Oscillation only at raising slope

okmog · 05:54 PM

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.

CBS240 · 1st September 2011, 10:11 AM

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.

okmog · 12:41 PM

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

sajti · 5th September 2011, 12:53 PM

And if You put the RC network directly to the output connectors? Over the LR network.

Sajti

janneman · 5th September 2011, 02:14 PM

How big is Cc?

jan didden

wahab · 5th September 2011, 02:16 PM

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.

okmog · 04:13 PM

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.

wahab · 5th September 2011, 04:52 PM

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.

1st September 2011, 10:11 AM	#2
CBS240 diyAudio Member Join Date: Aug 2005 Location: K-town	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. __________________ All the trouble I've ever been in started out as fun......

5th September 2011, 02:14 PM	#5
janneman diyAudio Member Join Date: May 2002 Location: Where Germany, The Netherlands and Belgium meet Blog Entries: 6	How big is Cc? jan didden __________________ /Another new issue: Linear Audio Volume 3!

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Raising filament winding above cathode with a EZ81 in the circuit OK?	G	Tubes / Valves	3	15th May 2008 08:29 PM
Raising amplifier supply voltage from 12 to 16V	Safe_Cracker	Car Audio	5	10th October 2007 06:04 PM
Donation/Fund Raising Idea	Apogee	Everything Else	3	5th January 2005 07:15 PM
Raising Qts with partial current drive?	Circlotron	Multi-Way	6	16th December 2003 01:26 PM
Suggestions on raising Qts	Mikett	Multi-Way	20	29th November 2003 12:55 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

5th September 2011, 12:53 PM	#4
sajti diyAudio Member Join Date: Apr 2003 Location: Vác, Hungary	And if You put the RC network directly to the output connectors? Over the LR network. Sajti

5th September 2011, 02:16 PM	#6
wahab diyAudio Member Join Date: Nov 2009 Location: algeria/france	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.

5th September 2011, 04:52 PM	#8
wahab diyAudio Member Join Date: Nov 2009 Location: algeria/france	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.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)