Amplitude modulation with JFET

Question

I have found a circuit that does AM modulation with a JFET transistor (2N5485). I modified it slightly and ended up with the circuit below. A sample output is also given.

I used Microcap 12 for the simulation.

For my message signal I used an AC amplitude of 0.5 V and I also gave a DC bias of -1.7 V to the gate of the JFET. The frequency of the AC is 500 Hz. For the carrier I used a 40 kHz AC signal of 0.5 V amplitude.

I noticed that if I play a little with the DC bias (close to the threshold voltage of -1.71 V) it changes the output noticeably and hence the amplitude modulation.

I also noticed that the message signal in red is not exactly the envelope of the blue (modulated output).

Is there anything I can change to have better modulation?

I am new in this field of electronics so I am sorry for anything I might have misunderstood about the whole process.

AM output (blue) and message signal (red):

Answer 1

Your AM signal is overmodulated. For AM signal the carrier signal amplitude must be higher than the amplitude of the modulating signal. The reason behind this is as follows:

The message signal amplitude rides up and down (to visualize consider sine wave) on the peak of the carrier signal acting as zero origin. That is the carrier signal peak is like the x-axis for the message signal. As an analogy, it is like a man (message signal) is running over mountains (carrier). When the man's height is less than the mountain, that is, message signal amplitude is lower than message signal, there is no overmodulation. Now, imagine that the message signal amplitude gets bigger and bigger and eventually gets higher than the carrier signal. Now the message signal on the upper half as well as from the lower half gets lost (phase reversal) and this condition is called overmodulation.

My blog post: AM modulator using JFET transistor has video showing simulation (animation) how overmodulation results in AM circuits.

I also would like to suggest some improvement on the circuit posted by the OP. First tip would be to apply the carrier signal into the source terminal side. The inputs (message signal and carrier signal) should be fed into the modulator using transformer (with/without capacitor for filtered input). The output AM signal should be taken from drain using transformer (with/without capacitor for filtered output). Doing this provides port isolation and hence less harmonic distortion (although I have not shown this in my blog post).