Can anybody help me? This amp oscillate at 10MHz?


2011-06-09 12:40 pm
just in yesterday I have built and tested Lazy Cat’s latest symmetrical topology with variation of n-most only output. The amp has almost all aspects meet my expectation, except the S/R and oscillation.. I initially used 22pF Miller cap. And my first pair of IRF530 stayed for couple of minutes before it broke down silently, together with one driver B649AC. And then after I replaced the B649, I also increase the Miller cap to 50pF. Even though, another two pairs of IRF530 did not sustain for couple of seconds after power on. I finally increase the Miller cap to 120pF, this time the amp starts to sing. My PSU is +/-35v. It can give a 28Vp swing, which is not bad. However, when I view the waveform, I see an high frequency oscillation at around 10MHz. This oscillation appear only to the negative circle. Can anybody tell me how to fix this problem? As you see that the slew rate is already low, maybe increasing the Miller cap is not a good cure. Can anybody give me a hint?

PS. Oscilloscope images are attached. You can see in that the amplified triangular waveform there is an oscillation at about 10MHz.








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I have seen such long connection wires as shown in the following SSA photo, but claimed to have no problem in hi-freq response neither issue with phase shift. :rolleyes: Is there any other possibility with the amp that caused the oscillation?

View attachment 334143

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But it all depends on how your amp is stable over phase and frequency span. Some amps so well built so you can feed them any rubbish and it would be stable, some of them is just sitting and waiting to pick up something from the air.
As I told you in my post above, you may need to increase C3 and C5. These two caps helps to stabilize the amplifier.
If 47pF does not help, then try 100pF.[/QUOTE/]

The best way is to simulate it in software like MicroCAP, it gives you a basic understanding if your amp is amp or just generator waiting for some frequency to arrive

Hi Devilsdance, thanks for your idea. :)

I have already done the simulation in LTspice. The gain-phase plot is attached, along with the 10KHz FFT. Any comments?

I know this amp is far from perfect, it needs some fine tuning. Your ideas are welcome.


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