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I'm trying to implement a low-pass filter using this circuit configuration:

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I first tried implementing in Cadence using an ideal op-amp:

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At 1 MHz I already have around -2.29 dB, so it's working as intended.

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Then U replaced that ideal op-amp with a LNA that I made:

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Here's what's inside the LNA:

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The frequency response doesn't make any sense. I want a response close to the ideal case.

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Why does this happen?

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You have a 1uF decoupling capacitor at the input of your LNA. That's why you see a bandpass behavior.

This produces a zero that, in turn, produces a high pass response. Therefore, you have made an amplifier that doesn't let DC through, so your full response has more of a bandpass behavior.

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