For small frequencies (20-20000Hz) the opamp has output more than -3dB
If we make a low frequency AM receiver , can we use an opamp for demodulation instead of a diode and for amplifications instead of a transformer?
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The transformer gives gain without noise, but you can replace the diode with an LM1496 balanced Demod with 85dB carrier suppression.
An Op Amp demod is too noisy and poor carrier suppression at 1MHz due to lack of GBW.
I think, looks like to me and forgive me if I am wrong, you confuse modulation signal (the information you want to obtain) and carrier that is the high frequency signal which its amplitude is modulated. Again, if I am not wrong, when you refer to the range 20~20000Hz you refer to audio signals that is the usual bandwith. The 20KHz can be managed by any opamp but the several MHz used for carriers not.
I found this circuit from wikiHow that is about a demodulator using opamp: