Oscillation due to (probably) feedback network

[Mambo]

I'm becoming crazy playing around a parastic oscillation in a small amp I'm building: it is located around 500Khz and thay are many and many days I'm trying to eradicate it.
Simulation tells that with the components You can see in the attached schema I should have about 25° of margin, because the frequency response of the amp cross the unity gain at about
-155°.
The oscillation is about 2V Peak to Peak and changing the bias of the amp it changes its frequency but the magnitude remains aroud 2V Peak to Peak.
Today, by following some suggestions in the forum, I tried to remove the compensation cap across the feedback resistor R16 and after connecting the amp to the supply I was expecting a higher degree in the amplitude of the oscillation with a quick overheating of the power devices; quite suprisingly the amp in this configuration doesn't oscillate even if, based on the simulation, I should completely be out of any phase margin.
without the compensation cap across R16 simulation tells about -200° @ unity gain.
Moreover the amp do not oscillate (with the input shorted) even if put a 2.2uF cap acros the 8Ohm load resistor.
Do You have any comments about this?
Thank You,
Mauro

[peranders]

What happens if you'll remove C2?

[Mambo]

probably i explained in the wrong way some sentences:

"I tried to remove the compensation cap across the feedback resistor R16" = C2

This is C2; if i remove it, as told in the post, the amp run fine

[Mambo]

Other interesting things are:
- If i put a 150pf between the bases and the collectors of the drivers Q11 and Q12 the amp runs fine
- if I change the output devices form MJL4281/4302 to MJW0281/0302 or MJL0281/0302 the amp runs fine also without the 150pf across base and collectors of the drivers

the 0281s and the 0302s hava an output capacitance of 400pf while the 4281s and the 4302s have 600pf as output capacitance


I have also others channels built in the same way but for hihger supply (40v) with 3 pairs of MJL3281/1302 and the behavior is the same

[peranders]

Originally posted by Mambo
probably i explained in the wrong way some sentences:

"I tried to remove the compensation cap across the feedback resistor R16" = C2

This is C2; if i remove it, as told in the post, the amp run fine

.... so what is the problem then? Adding such a cap in nothing you'll do on purpose, so why did you add it in the first place?

[megajocke]

This is probably not related to the oscillation, but the base of Q4 should connect to the emitter of Q3 instead. Your current configuration will have strange clipping behaviour and may latch up. R2 might need to be increased too.

[Mambo]

the problem is that without c2 simulation tells that the amp should be unstable because the unity gain in the response is at -200 deg, while in the real world it runs without problems.
theory tellls that to have stability the unity gain should be reached before the phase touch -180 deg, that's why i added c2.
whit it i found in the simulation that i should have about 25 deg of margin, while under test it oscillate.

[megajocke]

Adding C2 increases the loop gain at high frequencies, it is not the closed loop gain that matters. Small values of C2 gives a stabilizing effect though as it starts to provide phase lead a decade below its break frequency. Are you measuring closed loop or loop gain in the simulator?

If you do a transient simulation without C2 in the simulator, does it oscillate then? If it doesn't, then something is wrong with your loop gain measurment.

Someone on this forum posted this link:
http://eportal.apexmicrotech.com/mai.../app_notes.asp

It contains lots of useful stuff about stability!

[Mambo]

The imege below contains responses without C2
- red-> open loop
- blue -> closed loop
- grey -> input
- yellow -> output

[Mambo]

while the image below are the responses with c2= 22p

Colors are the same