I already have an oscillator with the carrier signal. Currently using a diode modulator. Can anybody advise me on how to convert it to a transistor modulator?

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Is this way correct? i seem to be getting a 9v output. Sorry i am a complete newbie.

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Are you trying to do AM modulation?

 

A diode is just a p-n junction, and a transistor is two of these junctions (either npn or pnp). So, a transistor is like 2 diodes put together. You can just use the base and emitter of your npn transistor (in the schem.) as a diode. Or you could use the base and the collector. Don't use all three terminals though.

I did a project just like this one this past semester in a comm. class. I'd be glad to help more, just keep posting and i'll keep checking.

A diode modulator like this is for AM modulation.

 

Your first circuit will not work at RF with a 1N4001, this is a low frequency rectifier diode. A 1N4148 would work much better.
You may also need a bit more than 1 volt from the oscillator to make it work in a reasonable manner.

The second circuit would produce AM if you drive the base of the transistor quite hard with your oscillator and then put the modulation on to the collector.
The best way to do this is to use an audio transformer, connect the modulating signal to the primary winding, and connect the secondary winding in series between the battery + and the tuned circuit.
You should then have a reasonable approximation to AM at the collector of the transistor.

JimB

 

Your diode and transistor circuit won't produce a symmetrical AM waveform. The circuit that JimB suggests will, and is commonly used in AM transmitters

 

My suggestion is to replace the resistor (next to your incoming signal source) with about a 0.1uF capacitor. Also, the 10K resistor should be connected to VCC (or the + side of the 9V battery) instead of the + of the function generator. Place a capacitor in parallel with it (hook the capacitor so that both ends touch each end of the 10K resistor).

The resistor and capacitor values need to be played with to get the best performance. I recommend a 2N3904 NPN transistor.

Also, play with the inductor value and the capacitor hooked in parallel with it to get the desired frequency.

 

It is for AM transmission. thanks for all the suggestions. Will let you guys know how it went once i test it out in the lab

 

Neither of your circuits look like modulators?, they are both mixers!.

As already suggested by JimB, an audio transformer is the commonly used method for AM modulation.

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Nigel

Mixers are modulators. Yes, really they are!

JimB

 

Very, very poor ones! :lol:

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If you are thinking of audio mixers, which are just adders, then that's true. However, mixer is a very old name for a modulator. Witness double balanced mixers, which are modulators.

 

No, I wasn't thinking of audio mixers :lol:

A double balanced mixer can be used as a modulator (quite sucessfully) because it is double balanced, so the output will be (near enough) the modulated carrier, and the sum and difference frequencies.

The crude non-balanced mixers above will also output both the input frequencies - so it's really rather nasty for a 'modulator'!.

As already suggested, using a transformer to increase and decrease the HT supply is the classic way of providing AM modulation, usually on the output stages so you can use class C RF amplifiers. For a signal generator type application, you can also do it 'nicely' by using a transistor in the HT feed, and feed the modulation input to this transistor, so the HT to the RF output stage varies up and down in time with the modulation.

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