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I was trying to make a op amp based AM modulator. I used a log anti-log multiplier to generate the below waveform.

Proteus Analogue Analysis This is the output I am getting as from this circuit

AM Wave What I intended to generate

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    \$\begingroup\$ A specific question would be good. \$\endgroup\$
    – Andy aka
    May 27, 2020 at 6:38
  • \$\begingroup\$ @Andyaka I have made a few changes to my question, hope it will help you help me :) \$\endgroup\$
    – Amal Pavithran
    May 27, 2020 at 7:16
  • \$\begingroup\$ What makes you think this should work without specs for each stage? \$\endgroup\$
    – Tony Stewart EE75
    May 27, 2020 at 8:17
  • \$\begingroup\$ You appear to have shorted input and carrier ... and why are you injecting them again at the last stage anyway? \$\endgroup\$
    – user16324
    May 27, 2020 at 14:37

1 Answer 1

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Your circuit idea might be flawed and I can't understand what your waveforms are in relationship to your circuit AND, your inputs into R4 and R5 don't appear to be sourced from anywhere so, start simple using the simplest form of AM circuit i.e. a diode like this: -

enter image description here

That will produce these waveforms: -

enter image description here

All the detail should be in the schematic above but, just in case: -

  • Carrier is 1 MHz sinewave 5 volt p-p superimposed on an offset of 2 volts DC
  • Modulation is 10 kHz 2.5 volt p-p with 1.25 volt DC offset
  • Output band-pass filter (L1 and C1) tuned to about 1 MHz.

I'm using a triangle wave modulation so that I can see it's fairly linear quite easily. Here's what sinewave modulation looks like: -

enter image description here

Start here and build on that. With a bit of circuit invention it can be made to be a full modulator with suppressed carrier if that is what you ultimately aim for.

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  • \$\begingroup\$ Thank you for the answer. I found out later that the only portion working was INPUT + CARRIER going into the final opamp the rest of the output somehow had become zero. I should have also included the input & offset values which would have made my question clearer. Thank you again :D \$\endgroup\$
    – Amal Pavithran
    May 27, 2020 at 10:19
  • \$\begingroup\$ Pretty much two resistors, a diode and some offset control does what you need plus of course a band pass filter. \$\endgroup\$
    – Andy aka
    May 27, 2020 at 11:01

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