Very simple MOSFET design in class A/A+/B

[skyra]

I built the version in post #2, and I don't seem to be set the idle current to 400 mA. Supply voltage 24V, R10 is 10 Ohms, the Schottkys are SB360, the Fet-s are IRF530-s, their gate is at 3.7V (Vgs), but the idle current does not go over 60 mA. I double checked everything, it works even this way but there is audile distortion at higher volumes (no oscillation). Any idea or advice?

 

[skyra]

Oh well.... Problem solved. The Vf of the Schottky diodes is crucial. Standard types are not usable in these positions. I tried it with lower Vf types, and now it works perfectly. 😎

 

[des_chan]

Some update:

I have another class AB amplifier circuit using IRF540/9540 as mentioned before. I removed the MOSFETs from the circuit board and using flying leads to connect, and try to remove all overlapping wires (of course not shorted) within the circuit board. This amplifier can function properly with reasonable power output, provided that I connect the loudspeaker (4 Ohm) and the input to the audio jack of mobile phone BEFORE connecting the power supply (24 V). Also I found that the power rockets with noise. For safety purpose I connect a fuse to protect the board.

For this amplifier circuit, I still cannot find a way to resolve the problem of huge power.

Any idea? Is this related to stray capacitance?

 

[Dupt]

.... The gate capacitance certainly affects the parameters of the amplifier. In the final version I used 1.4 nf transistors and got very good results. I would recommend using field effect transistors with a gate capacitance of no more than 1.5 nf.

I'm considering building your amplifier. Class A at low power and class B with higher output always makes a nice balance between efficiency and (low distortion) needs.

In the schematic you indicated VGth =3V, and input capacitance less than 1,5nF.
I made a selection filter at Digikey with the following parameters:

• Through hole
• Ids = 4 to 20Amp
• Input cap 500-1500pF
• Vds = 60-300V
• VGth = 2,5-3,5V (@250uA)

And I got some 18 FETs as result. Almost non of the IRF... type, many of Taiwan Semi, Alpha&Omega, Goford, 1 or 2 Onsemi

Are there other specs that are essential in this application?

For example this one seems quite OK

https://www.digikey.pt/en/products/detail/goford-semiconductor/18N20F/17112566

Total gate charge 18nC ok?

Any advice or tips appreciated.

Thanks for your nice design!

Greeting, Tomas

 

[abstract]

Also I found that the power rockets with noise.

Not sure what you mean. Could you post a schematic? Output current noise will climb if the bias current climbs.