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This is what I mean by simple slope detector:

enter image description here

Why can't it be like:

enter image description here

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Why is transformer coupling used in a simple slope detector?

You don't need a transformer for a simple slope detector. This circuit would work providing you select the centre frequency to be one side of the LC tank resonant frequency: -

enter image description here

Or maybe this diagram from an Analog devices article: -

enter image description here

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  • \$\begingroup\$ just from curiosity: what is the use of the transformer? some kind of sensitivity enhancement? antenna matching? \$\endgroup\$
    – schnedan
    Oct 1, 2020 at 10:36
  • \$\begingroup\$ @Andy aka Do transformer provides some additional advantage ? \$\endgroup\$ Oct 1, 2020 at 10:42
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    \$\begingroup\$ I reckon that a moderately coupled (but not excessively coupled) transformer might be able to produce a double parallel tuning effect that gives more slope hence a higher gain for the demodulation to an AM signal. I'm not 100% on this so maybe if you have some time try and simulate it. \$\endgroup\$
    – Andy aka
    Oct 1, 2020 at 10:52
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    \$\begingroup\$ @Andyaka Not only that, but double tuning can keep the slope more linear across the required range, reducing distortion on the demodulated signal. They can be a right b*tch to tune for minimum distortion. \$\endgroup\$
    – user16324
    Oct 1, 2020 at 12:44
  • \$\begingroup\$ @BrianDrummond ta for that dude. I had a suspicion it was that but couldn't find anything explicit on t'web \$\endgroup\$
    – Andy aka
    Oct 1, 2020 at 12:50

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