Help on OP AMP unwanted oscillation

[air]

The noninverting input is connected to ground, inverting input connected to a voltage source through a resistor and another resistor form a feedback loop from output to inverting input. when the feedback resistor's resistance is huge like 100 M Ohm and above. there is no oscillation. but when the feedback resistor's resistance get smaller than tens M Ohm to the minimum like 10K. the output oscillation begin increase and getting bigger and bigger. How to fix this? Are there some references dealing with OP Amp trouble shooting practice? Thanks!

 

[IanP]

Could you double check if you don't have opamp's inputs reversed --> (inv) to (non-inv)??? ie. ground to inv (-) and feedback to non-inv (+) ..
Regards,
IanP

 

[sunking]

the PM of the opa?

 

[suria3]

Pls check the PM of the Opamp using the Midde Brook method for both the High and low value feedback resistor. I think the capacitance contributed by the feedback resistor could alter the PM of the loop.

 

[seadolphine2000]

I think using -ve feedback will solve the problem.

 

[ptmsl]

What is the value of the resistor between the voltage source and the inverting input of the opamp? What type of opamp do you use -is it unity gain stable? do you use a trimmer pot for feedback resistor? what is the frequency of the oscillation? REMEMBER do not use bipolar input transistor opamps with large resistors (100Meg is a large resistor) - the biasing current and the noise current can spoil a good circuit

 

[yuyangjp]

I think the pm is poor too. and maybe the dominant pole is colse to the second dominant pole

 

[Kral]

air,
A few things to check:
~
Are the connections to the OP-Amp inputs short?
~
Is there a resistor between the NI(+) input to ground? If so, you can try bypassing it with a capacitor to minimize positive feedback to the NI input.
~
Do you have power supply bypass capacitors? If so, are the leads short? Some OP-Amps, such as the 741 are tolerant of high supply impedance; others, especially high-speed Op-Amps are not.
~
The common to the input voltage source should connect to the NI input (or the common side of the resistor between NI input and ground) by as direct a path as possible to prevent ground loop problems.
Regards,
Kral

 

[Borber]

You should post the type of OPAMP, manufacturer and the kind of load to OPAMP. Some specific OPAMP's tends to oscillate without obvious reason. One of such is ST TL071 when even slightly capacitively loaded.

 

[air]

The amp I used are LT1008 and CA3140E. the circuit is routed on breadboard. I added a .2 uf cerimetic cap accros pin 1 and pin 8. It really help a lot. I don't have any bigger cap for now to try further. Can I get a straight line using this method or any other good method?

 

[Borber]

What is the frequency of oscillations?
You may try placing 100pF-1nF in parallel to feedback resistor. In this way you will lower first pole.

 

[air]

about 50 Hz ripple at output with 0.5 ~ 1 v peak to peak.

 

[Borber]

Obviously you have trouble with hum and not with oscillations. Gain of your amplifier must be very high and any ground loops or bad grounding may cause this. Check your oscilloscope grounding.

 

[leomecma]

Look for more details on THE ART OF ELECTRONICS on pag 242 until 248. In forum has the book for download. Look this link too, it's an Analog devices simulator to see the effect of driving capactive loads and modes of avoid the effect of non-stability:

**broken link removed**

 

[air]

Hi
I am so sorry about my describtion on the top. Now the thing had changed to just the opposite. When the feedback resistor in the range from 10's M to morn than 100 M ohm, the output has big noise ripple. I found that the noise source is come from inverting input (- input). When the feedback resistor are smaller like one M Ohm and below, the output look prety nice. Anybody can help me?

 

[Borber]

Why do you need such big resistor, 1e8 ohm. You should post your schematics and wanted specifications of the circuit.