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I created an AM modulator circuit with BJT 2N3904 for a school project. enter image description here

V1 is the modulating signal (sine wave, amplitude 5V, frequency 1KHz) V2 is the carrier signal (sine wave, amplitude 30mV, frequency 600KHz) With Tina simulation I have this output:

enter image description here

The oscilloscope show me this situation:

enter image description here

I have calculated the amplitude of the modulated signal (about 1,30V) Why I have this behaviour? How can I calculate the modulation index?

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You are approaching the sensible modulation-depth limit for this simple type of amplitude modulator. If you looked at the signal on the collector (blue) you'd see what is happening with slightly more modulation (Vm = 5.5 volts peak) applied: -

enter image description here

The blue trace (voltage on the collector) is limiting on the troughs and this causes distortion on the bottom of the filtered signal. My sim does exaggerate this slightly because Vm = 5.5 volts peak but I do so to explain that you are are reaching the circuit limits for the modulation index i.e. this circuit cannot handle modulation indexes this big.

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  • \$\begingroup\$ So, can I calculate m index? \$\endgroup\$ – Rik99 Feb 24 '18 at 16:42
  • \$\begingroup\$ Yes, if you use the correct method: See this wiki page with examples about 75% down. If you want to calculate it with a distorted waveform then you are facing problems as to interpreting the results. \$\endgroup\$ – Andy aka Feb 24 '18 at 17:21

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