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When making AM radio, I know that Band pass filter is necessary. As you know, there's 2 ways to make R L C Band pass filter, one is parallel R with parallel LC and the other is series R L C. But almost every AM radio circuit I saw uses R with parallel LC. Is there any special reasons?

*****These are 2 circuits that used different R L C Band pass filter**** enter image description here

enter image description here

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  • \$\begingroup\$ Draw some schematic of what you mean. Just from what you've given, I'd expect either sensitivity to component tolerance, and/or required values of components could be the reason. \$\endgroup\$ – Joren Vaes May 1 '17 at 14:21
  • \$\begingroup\$ I'd also like to point out that there are many more than "2" ways to make a bandpass filter. I'm not quite sure what you mean by that. \$\endgroup\$ – Joren Vaes May 1 '17 at 14:25
  • \$\begingroup\$ I added 2 different AM radio circuits(with no amplifier). \$\endgroup\$ – youngjae May 1 '17 at 14:31
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    \$\begingroup\$ Are these two circuits equivalent in terms of band pass and band rejection characteristics? If not then your question becomes difficult to answer because values play a big role in the decision. Do you reasonably expect the output of a real mixer to work into an input impedance of 11 ohms? This is beginning to look like a waste of time. \$\endgroup\$ – Andy aka May 1 '17 at 15:01
  • \$\begingroup\$ And do you expect the 2nd filter to be able to deliver any appreciable power to a load with 86 kohm output impedance? \$\endgroup\$ – The Photon May 1 '17 at 15:34
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Here are some things to look at when looking for differences between these circuits:

  • What is the input impedance of the filter, in both the pass-band and the stop-bands?

  • What is the output impedance of the filter, in both the pass-band and the stop-bands?

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Let me confuse you more... Would you say the following circuit is series-resonant or parallel resonant? It is certainly resonant at 455 kHz., and delivers power from the generator to the detector load (R2).
An LC resonant circuit at 455 kHz. You'll find that many LC circuits used in AM radios have tapped coils, or coils having different number of turns mutually linked, so that source impedances and load impedances can by matched. Yes, most of their inductance resonates with a parallel capacitor, but it is limiting to say strictly "it is a parallel resonant circuit", especially where greatly-differing impedances are involved.

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