Hello,

In the "old" days with tube amplifiers, they used an audio transformer in the anode of the tube to modulate.
The power supply voltage and the modulation voltage will superponate.

This picture come from the wiki:
http://en.wikipedia.org/wiki/Radio_transmitter_design

Greetings,
Bertus

 

Thanks for all that I am curious about one thing when I was studying transistors on the allaboutcircuits book....

It is in the attached picture....

The transistor can amplify the total sinewave at the base if their is a suitible foward basised voltage to keep the transistor from going into cut off or saturation ..... (in the picture the base sine wave is amplified and inverted)

What I want to know is what would happen if the 15volt dc emitter-collector supply voltage was an AC sinewave voltage? ( then would the base wave by am modulated/amplified instead of just amplified )

Would the output from combineding the base sinewave and the ac sinewave voltage be equivalent to Amplitude modulation of the base sinewave.... Would it be in theory the product of these to waves ????

If the common-emitter inverting is going to be a problem then how about the same question but using a common-collector ???

Or will this also somehow effect the frequency/period of the output wave....

Curious because in the AAC book they never talk about if the voltage connect to the emitter-collector is sinewave....
And weather being sinewave will effect the frequency/period of the output wave?

Thanks for any help
And when I get a chance I will build the above schematic you just gave me.
Attached Thumbnails

Still curious about this question?

 

Hello,

When the powersource of the amplifier is changed to an AC source the power also can go negative.
As transistors do not work correctly at negative power, you need a positive DC offset to the AC source that is a little bit greater than the negative value of the AC voltage.
In that way you can modulate AM with an transistor.
The base of the transistor can have the carrier signal.
Also here goes the offset as needed.

In the tube example I gave you this is also done.

Greetings,
Bertus

 

You might be able to put a transformer in the collector circuit to modulate. Be aware that it goes both ways though, the RF will also go back through the transformer. This could be a bad thing for your sound card, so some buffering might be in order.

 

Hello,

When the powersource of the amplifier is changed to an AC source the power also can go negative.
As transistors do not work correctly at negative power, you need a positive DC offset to the AC source that is a little bit greater than the negative value of the AC voltage.
In that way you can modulate AM with an transistor.
The base of the transistor can have the carrier signal.
Also here goes the offset as needed.

In the tube example I gave you this is also done.

Ok so let me get this straight.
If the DC source was AC and I was able to forward shift the voltage so the base never goes negative or below cut off then this would be AM modulation?
So one transistor can be used for a product mixer/ Amplitude modulation?

If this is all true then AM transmitters amount to nothing more then creating an ossilator circuit , then using a transistor to modulate AM , then creating filters/ amplifiers and sending it out an anntena.
Ofcourse you could do this in an infinity amount of ways an some way/components are less buggy/better quality ... can be used in ranges of different tempatures ,...etc
(the basic princples are create an ossilation , AM modulate it with a transistor/mic or IC or whatever ..., amplify/filter...,Send it to an antenna... your done!)

Thanks

 

You don't even need a transistor. Did you look how a balanced modulator works? 2 transformers, 4 matched diodes.

 

You don't even need a transistor. Did you look how a balanced modulator works? 2 transformers, 4 matched diodes.

If this is what you are refering to then yes see attachment.

Question in this circuit their is LO , IF , and RF is the LO the carrier/ossilating circuit or is it the IF (intermediate frequency )...

I am just unsure with this picture where the audio signal should go into and where the carrier wave should be?

And I am still looking for confirmation on the correctness of a transistor with base with audio signal and carrier being the large ac voltage between collector and emitter ===> output at the collector is going to be AM modulation... I think it is true "provide that you basis the transistor so it never goes below cutoff but in the AAC book they never varied the main voltage only the base voltage/current so I am wondering if varying both modulates as AM still? Below is the orignal post look back a few post to get a better picture of what I am asking....

Quote:

Hello,

When the powersource of the amplifier is changed to an AC source the power also can go negative.
As transistors do not work correctly at negative power, you need a positive DC offset to the AC source that is a little bit greater than the negative value of the AC voltage.
In that way you can modulate AM with an transistor.
The base of the transistor can have the carrier signal.
Also here goes the offset as needed.

In the tube example I gave you this is also done.
Ok so let me get this straight.
If the DC source was AC and I was able to forward shift the voltage so the base never goes negative or below cut off then this would be AM modulation?
So one transistor can be used for a product mixer/ Amplitude modulation?

If this is all true then AM transmitters amount to nothing more then creating an ossilator circuit , then using a transistor to modulate AM , then creating filters/ amplifiers and sending it out an anntena.
Ofcourse you could do this in an infinity amount of ways an some way/components are less buggy/better quality ... can be used in ranges of different tempatures ,...etc
(the basic princples are create an ossilation , AM modulate it with a transistor/mic or IC or whatever ..., amplify/filter...,Send it to an antenna... your done!)

Thanks

 

Hello,

The attached picture is indeed a ringmodulator.
The LO is the signal from the oscillator.
The IF is the modulation signal.
The RF is the modulated output.

http://en.wikipedia.org/wiki/Ring_modulator

Greetings,
Bertus

 

Is this different in anyway to a balanced modulator that Bill_Marsden was refering to?

And can anybody answer my transistor question in previous post.
I just want to be positive I got this straight.

My next question is if AM modulation is done with product mixers either built by transistors or diodes/transformer pairs....

Then how do you modulate a carrrier with FM modulation? Another words how do you use a transistor , or diodes/transformers to vary the frequency while keeping the amplitude the same.... Would this be called a frequency mixer (or what type of mixer is this for FM modulation called if any) And how do you go about making one....

Thanks

 

And can anybody answer my transistor question in previous post.
I just want to be positive I got this straight.

Thanks

You would not be biasing the transistor if you used AC. Bias is a stable DC current.
For proper reproduction of a complete AC signal (also known as class A), you need to bias the transistor with DC current between the cutoff and saturation points of the transistor. Check out diagrams of transitor characteristic curves. An NPN will need a need positive current to bias and a PNP would need the opposite.

If you were trying to make a sine wave from a jump rope, would it work if you were lying on the ground? You could get a postive (upward) movement, but it wouldn't go negative, right? You have to raise the midpoint of the wave so that it can give a downward wave without touching the ground.



http://www.allaboutcircuits.com/vol_3/chpt_4/4.html talks about biasing much better than I could hope to.


There are other arrangements (class B C D etc etc) but none of them involve injecting power supply AC current into the base.

 

There are other arrangements (class B C D etc etc) but none of them involve injecting power supply AC current into the base.

well I meant if you used biasing and had the ac at the base of the transistor shifted up by the dc battery...etc assuming this was done then if the supply source between the emitter /collector was ac would the output at the collector be AM modulation?

I have read the transistor chapter and know all about class A B AB ,...etc but nothing in this chapter covers if the main source between collector and emitter was AC at the same time as the base has shifted up ac to remain forward biased.

Thanks for any help here.

As for my other questions about FM modulation still waiting on this one ...?

 

Hello,

I found an interesting page for you:
http://www.users.cloud9.net/~stark/commbook.htm

See chapter 8 for FM

Greetings,
Bertus

 

Thanks for the link I will look into FM modulation more and post another thread specific to FM at a later date...

But I still have my transistor question to be confirmed above? Yes/or No?
I believe this would be amplitude modulation if basied correctly...(i.e I beleive one transistor could be used as a product mixer)

 

Hello,

Take a look at page 2 of chapter 7 for AM modulation with a transistor.

Greetings,
Bertus

 

Ok , but now I am curious for this ring modulator (product mixer )
http://en.wikipedia.org/wiki/Ring_modulation

Would switch diodes be ok to use for making the diode ring?
And what 2 transformers or at least how many turns of a coil of where should I use?

And should the taps for carrier be made in the center of the coils or does it not matter that much to be exactly centered?

Thanks

Also wondering is their any other way to modulate AM without using a ring modulator , or a transistor circuit.... (leaving out for the moment useing vaccum tubes in place of the transistor )?

I would think only product mixers are the only way to AM modulate ?
Curious to know if I am wrong and if their is a better/best way to modulate AM other then using product mixers.... My thoughts is probably not...

 

Hello,

Here I have a link on making a ringmodulator.

http://www.cgs.synth.net/modules/cgsrr.html

For RF use other transformers.

Greetings,
Bertus

 

For the link you gave me I don't have the abilty to get those germanium diodes could I use in their place switch diodes 1N914 - 1N4148???

And for the transformers would any audio transformer do?
I have only radioshack 273-1380 audio output transfomer or radioshack isolatation transformer part 273-1374?

By looking at the ring modulator circuit it seems the only thing that would be significant is that the components don't drop significant voltage i.e diodes don't drop to much forward voltage and audio transformer don't resist to much. Eitherway If this occurs I should beable to amplify it after it is modulate with an LM386 or something...

But I am wondering if anybody see's a reason why the parts have to be specific.... before I try it and if it doesn't work try amplifying it????

Thanks for your reply
I think I am save to use almost any thing as long as resistance isn't to much and the transformer can respond to the frequency. Now that I think about it frequency response could be an issue.

Looking at the spec's I see nothing that stands out differently then the audio transformers of radioshack that I have from theirs
http://www.mouser.com/catalog/specsheets/XC-600134.pdf

 

You could also use Shottky diodes, which drop even less voltage than germanium. Generally toriod transformers are used for RF, the kind you wind yourself.

 

ok , last thing will the switch diodes be ok I see picture of them used here
http://www.electronics-tutorials.com/basics/diodes.htm

Half way down the page....
seems I am all set then just need to know how many turns for the coils of wire?

 

Actually that isn't as critical as you'd think. It is the ratio that is more important. The more turns the tighter the coupling, up to a point. Too few and it can still work, but efficiencies go down.