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| Weird signal instablity of an opamp amplifier |
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| VanitarNordic:
I have configured a well known TL071 opamp in single supply and AC coupled inverting amplifier. with 10K and 1M resistors, it produces around 100x gain. I inject a 10mV sinewave signal to the input. for the 10mV signal, I should get around 1V after amplification, it is almost okay, but look at that weird instability (the video has attached). The frequency is just 300Hz and if I increase it, this instability would be stronger. The supply is regulated. oscilloscope has no noise. The signal generator is the Siglent brand and no noise. I have tested the circuit also with a battery but the same results. with this setting, The GBW is about 300 * 100 = 30Khz, which is far away beyond opamp limits, but why this happens? :o :o Has anybody experienced this? is this a fake opamp? |
| StillTrying:
It looks like mains hum to me, see what it looks like with the scope's trigger on AC LINE. I'd start with 1Vpp and divide it by 100 on the board right at the op amp's input to give a clean 10mVpp. |
| VanitarNordic:
--- Quote from: StillTrying on November 19, 2018, 10:45:10 am ---It looks like mains hum to me, see what it looks like with the scope's trigger on AC LINE. I'd start with 1Vpp and divide it by 100 on the board right at the op amp's input to give a clean 10mVpp. --- End quote --- I'll try your suggestion but if it is the mains hum, then why still exists when I use a battery? --- Quote ---see what it looks like with the scope's trigger on AC LINE. --- End quote --- You mean I should put the scope input coupling on Ac? it is already on the AC coupling. |
| StillTrying:
"You mean I should put the scope input coupling on Ac?" No, Trigger Input to AC LINE to see if the wobbles are synchronized with the mains, if I stop the vid and look at 2 very similar peaks they're often 20ms apart. "why still exists when I use a battery?" Mains noise maybe coming from the signal generator, cables or GND connections. |
| VanitarNordic:
--- Quote ---I'd start with 1Vpp and divide it by 100 on the board right at the op amp's input to give a clean 10mVpp. --- End quote --- I did. I did set the generator output to 1V and divide it by a multiturn pot to 10mV to handle 1V at the opamp output. The instability is still present. Yellow: channel two (1V from the generator) Red: Opamp output |
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