Oscillating Op-Amp - diyAudio
Go Back   Home > Forums > Source & Line > Analog Line Level

Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

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
Reply
 
Thread Tools Search this Thread
Old 3rd May 2013, 02:54 PM   #1
Loren42 is offline Loren42  United States
diyAudio Member
 
Join Date: Sep 2008
Location: "Space Coast" Florida, USA
Default Oscillating Op-Amp

I don't know enough about op-amps to skin this cat, so I was looking for ideas that might help from the gang.

The following circuit has a bad 781 kHz oscillation on the second stage of the chip. The first stage is oscillation free and clean.

The circuit is as follows:

Click the image to open in full size.

I have also tried removing resistors R116 and R117. I also tried that and shorting R118 to make a traditional inverted op-amp buffer.

However the second stage still oscillates and I do not know why.

The chip is bypassed by ceramic .1 uF caps (C111 & C112), which are jammed as close to the supply pins as I can physically get them.

The layout may be the cause, but I am just guessing. Board size is 2.5" high by .9" wide, so it is small. Here are the three layers. The ground plane is a single plane with the only external ground connection at the top of the board going to a star ground. The other two "ground" connections are part of a twisted pair that simply terminates as a dead end wire on the input and output to prevent ground loops.

The power plane is split so that it supplies + and - 15 VDC supply. C15 & C16 are 10 uF. C17, C18, C19, & C20 are by-pass caps (.1 uF and .01 uF for each rail).

The wing shaped polygons are connections to two 100K pots. This board sits just behind the pots.

Click the image to open in full size.Click the image to open in full size.Click the image to open in full size.
  Reply With Quote
Old 3rd May 2013, 03:18 PM   #2
Loren42 is offline Loren42  United States
diyAudio Member
 
Join Date: Sep 2008
Location: "Space Coast" Florida, USA
After some more experimentation I think this is due to phase shift feedback.

The oscillation frequency changes with the treble setting (not the bass control). Minimum treble stops the oscillation. Mid position is abut 780 kHz, and full is over 1.5 mHz.

What I don't know is how to fix it.
  Reply With Quote
Old 3rd May 2013, 03:23 PM   #3
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: Göteborg, Sweden
Blog Entries: 4
Just wondering, have you found this circuit on the net? What is R115-117 doing? Pot values?
__________________
/Per-Anders (my first name) or P-A as my friends call me
Super Regulator SSR03 Group buy. Still time for signing up.
  Reply With Quote
Old 3rd May 2013, 03:30 PM   #4
Loren42 is offline Loren42  United States
diyAudio Member
 
Join Date: Sep 2008
Location: "Space Coast" Florida, USA
R115, 116, & 117 form a network that reduces the boost and cut of the tone controls to about ±9 dB. Removing them yields ±16 dB, which is way more than I would ever need.

This is part of Rod Elliot's Project 97 preamp.
  Reply With Quote
Old 3rd May 2013, 03:36 PM   #5
DF96 is online now DF96  England
diyAudio Member
 
Join Date: May 2007
Is the opamp unity gain stable? If so, it is a layout problem. If not, read the datasheet to see how to add compensation.
  Reply With Quote
Old 3rd May 2013, 04:00 PM   #6
Loren42 is offline Loren42  United States
diyAudio Member
 
Join Date: Sep 2008
Location: "Space Coast" Florida, USA
Quote:
Originally Posted by DF96 View Post
Is the opamp unity gain stable? If so, it is a layout problem. If not, read the datasheet to see how to add compensation.
According to Texas Instruments the OPA134 family is unity gain stable.

Then the next question is, what is wrong with the layout?
  Reply With Quote
Old 3rd May 2013, 04:10 PM   #7
Loren42 is offline Loren42  United States
diyAudio Member
 
Join Date: Sep 2008
Location: "Space Coast" Florida, USA
Quote:
Originally Posted by DF96 View Post
Is the opamp unity gain stable? If so, it is a layout problem. If not, read the datasheet to see how to add compensation.
Is stage two really unity gain?

Stage one is, but that does not oscillate. Stage two has the tone network as part of the feedback loop and that can't be unity gain.

The oscillation is pretty high (781 kHz or a 180° phase shift of 500 nS). There would be some phase shift from the caps, but not enough to produce positive feedback, correct?

How do I test if this is a phase shift induced oscillation?

And how do I fix it if it is?
  Reply With Quote
Old 3rd May 2013, 04:15 PM   #8
AKN is offline AKN  Sweden
No snake oil
diyAudio Member
 
AKN's Avatar
 
Join Date: Aug 2005
Location: In the middle of Sweden
Send a message via MSN to AKN
Hi,

No resistor at output node (w6)? There should be a resistor before output, such as 50Ohm or so. Does oscillation occur even with no cable (capacitance) connected to output?
__________________
/ Anders
  Reply With Quote
Old 3rd May 2013, 04:32 PM   #9
Loren42 is offline Loren42  United States
diyAudio Member
 
Join Date: Sep 2008
Location: "Space Coast" Florida, USA
Quote:
Originally Posted by AKN View Post
Hi,

No resistor at output node (w6)? There should be a resistor before output, such as 50Ohm or so. Does oscillation occur even with no cable (capacitance) connected to output?
Yes, the oscillation is there when the cable is desoldered.

It only stops when the treble control is turned to either extreme - or the power is switched off.
  Reply With Quote
Old 3rd May 2013, 04:38 PM   #10
Mooly is online now Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Try a small (say 22pf) cap between pins 6 and 7.
__________________
-------------------------------------------------------
Installing and using LTspice. From beginner to advanced.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Connecting Multiple op amp inputs from a single op amp output ZLyzen Analog Line Level 7 8th August 2012 12:17 PM
Oscillating swapped OP Marmotus Solid State 6 16th September 2010 08:05 PM
Difference between self oscillating and non-self oscillating D-amp. sildex Class D 36 4th August 2010 03:30 PM
741 op.amp works, tl 071 is oscillating? zoranaudio Solid State 42 16th February 2009 10:43 AM
Oscillating amp. DaViruz Solid State 23 5th December 2002 01:50 PM


New To Site? Need Help?

All times are GMT. The time now is 08:45 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2