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AM demodulator

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sniper007

New Member
Hi!

Please let me know why need this last stage (Non-Inverting opamp with gain 2) ?

Is is possible to connect without this last stage but only one capacitor to eliminate DC ?

Regards,
 

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BrownOut

Banned
It's an "impeadance transformer." You need it to drive the speaker's impeadance.
 

crutschow

Well-Known Member
Most Helpful Member
You won't get much volume from an 8 ohm speaker using a 741 op amp. You need an audio amplifier for that.
 

audioguru

Well-Known Member
Most Helpful Member
1) The horrible circuit has its first opamp as an inverter that has a low input impedance of 5.1k ohms. Therefore it causes the selectivity to be low because the tuned circuit is a heavily loaded. If the opamp has a non-inverting input then its input impedance would be very high and the selectivity would be much better.

2) The circuit has only a single tuned circuit so its selectivity will be pretty bad when compared to a normal radio that has many tuned circuits.

3) The very old opamp used as the RF amplifier has a poor high frequency response. Its max gain at 1.6MHz (the top of the AM broadcast band) is only 3.
Its max output level at 1.6MHz is only 2V p-p.

4) The detector diode should be inside the negative feedback loop of a good RF opamp for good sensitivity.

5) The circuit is missing automatic-gain-control like all normal AM radios have so that weak and strong stations have the same volume level.

6) The circuit is missing a power amplifier to drive a speaker.

I wonder why the circuit needs two 18V batteries??
 

Hero999

Banned
Don't tell me your college lecturer tasked you with building this crappy circuit? If so he needs to be sacked.
 

sniper007

New Member
I found this circuit on internet (one random page). I need (for college) to design and explain one simple AM demodulator + amplifier. (how does it works...).

Can you give me one simple circuit like that (which works :D )

Thanks,
 

JimB

Super Moderator
Most Helpful Member
I found this circuit on internet (one random page). I need (for college) to design and explain one simple AM demodulator + amplifier. (how does it works...).

Can you give me one simple circuit like that (which works :D )

Thanks,
Finding a poor circuit on the internet, and then asking for a working circuit does not equate to designing a circuit.

JimB
 

Mikebits

Well-Known Member
I found this circuit on internet (one random page). I need (for college) to design and explain one simple AM demodulator + amplifier. (how does it works...).

Can you give me one simple circuit like that (which works :D )

Thanks,
So, can you explain how you think this circuit is supposed to work? eg. what is the diode and cap for?
 
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sniper007

New Member
I need to desing whole telecomunication system like:

thermistor -> wheatstone bridge -> instrumentation amplifier -> AM modulator

and

Am demodulator -> amplifier


I don't know if desing is right words, however i need to understand how this process works. (why bridge, why modulator, etc).
 

Mikebits

Well-Known Member
I need to desing whole telecomunication system like:

thermistor -> wheatstone bridge -> instrumentation amplifier -> AM modulator

and

Am demodulator -> amplifier


I don't know if desing is right words, however i need to understand how this process works. (why bridge, why modulator, etc).
Your original posted circuit would be useless for what you propose.

As in your above flow diagram, design each block accordingly. Find a thermistor, figure out how to put it in a wheat stone etc...

I suspect you are expected to make this design yourself rather than take a bogus circuit off the internet.

If your unable to complete this assignment without outside help, then perhaps you might be better off re taking the class.
 

microtexan

New Member
"If your unable to complete this assignment without outside help, then perhaps you might be better off re taking the class. "

Maybe rethinking your field of study! :eek: :(
 
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sniper007

New Member
"If your unable to complete this assignment without outside help, then perhaps you might be better off re taking the class. "

Maybe rethinking your field of study! :eek: :(

 
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