Inverting Amplifier Input Distorted

operational-amplifier

enter image description hereI am building a small project and I used an inverting amplifier to amplify AC signal.

When R2= 220R and R1= 33k, It can work with input signal of upto 600mV but when I reduced the R2=100R and R1= 15K the maximum input voltgae (AC) I can input is 60mV. I am giving an AC signal from a frequency generator(Can there be an impedance issue between the freq gen and my input opamp).

I want to reduce the resistor because of input noise. I can see a good input noise when I reduced R2 to 100R but I don't understand why its clipping the input voltage when I increase the input.

Appreciate if you can help me understand this phenomenon.

I attached the image when R2=100R and when input is >60mV ch1=input signal ch2=output signal, Capacitor value is 10uF
enter image description here

When R2=100R and input ac voltage is >60mV input signal is ch1 and opamp output is ch2

Best Answer

The gain of your amplifier is R1/R2. In both cases, the gain is 150, so that's not the issue.

The impedance is lower in the second case. The opamp has to supply more than twice the current at the same output voltage. However, even with a 15 kΩ load, most opamps will be fine. This is unlikely the problem either.

There is therefore something else going on. Something is not as you think it is, but with the sparse information you have supplied I can only guess at possibilities:

  1. In the second case, R2 isn't really 100 Ω. My guess is that you found a resistor labeled "100", but didn't realize that actually means 10 Ω. Ordinary 5% resistors are labeled with 3 digits using a floating point format. The first two digits are the bare number (mantissa in floating point lingo), and the last digit the exponent. "XYZ" means XY x 10Z. "100" therefore means 10 Ω. A 100 Ω resistor will be labeled "101" (10 x 101).

  2. Something is happening to the power supply. You didn't say how the opamp is powered at all. Compare the + and - power of the opamp in the non-clipping and clipping cases.

  3. What exactly is it doing when you say it is "clipping"? Is the opamp output really close to the supply limits in each case? If not, something else is going on. Show a scope trace of this clipping output together with one of the supplies.

  4. You were getting more than guaranteed in the first case, and closer to the guarantee in the second. Some opamps require significant headroom, like the TL07x for example. At the low output currents of the first case, the opamp was able to swing somewhat into the headroom range, but couldn't when the larger output current was required in the second case.

Again, supply some real data and maybe we can figure this out.

Added

Now that you've shown a scope trace, we can see that there doesn't appear to be anything wrong. You are putting in a signal about 70 Vp-p, and your amplifier has a gain of -150. That would result in 10.5 Vp-p out, but your amplifier can't do that since it is running from 0 and 3.3 V supply. As a result, it clips.

How is any of this not as expected?

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