-2
\$\begingroup\$

EDIT: Glen_Geek thank you for your answer it was helpful.

I didn't add many details originally because I was looking for general things that could cause those oscillations, and to determine if those were the classic "oscillations" that are problems with op-amps. However, I can add more details now.

I believe the problem was a capacitive load, and I was able to remove the majority of oscillations by adding a 50 ohm isolation resistor and limiting the bias voltage applied, especially to one polarity, i.e biasing it with +2V/-7V removes oscillations and still passes negative polarity signals unaltered. I did add bypass capacitors but this did not seem to do much.

Moving on past this, I am struggling to set the gain as I wish. For simple testing I am using a function generator as the input with an impedance of 50 ohms, and simply producing a 1 MHz sine wave. The circuit I am using is this (not shown are two bypass capacitors in parallel of .1 uF and .01 uF for each bias pin):

enter image description here

R1 alters the output but not as expected fully. When R1 is equal to 820 ohms again, I observe a signal. When R1 is equal to 50 ohms, I observe a signal ~10X larger.I tried multiple resistors for R1 between 50 ohm and 820 ohm (200, 330, 560, 670 ohm) and all yielded the same severely attenuated signal (changing the resistor does not alter the signal). In fact, this signal is the same as when R1 is removed. Interestingly, the amplitude of this signal increases when the frequency of it increases too. I am using an LM6181 op-amp and implementing this circuit with a breadboard with the actual setup is shown below.

enter image description here

What could be causing these problems such that only 50 ohm and 820 ohm resistors produce an output? I also tried a different signal source, using a silicon photomultiplier (essentially a photodiode) that to my knowledge does not have exactly a 50 ohm output impedance and observed the same effects. I'd add more pictures but I can only have two links, sorry!

Thanks!

\$\endgroup\$
  • 3
    \$\begingroup\$ We need a schematic and in particular the part number of the opamp to help you. \$\endgroup\$ – Kevin White Mar 9 '17 at 4:16
  • \$\begingroup\$ This could be from a number of things, please edit your question, draw a schematic of your setup with the tool (button by the img button) \$\endgroup\$ – Voltage Spike Mar 9 '17 at 4:58
  • \$\begingroup\$ Your scope is capable of measuring and displaying frequency and amplitude, as well as taking a scope capture directly to a USB drive. This greatly improves the clarity of your scope captures. \$\endgroup\$ – uint128_t Mar 9 '17 at 5:40
  • \$\begingroup\$ Try putting decoupling capacitors on the power pins of your op-amp. \$\endgroup\$ – Andy aka Mar 9 '17 at 11:43
  • \$\begingroup\$ Ah, breadboard, there is part of the problem \$\endgroup\$ – Voltage Spike Mar 10 '17 at 5:34
1
\$\begingroup\$

Oscillations like this can be caused when the op-amp must drive a load having a significant capacitive component.
Almost all op-amps are "unity-gain stable". A few are not, and will oscillate if their closed loop gain is low, as in your situation.
Another possible cause is capacitance at the inverting input.
Have you added power supply bypass capacitors? Sometimes long leads to the power supply have enough inductance to cause instability. Add power supply bypass capacitors close to the op-amp. A complete circuit diagram would help us, including signal source and load.

\$\endgroup\$
  • \$\begingroup\$ And what does the supply look like... \$\endgroup\$ – Trevor_G Mar 9 '17 at 4:24
  • \$\begingroup\$ ALso what does no input mean? Floating? \$\endgroup\$ – Trevor_G Mar 9 '17 at 4:26

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.