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Strange OP-amp behaviour - For amplifier experts.

Hello All

We have an LM224DG (ON semi) op amp amplifying the audio out of a GSM module. Differential stage followed by inverting amplifier.

We always have problems with these designs trying to keep the hum caused by the 217Hz pulsing of the GSM system (pulsing 2.5 - 3A for the module when Tx) out of the audio circuits.

Whats happening in this design has me confused. The gain phase margin of the inverting amplifier is causing oscillations at 40KHz. Ok no big deal, change the gain and capacitive loading on input, output or feedback and problem goes away.

The problem is that once I remove the 40KHz oscillations the GSM Hum is much worse. The thing is you cant hear the 40KHz component and with no GSM Hum you would nearly be tempted to leave the oscillations.

Does this operation make any sense? I just dont see it. Any sugestions.

Many thanks for any input.

Regards

Denis ________________________

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Reply to
dgleeson3
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Post your schematic, either via a URL link or place on newsgroup alt.binaries.schematics.electronic

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave
Reply to
Jim Thompson

LM224, LM324, LM358, whatever, a very poor choice for audio, most folks would say. That's in part because it has a horrible huge deadzone in its class AB output stage, where neither the pullup or the pulldown transistor is significantly working. This deadzone causes distortion at higher audio frequencies, etc.

I'll take a guess about your problem, and say the 40kHz oscillation caused a dither so one or the other output transistor was always on, pulling up or down, thereby creating a low output impedance, like you would have with an ordinary opamp. But, when you killed the 40kHz dither, now the opamp's effective output impedance went up and the output wiring became susceptible to the GSM hum (whatever that is). You can test my theory by subtituting a better part for the LM224, such as LTC's LT1014i, which has a properly biased output stage.

Reply to
Winfield Hill

Bipolar amps can be really nasty with cell phone exposure. Their numerous b-e paths act as rectifiers. Especially GSM is bad.

Try a FET opamp. Or at least bypass the inputs against each other and/or GND really well. Just a few hundred pF, you want the ESR really low all the way up to 2GHz and no strange resonances up there. Very small inductors right in front help as well. Ferrite beads also but they aren't too effective above a GHz.

Do you have a full ground plane? If not this could become a little nightmare. Not hopeless but tough.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

Bipolar opamps love to rectify RF. Try a jfet or cmos amp and see if that helps. LF347 is cheap and behaves pretty well.

John

Reply to
John Larkin

Hi guys

Many thanks for the input. Will try a fet or CMOS opamp.

Denis ___________________ http://www.Centr> > >Hello All

Reply to
dgleeson3

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