# Why is inverting op-amp behaving like a low pass filter?

can anyone explain why this circuit behaves like a low pass filter despite having no capacitor in the feedback loop?

• Have you not heard of GBW product? All stable op Amps are integrators with internal RC. So if your example has GBW of 4MHz with gain of 40 has BW = 100kHz – Tony Stewart Sunnyskyguy EE75 Dec 5 '19 at 8:49
• An LM741 costs about 50 cents, and has performance to match. You can easily get ten or twenty times the performance of that op-amp, at about ten to twenty times the price. – J... Dec 5 '19 at 21:45
• Heck, even a piece of wire is a low-pass filter. – Hot Licks Dec 6 '19 at 1:03
• @J... You can get much better op amps for 50¢, too. – Hearth Dec 6 '19 at 15:46

can anyone explain why this circuit behaves like a low pass filter despite having no capacitor in the feedback loop?

All op-amp amplifiers will behave like low pass filters due to parasitic capacitance between the output and, in your circuit, the inverting input. If there is 2 pF present then it forms a low pass filter with the 400 kohm feedback resistor having a 3 dB point of about 200 kHz.

This problem would be improved by choosing an input resistor of 1 kohm and a feedback resistor of 40 kohm. However, the 741 is pretty crappy so you will be unlikely to get a ten times improvement in frequency response.

Inside (virtually) every op-amp are compensation capacitors; these limit the frequency response in order to maintain closed-loop stability. As Tony Stewart mentioned in his comment, this has a significant effect on frequency response when higher closed-loop gain is required.

See the picture below for the closed loop gain (40 dB) of a 741 and note that your circuit has a closed loop gain of 32 dB: -

So, the closed loop response starts to fall from 40 dB gain at around 50 kHz. With your circuit (gain of 32 dB) you might see the fall in gain around 70 kHz.

Picture taken from this useful document entitled "Frequency response of op-amps". See also my answer here on a very related subject.

1. basically, every physical system in this world has a finite bandwidth and hence exposes some low-pass behaviour, so in the larger scheme of things, this is to be expected
2. You use an ancient (LM)741 series opamp. These things have very modest bandwidths – in fact, the current Texas Instruments LM741 datasheet only claims 0.5V/ms as slew rate, and doesn't specify a bandwidth for most device variants, at all.

You simply use a model of a very low-passy opamp.

Other answers are correct, but they miss the biggest point.

Operating a 741 with a single 5 volt power supply is so far beyond the requirements of the IC that I'm a bit surprised that you get anything out at all. For that matter, it would not surprise me if most of the output is simply due to the resistors connecting the input to the output. The 741 is only specified for a power supply range of +/- 5 to +/- 15 volts. Not only are you violating power supply level requirements, if you are applying a positive input signal, your output needs to be less than zero (this is an inverter, right?) but the op amp CANNOT drive to less than the lowest power supply voltage, which is zero. Additionally, look at the data sheet such as this one and find the Common Mode Input Voltage range. Note that for +/- 15 volt supplies it must be within the range +/- 12 volts to guarantee operation. In other words, at +/- 15 volts the + and - inputs must be about 3 volts lower than VCC and 3 volts higher than VSS. Think about what this means for a VCC of 5 and VSS of zero.

So. Replace the single power supply with a +/- 5 to 15 volt supply. Until you do that, you are wasting your time.