Sinusoidal (opamp 741) Amplitude Modulator

[moicez]

Hi I want to make a low frequency AM circuit, since I wont be using this circuit for air transmission it ok to use low freq., just as long as its above 20kHz for the carrier. I will be transmitting audio signals.

The circuit that I'm showing is a design example of a 10.5-kilohertz Wien-bridge oscillator. What I need help is selecting a way of controlling the "Optional amplitude control" resistor through a microphone or whatever audio signal. I am not sure if should use a PWM, a Voltage Controlled Resistor using FETs or opamp Aplifier.

Any suggestions would help and also an example circuit would be very nice.

Thank you.

**broken link removed**

 

[audioguru]

a lousy old 741 opamp is 43 years old and has trouble above only 9kHz.
Use a better opamp like a TL071 that costs almost the same and is available almost everywhere. It goes to 100kHz.

 

[moicez]

Cool that helps, I figure that I would use a different opamp but now I know witch on to use. Any ideas on how to modulate the amplitude?

 

[carbonzit]

Try a Google search using these terms (be sure to include the quotation marks to narrow your search):

"am modulator" circuit schematic

You'd be surprised what you can find this way.

F'rinstance, there's this simple (1-transistor) modulator.

 

[moicez]

Thanks carbonzit,
There are reasons for using an op amp, like really need to control the quality of the carrier and it needs to be a consistent frequency through use. Also I want to completely remove the audio signal from the output. That is the problem that I'm finding with these simple AM circuits, they usually mix both signals.
So an Op Amp is something that I know how they work and can count on them for generating the sinusoidal signal that I need, I just need to modulate the amplitude.

 

[carbonzit]

So do you really need us to tell you how to add "op amp" to your search terms?

 

[moicez]

well that's what im no sure of " if should use a PWM, a Voltage Controlled Resistor using FETs or opamp Amplifier", to control the amplitude.

 

[crutschow]

For low distortion AM modulation you could use an analog multiplier (Analog Devices makes many models). The carrier goes to one input and the modulation signal to the other.

You could also use an MC1496 modulator chip (see Fig. 28 in the data sheet).

 

[audioguru]

Cool that helps, I figure that I would use a different opamp but now I know which one to use.

I told you that a TL071 opamp works well up to 100kHz, is inexpensive and is available almost everywhere.

Any ideas on how to modulate the amplitude?

Your Wien bridge oscillator already has a voltage controlled resistor using a Jfet. If you modulate the gate voltage of the Jfet then the circuit will produce AM.

 

[moicez]

Cool JFET it is, thanks.

 

[atferrari]

I do not know why I cannot see the Wein bridge circuit.

I would like to undesrtand the Audioguru suggestion.

 

[crutschow]

I do not know why I cannot see the Wein bridge circuit.

I have the same problem. The picture just shows as a small rectangle with a red x through it.

In my case I believe it's my computer firewall that prevents certain picture types it deems dangerous from going through.

 

[ericgibbs]

I have the same problem. The picture just shows as a small rectangle with a red x through it.

In my case I believe it's my computer firewall that prevents certain picture types it deems dangerous from going through.

hi Carl,
Uploaded and saved as png, reposted, I hope its the one you mean.

 

[crutschow]

hi Carl,
Uploaded and saved as png, reposted, I hope its the one you mean.

Eric,
That's it.
Thanks.