Why is the non-inverting amplifier's bandwidth rating lower than expected?

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Discussion Overview

The discussion revolves around the unexpected bandwidth performance of a non-inverting amplifier compared to an inverting amplifier using the LM741 op-amp. Participants explore the reasons behind the significant difference in bandwidth ratings observed during testing, focusing on configurations, gain settings, and the implications of the op-amp's specifications.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that the closed-loop gain capabilities depend on input frequency and suggests that the gain bandwidth product from the specification sheets is relevant for understanding performance.
  • Another participant questions the gain settings used during testing and requests schematics to better understand the configurations.
  • It is mentioned that the feedback factor, which is calculated based on the non-inverting gain, may differ significantly between the two configurations, potentially affecting bandwidth.
  • A participant calculates the maximum frequency without distortion based on the op-amp's slew rate and suggests that the small signal bandwidth should be around 375kHz for the non-inverting configuration with a gain of 4.
  • One participant raises the possibility that using smaller input signals might yield different bandwidth results.

Areas of Agreement / Disagreement

Participants express differing views on the reasons for the bandwidth discrepancy, with no consensus reached on the underlying causes. Some propose that configuration differences and feedback factors are significant, while others suggest that the op-amp's specifications may not fully account for observed performance.

Contextual Notes

Limitations include potential dependencies on specific circuit configurations, assumptions about gain settings, and the impact of signal amplitudes on bandwidth performance. The discussion does not resolve these factors.

Who May Find This Useful

Readers interested in operational amplifier performance, feedback configurations, and bandwidth analysis in electronic circuits may find this discussion relevant.

tomizzo
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Hello, I have a question that I thought some of you may know an answer to.

I was recently testing an op amp is several closed-loop feedback configurations. Primarily inverting, and non-inverting.

When testing the output voltage of an invertin amplifier, I found that the output signal became attenuated at around 1.6MHz. The op-amp's data sheet says the bandwidth is 1.5MHz so this makes sense.

However, when I did the similar test with a non-inverting amplifier, I could only get up to 29KHz before bad deformation occurred. Why is this? I thought the op-amp was rated at 1.5MHz?

Does it have something to do with how many capacitors within the op-amp are being used in a non-inverting configuration?

I haven't been able to find a good answer so I will appreciate any help!
 
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tomizzo said:
Hello, I have a question that I thought some of you may know an answer to.

I was recently testing an op amp is several closed-loop feedback configurations. Primarily inverting, and non-inverting.

When testing the output voltage of an invertin amplifier, I found that the output signal became attenuated at around 1.6MHz. The op-amp's data sheet says the bandwidth is 1.5MHz so this makes sense.

However, when I did the similar test with a non-inverting amplifier, I could only get up to 29KHz before bad deformation occurred. Why is this? I thought the op-amp was rated at 1.5MHz?

Does it have something to do with how many capacitors within the op-amp are being used in a non-inverting configuration?

I haven't been able to find a good answer so I will appreciate any help!

What gains were you testing at? Can you share the schematics of your test configurations?
 
The closed loop gain capabilities are a function of the input frequency. If I am not mistaken what would be supplied to you on the specification sheets are the gain bandwidth product of the chip. This can be used, along with your resistors (see 'feedback factor') to find the gain for varying input frequency. The derivation is a little cumbersome, especially in the case of the inverting amplifier, but the end result is that closed loop gain for both configurations only differs by 1 for the same feedback factor. So, for different instances of the same chip, it is odd I believe that you have received such a dramatic bandwidth decrease. As berkeman said it would be a good idea to post how the circuits are connected, I expect these feedback factors are wildly different. The feedback factor is calculated as one divided the non-inverting gain for both an inverting and non inverting amplifier arrangement. This is an oddity but not a unique one in electronics, many things that are common to both configurations (inverting and non inverting) are determined by the non-inverting configuration only. It appears not only here in frequency response but also in noise calculations and beyond.

Hopefully I didn't lead you astray if I did I am sure someone will set us both straight
 
The LM741 op-amps were powered with +/ 15V. The gain for the inverting amplifier was -3 and the non-inverting was 4.

I've posted images below of the circuits.

For the inverting op-amp, Rin = 3.3K ohm and Rf = 10K ohm.

Here is a screen shot of the output being shifted/deformed at 1.75 MHz which is outside the bandwidth of the op-amp. Refer to the attached screenshots from the oscilloscope.


The non-inverting op amp has Rg = 3.3K ohm and Rf = 10K ohm. The non-inverting output became deformed at 29Khz. The op-amp is rated at 1.5Mhz. Why is the output signal being distorted this much? Refer to the second attached screenshot from the oscillscope featuring the triangle wave.

images?q=tbn:ANd9GcQD70c1nUYza0Ed6zb6IPRKS1YZSqSrOaxcPkzstrbafsl0wpMe.png


220px-Operational_amplifier_noninverting.svg.png
 

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Strange. If you use smaller signals, like around 1V max output voltage swing, what do you get for the bandwidths?
 
LM741 has a slew rate around 0.5V/us So the Fmax without distortion for 6V peak output is equal to
Fmax = (0.5V/us)/( 2 * pi * 6V) = 13.4KHz
So your op amp has slightly better SR then 0.5V/us.
Also the small signal bandwidth should be equal to.
fc = 1.5MHz/gain = 1.5MHz/4 = 375kHz
 

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