Amplifier N-FET

[mgm2000ro]

Hello, i have a problem for experts
I simulate this schematic into Orcad and it worked perfectly well, but when I build it fizicly i'm having a few problems when i rich up the IN volume level. I put the osciloscope on exit and when i'm increasing the volume the shape of the sin wave is becomeing strange.

If you cand recorect the schematic, please, with less modifications possible, because the PCB is allready made and cannot admit to many modifications, i mean I want if you can to recorect the actual value of the components(R,C, so on)

Please post(draw on to the original schematic) the modified schematic because if not is difficult to understand in words the new improvements you made.
Thank you very much.



image scop:
http://img176.imageshack.us/img176/9...0resizepb0.jpg

http://img176.imageshack.us/img176/3...6resizefk6.jpg

[richie00boy]

That is oscillation. It could be your PCB just as well as your schematic.

D9 should not be there, but I don't think it is the cause of your problem.

[x-pro]

On a first glance:

Increase gate resistors to 100 Ohm, move R33, 34, 35 into the sources of M4-M6, remove C10, perhaps connect (if required) a small capacitor (2-10 pF) between collector of Q3 and bases of Q8/Q2. You may need to reduce the amount of feedback - I would think the gain of 30-50 is more appropriate in this kind of configuration.

Cheers

x-pro

[roender]

Make C10 as small as possible, the right value is one which produce nice square wave output, without ringing, and no oscillation

BTW, nice design!

[mgm2000ro]

without c10
pictures-scop

[mgm2000ro]

[Nelson Pass]

start with x-pro's advice, and just for grins, also try removing D6.

[x-pro]

On top of what I've said earlier one more question: what are your power supply arrangements for the circuit? Is there sufficient decoupling on the supplies?

Cheers

x-pro

[mgm2000ro]

the power supply & all aranjament
http://img159.imageshack.us/img159/5725/img6343rt2.jpg
+/-42v at 12v car battery
I will try all changes and i'll post results.
Thank very much all of you!

[analog_guy]

I have seen such behavior in simulatons before. It is very likely that some component parameters in the actual circuit are different from those assumed in the simulation.

I would start by seeing if the observed effects in the real circuit can be replicated in simulation by adjusting simulaton parameters. For starters, you might consider adjusting down the idle currents in the simulation VAS stage. The waveform you posted does not look like a classical case of HF oscillaton as much as a predriver or driver stage running out of current. If you are able to replicate the problem in simulaton then it should not be too difficult to determine a fix.

Hope this helps.