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I am doing AM modulation simulation in pspice but the graph output is in error. The output is somehow rectified(so to speak) above and below the signal seems to be correct, please see the ckt and output display. what am i doing wrong?
At a quick glance, the modulation amplitude of 1V at the emitter exceeds the transistor's 0.6V Vbe so is switching the transistor off. Try reducing the amplitude to, say, 0.1V.
Edit: On running a sim in LTspice it is the amplitude of the V1 source (rather than the V2 source) which is too big; it is switching Q1 off.
BTW, your circuit is not a true AM-modulator. It merely combines the two voltage waveforms additively instead of multiplying them together.
Hi,
I tried both of your suggestion, and permutation of the two, the output is no good. attached is pic of the output when V1 source amp is 3.5V and V2 source amp is 1V, and when V1 is at 3.5V and V2 is at 0.6V.
I got this circuit from a lab manual of some university and doubt that it is wrong. I have seen this configuration in many places but I also know that it is not true AM circuit. However, this circuit should give the am waveform isn't it?
Reduce V1 to 3V to prevent Q1 going into saturation. You may also want to increase the value of C6 to, say, 0.1uF to prevent significant attenuation of the 1kHz component of the output signal.
I changed the values still no good. Do you have the calculation involving the input carrier, modulation signal amplitude and frequency required for a given transistors? Please share, I did search but without luck
I got this circuit from a lab manual of some university and doubt that it is wrong. I have seen this configuration in many places but I also know that it is not true AM circuit. However, this circuit should give the am waveform isn't it?
It may be in a lab manual but that doesn't mean it's correct.
That circuit adds the two signals which means that the output "carrier" positive and negative peaks go up and down together, the modulating signal is just riding on the carrier. The carrier pk-pk signal value stays the same.
A true AM modulated waveform has the carrier pk-pk amplitude changing with the amplitude of the modulating signal such as this.
Edit: OK, after some further reading and simulation I see how the circuit should work. It modulates the emitter current which varies the transconductance gain of the transistor thus amplitude modulating the carrier. But you need the proper mix of frequencies and amplitudes, along with the proper high-pass output filter to see the desired results. I increased the input carrier frequency to 100kHz and reduced the output capacitor to 470pF. This output capacitor passes the high frequency modulated carrier signal, but attenuates the low frequency modulating signal which also appears as part of the collector AC signal.
thanks to all,
I could not return here as I was busy. crutschow thanks for your explanation, yes, this circuit seems ok and is doing AM modulation. I found another circuit with other values of components and its output was fine. I will simulate this circuit with the values you have provided, surely it will be ok.
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It simulates fine in LTspice. 