transistor AM modulator

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mif_tronics

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i m trying to make an AM modulator using 2N3904 (common emitter, A class)but i dont know how to set the operating point in both AC and DC analyze. i need to modulate an small AC signal at about 38-42 KHz (on transistor base) and mid AC signal of 40KHz(on emitter) to get a beat frequency between them. i work on 12V power supply. is there anybody can help me?
 

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i got that circuit from "Electronic Principles 2nd Edition" McGraw-Hill. i thought that it's just like an amp A class with a variable gain because of swinging emitter current. i couldnt understand much about this circuit.
in essentials, i need to get a beat frequency between two AC sources(40KHz & 38-42KHz).can you give a good solution?what should i do after modulate them?using a filter or an envelope detector?
 
There is nothig wrong with this circuit when used as a common emitter amplifier but it's not supposed to be used as a modulator.

You don't seem to understand how it works.

C2 passes AC but not DC.

C2 will have a very low impedance to the 40kHz from your oscillator, meaning it will short it to ground.
 
AM modulator

that's what i ve got from the book. i know the capasitor will connect it to gnd but i thought it used to give a fluctuated current to emitter so the gain will vary too.
so what should i do? do you have a better solution?
 
No, the capacitor bypasses the emitter resistor, removing the negative feedback and increasing the AC gain.

The circuit you've got from your book is not designed for this purpose, it will not work as an AM modulator.
 
mif_tronics said:
that's what i ve got from the book. i know the capasitor will connect it to gnd but i thought it used to give a fluctuated current to emitter so the gain will vary too.
so what should i do? do you have a better solution?
Click to expand...

Take another look at the schemo you attached. Connect the emitter resistor between the emitter and ground. Disconnect the ground side of C2, and use that as the LO injection point. It should work as a mixer just fine.
 
Miles Prower said:
Take another look at the schemo you attached. Connect the emitter resistor between the emitter and ground. Disconnect the ground side of C2, and use that as the LO injection point. It should work as a mixer just fine.
Click to expand...

Exactly, it's drawn wrong, as you say the resistor goes to ground, and the capacitor goes to the other oscillator.

I presume the OP knows this is a totally unbalanced mixer?, and will need filtering on the output.
 
Nigel is correct some filtering will be required as the output will have four frequencies.
The two input frequencies, plus the sum of the two frequencies, and the difference of the two frequencies.
 
i m trying to use diode modulator, here i up load the scheme.
anyone can explain me explicitly how this circuit works?
 

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Miles Prower said:
Take another look at the schemo you attached. Connect the emitter resistor between the emitter and ground. Disconnect the ground side of C2, and use that as the LO injection point. It should work as a mixer just fine.
Click to expand...

That will work. I'm embarrassed that I didn't notice that.

One thing I'd like to add is that the emitter will have a fairly low impedance so just make sure your oscillator is up to driving it - you might need a buffer.
 
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