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Here is the circuit Simple Diode Amplitude Modulator

I am a beginner so I'm not sure what exactly could be done to improve the circuit but let me go through what i understand about the circuit.

The circuit is comprised of the following

  1. A resistive mixing network to algebraically add the carrier signal and the modulating signal (I dont fully understand why the resistors are required, can't we just connect them same node directly ? Also i dont see any difference upon removing the resistor R4 despite the references i read saying its need and that the mix signal appears across it, which i mean sure but why would that have any effect on the mixing of the two signals ?)

  2. A diode which causes halfwave rectification, this makes the current running through the diode a series of pulses proportional to the amplitude of the modulating signal (I dont understand why its only proportional to the amplitude of the modulating signal and not to the carrier signal aswell)

  3. Finally the LC tank tuned to resonate at the same frequency as the carrier signal 1.85Mhz That creates a negative half cycle for ever positive half cycle of our modulated signal as the pulses of of current cause the charging during the positive half cycle and discharging during negative

If there is any issue with my understanding of the circuit please let me know

Here is the output of my circuit compared to Ideal AM modulation

enter image description here

The output is definitely modulated, however i fear that the quality simply isn't good enough

My personal suspicions as to why this is the case based on my understanding are the following

  1. The resistive mixer part of the circuits values were chosen arbitrarily through trial and error as i have no idea what exactly they should be, tunning these could improve the output.

  2. The diode i am using could simply not have the correct characteristics + my input signal may be too high of a voltage and i am aware that this would cause no-linear behavior in the diode.

  3. I miss understand the concept of the D.C offset required to offset your input signal such that through out its cycle is it positive.

Thank you in advance for the help

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  • \$\begingroup\$ My simulation of a slightly different circuit doesn't appear to create those strange distortions: electronics.stackexchange.com/questions/650895/fet-am-modulator/… \$\endgroup\$ Commented Dec 15, 2024 at 16:37
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    \$\begingroup\$ Your point number (2.)... all diodes suffer from an offset voltage of approximately 0.6V before they conduct much current. Depending on how you use the diode, this D. C. voltage might interfere with the modulating base-band voltage (10 kHz in your example), especially its own D.C. offset voltage. You might try reducing the diode's offset disturbance by increasing the A.C. amplitude of both carrier and modulating sine waves. \$\endgroup\$ Commented Dec 15, 2024 at 19:50
  • \$\begingroup\$ @glen_geek I think i understand your point, however wouldnt increasing the A.C voltage push us further into the nonlinear region of the diodes operation, and my understanding is that this modulation technique is applicable only for small signals where we could approximate the diode behavior to be linear ? Also my understanding is the D.C offset must be equal to the voltage of my modulating signal as such increasing the voltage of the modulating signal would also need a proportional increase in its D.C offset. thank you for your reply \$\endgroup\$ Commented Dec 16, 2024 at 15:48

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OP's circuit seems to use the diode as a series non-linear element to feed current from the base-band sine wave to the LC resonator. In this answer, the load is simplified to a resistor rather than LC resonator, to simplify the concepts...all that is needed is the load resistor, and two signal sources: one sine wave at base-band frequency, the other a square wave at carrier frequency.

  • Modulation here is a 1 kHz sine wave, peak voltage of 2V, DC offset of +2V.
  • Carrier is a square wave at 20kHz whose amplitude extremes are specified.

I use a square wave to make clear that the diode will act as a switch. If an actual electronic switch were to substitute for the diode, then diode offset voltage (roughly 0.7V) doesn't cause bias problems.

A first attempt ignores the diode offset: the diode's anode must be roughly 0.7V more positive than its cathode.
Modulation is a sine wave of 1kHz frequency, whose peak amplitude is 2V. A +2V DC offset is added, so that the waveform swings from 0V at its minimum peak, up to +4V at its maximum peak.
A square wave is added (20kHz carrier). Its most positive peak is 0V, switching to a negative peak of -4V. The modulator output \$V_{out-series}\$ is offset by roughly -0.7V because of the diode's forward voltage drop. This causes distortion when modulation reaches its full 4V pk-pk amplitude...for modulation amplitude lower than about 3.6V pk-pk, there will be no distortion:
Spice circuit modulator series-gate modulator waveform
Distortion caused by diode offset can be cured by increasing the carrier's upper voltage limit by +0.7V. Now when modulation reaches its +ve peak @ +4V, the sum of modulation + carrier will be +4.7V, resulting in V(out_series) to be +4V.
At negative peak of modulation (0V), V(out_series) just "kisses" 0V:[![corrected

A diode can also be used as a shunt-modulator. In this circuit, the object is to use the diode to short to ground the modulator signal on one carrier phase, or let it pass to output on other carrier phase.shunt modulator circuit
Consider the case where we ignore the diode's 0.7V anode-to-cathode offset voltage: again, distortion results because the carrier waveform cannot drag output down to 0V through the diode:shunt modulator with offset waveforms
Again, an offset of the carrier waveform reduces distortion substantially. The lower limit voltage of carrier waveform is boosted from 0V to -0.7V: shunt modulator corrected waveform

These simplified modulators require a linearizing buffer stage before driving the L-C resonator that rejects unwanted frequencies: the series modulator has output impedance that switches radically from near-zero ohms to near-infinite ohms. Shunt modulator has similar variable output-Z. The buffer stage should allow an L-C resonator to be driven from a constant, controlled output impedance.

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