ @GFrank - A LM317 couldn't do a high enough current and would have too high drop-out voltage. A P-FET or a PNP bipolar would be most appropriate. \$\endgroup\$ – Kevin White Jun 19 at 14:13

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