[aaaahmed]

hiii there !! plz anyone can help me?? i am trying to design analog multiplier to use as AM modulator for communication system using op-amp with multisim

 

[shrtrnd]

What's 'multisim'?

 

[aaaahmed]

it is a software used for simulation
http://en.wikipedia.org/wiki/NI_Multisim

 

[CDRIVE]

Are we talking about audio modulation here?

Chris

 

[aaaahmed]

Yes sir I need to get an output which is multiplication of the message signal with (low frequency) by the carrier signal with (high frequency)
Output= m (t) . c(t)

 

[CDRIVE]

Yes sir I need to get an output which is multiplication of the message signal with (low frequency) by the carrier signal with (high frequency)
Output= m (t) . c(t)

That is rather... no, ... it's extremely odd. AM is always impressed on the finals for a very good reason. If you inject it prior to carrier multiplication, which is usually operated class B or C, the audio will be garbage when it gets to the finals. Even if the multipliers were operated class A the audio would still be garbage after the first multiplication.

Chris

 

[aaaahmed]

Sorry sir in fact I did not understand your intention

 

[CDRIVE]

AM prior to the multipliers or non class A (effective) stages will generate copious levels of spurious signals across the RF spectrum. It will not endear you to your governing communications agency either. In the old days, prior to cable and sat TV, it would have your neighbors gunning for you because missing a segment of the Lone Ranger was unthinkable!

Chris

Edit: Oops, I see your in Iraq. You may have to Google the Lone Ranger.

 

[aaaahmed]

Thank you sir for your information but still I have to complete my design it is required from me by my supervisor do you have any idea about this design using op-amp??

 

[CDRIVE]

We need more information. What is fo? Is it modulated by voice?

Chris

 

[Laplace]

Try googling "Gilbert cell amplitude modulator". It might give you some ideas.

 

[aaaahmed]

I will use a software for simulation to design a schematic consists of op-amp,,, I try to multiply message signal and carrier I want to get the result and compare it with amplitude modulation source result which already exist in the software I must get approximately the same answer from my designed circuit and the AM source,,,, no sir I will not apply voice I will use a signal of low frequency from a function generator in the software that I use

 

[aaaahmed]

thank you Laplace but i need to use op-amp not transistors

 

[Harald Kapp]

aaahmed:
1) please don't shout using bold and colored lettering.
2) what is "low frequency" ? Anything between say 10 Hz and 20 kHz? If so, you can treat it as audio, even if it comes not from an audio source but from e.g. a signal generator.

If you insist on using OpAmps for the multiplier, look up this article or any of these:
http://2005.iccas.org/submission/pap...multiplier.pdf
http://en.wikipedia.org/wiki/Analog_multiplier
http://www.daxia.com/bibis/uploadasp...2%B7%CD%BC.pdf

 

[CDRIVE]

Well I can see that we're talking about a different animal here (Balanced Mixer) and not a classic AM envelope and a totally different application than I thought. That said I would be remiss if I didn't comment that this is sounding more like home work with each post from the OP.

Tread with caution mates. <<< I'm morphing into an Aussie .

Chris

 

[duke37]

Horrowitz and Hill give a circuit (p98 in my copy) of an operational amplifier with a gain of +1 or -1.
It is a virtual earth amplifier with the + input conncted either to the input or ground.

This gives a balanced modulator but frequency response will be limited.

 

[Laplace]

If you are limited to the op-amp as your only active element, then the attached technical paper (first 2 pages) may be of interest. It shows how to make an analog multiplier using only op-amps.