Project Circuit Help

[rheian2]

View attachment 67895

I would like to ask what would be the theory of operation of this circuit

 

[audioguru]

The value of the 2F (2 Farads!) capacitor is much too high. Maybe it should be 0.2uF or 2uF.
The old opamp is not powered and has poor performance above 9kHz.

It looks like the 1kHz input causes the Jfet to conduct more or less. When it conducts then the gain of the opamp increases so its 20kHz output is higher. When it conducts less then its 20kHz output is lower.
So the circuit is an Amplitude Modulator.

 

[rheian2]

Yes Thank For The REpLy .. hehehe..........

"If I put power to the op amp what would happen??....."


i have resimulated the circuit using the newer version of multisim and it does work in 0.2uF and 2uf

but the older version of the multisim has an error and that circuit simulation does does gave me an error

 

[Megamox]

Hi rheian,

This looks like an inverting amplifier. The N-Jfet is essentially a variable resistor and so when you couple the oscillations on to the gate, you vary the resistance between the drain and source. This gives the op amp a variable gain output. The input sinewave will then be amplified more when the resistance between the drain and source of the n-Jfet is low so you'll get amplitude modulation. The n-Jfet's resistance goes up when the Vgs voltage goes negative. I've never built or simulated one of these, so it's just a guess!

Megamox

 

[rheian2]

Thank very much for the reply .................. actually i'm still a student............. and i dont know the theory of operation......of the circuit...that why i need help from the experts here.....^^..

thank you again for your answers............^^....

 

[unclejed613]

for a 1khz input to the jfet, use a 2uf or 10uf cap. also, i fail to see why in this day and age of op amps capable of 300Mhz or more of bandwidth (and there have been excellent audio op amps since the late 70's, that are readily available just about ANYWHERE) people continue to try to use 741's for audio. 741's are not any good above 9khz or so, they have huge amounts of input noise, and relatively large input bias currents and input offset voltages, and are (if you happen to use one without internal compensation) unstable and prone to oscillation (in the circuit you submitted above, the compensation pin is left open). while this may be a great training aid to learn the differences between an "ideal" op amp compared to a "worst-case" real-world device, it's not suitable for many applications that are filled by better types of op amps. try this circuit with a TL072 or LF353, or even better, a TLE2072 or other op amp designed for audio applications.

 

[mines]

im sorry ...anybody here can help me...how to find source code for wireless smoke detector,,,,
i need it A.S.A.P ...plez help me?,,
thank you