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Old 31st August 2011, 05:51 PM   #1
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Default Oscillation only at raising slope

Amplifier:
MOSFET amp module AMP38 (mono)
LME49830 + Renesas 2SK1058/J162 MOSFETs
PSU: +-60V

I have oscillations only at the raising slope. The input signal is 1 khz square wave with 1.5 Vpp. No load at the output, just the oscilloscope connected.
The oscillation is also visible at the input signal when the amp is connected. It disappears at the input when the amp is disconnected. So it's probably a grounding problem. Can somebody agree, or what could be the problem? Wiring?

Please have a look at attached photos.

Red is input. Yellow output.
Attached Images
File Type: png oscillation.PNG (78.9 KB, 125 views)
File Type: png oscillation-zoom.PNG (9.2 KB, 119 views)

Last edited by okmog; 31st August 2011 at 05:54 PM.
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Old 1st September 2011, 10:11 AM   #2
CBS240 is offline CBS240  United States
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It is possible there is a ground problem, it could be wiring. Also it could be instablility and ringing from one of the outputs. What frequency is the ringing? HF ringing can spoil a not so good grounding scheme, make the HF noise show up on the input also. If it is the mosfet ringing, one way to dampen the occilator (all mosfets make great HF occilators) is to include a zero, a small cap in series with a resistor like 20-50pf and 50-150R range, from gate to drain, as close to the device package as possible.
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Old 5th September 2011, 12:25 PM   #3
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Thanks for your reply!

The oscillations are about 15 Mhz. It's really strange, when I disconnect the snubber network (Rn=10R, Cn=100nF) the oscillations disappear, but without any stability. I've already tried higher values for the Cn (150n, 330n 470n) made it worse, also with higher values for the Rn.
I've tried different impedances 50R, 600R of the input signal nothing changed. Also some shorts in grounding points on the PCB didn't bring improvement.
I've tried higher values for gate resistors Rzn/Rzp 330R instead 220R nothing changed.
How could it come that the snubber network let the mosfet oscillating and only when the slope is raising ...
I'm not such a big fan of the solution with a zero between the gate and the drain, because this would decrease the bandwidth but would not kill the trigger of the oscillations... any other ideas?
TIA
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File Type: png sch-amp38.PNG (135.7 KB, 68 views)

Last edited by okmog; 5th September 2011 at 12:41 PM.
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Old 5th September 2011, 12:53 PM   #4
sajti is offline sajti  Hungary
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And if You put the RC network directly to the output connectors? Over the LR network.

Sajti
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Old 5th September 2011, 02:14 PM   #5
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How big is Cc?

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Old 5th September 2011, 02:16 PM   #6
wahab is offline wahab  Algeria
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What is the value of the compensation capacitor Cc?..
Best it to use National s recommended value , but in any case
not a lower one.
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Old 5th September 2011, 04:06 PM   #7
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Cc is 20pF as recommend in the LME49830 datasheet. But could that be the bad guy? I will try a bigger one as soon I get one.
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Last edited by okmog; 5th September 2011 at 04:13 PM.
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Old 5th September 2011, 04:52 PM   #8
wahab is offline wahab  Algeria
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Try 33pf and 47pf , this should be enough , otherwise, you ll
have to look at your layout and particularly the way the mosfets
are connected , since the tracks must be as short as possible.
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