2016年12月23日金曜日

Checked if OP-Amp could generate AM modulated voltage signal

I watched a tutorial video on the internet which explained an AM modulation circuit used  two Operation amplifiers "TL-072".

I checked if Operation amplifiers could generate AM modulated voltage signal.
Now the result is  in the negative.










Consideration:

The circuit above generates RF carrier as square wave but it is not sin wave.
There is no AM modulated signal when I use the circuit of  the tutorial.
As operational amplifiers has no characteristics of analog multiplier,  it is very difficult as far as I know.

To fix this subject, I think following circuits are required.
(1) One oscillator which can generate sine wave 1[MHz]. V(t)=Asin(ωc*t)
(2) One multiplier known as "Gilbert-cell" multiplier such as NE-602 or NE612.
(3) One baseband amplifier which can amplify audio MP3 player. 
(4) One adder. It is possible to configure it to use only two registers such as 10k ohm before #(3)'s amplifier.
(5) DC voltage source to be added with audio baseband voltage signal.

Vam(t) = {Vdc+x(t)}*Asin(ωc*t) ...(A)
Equation (A) is AM voltage signal's. 

One "adder" in the operation {Vdc+x(t)} is required.
One "multiplier" in the operation in "*" is required.
(OP amps can not do multiply here.)


Reference:
"Amplitude Modulation tutorial and AM radio transmitter circuit " 
a tutorial video on the internet