help with this very simple Op-Amp FM receiver

Thread Starter

tsukaranz

Joined Aug 23, 2011
24
i found this on the net while googling...
i was amazed of it cause it has a few parts in it...


my question is does it really work? because i made my own, but it didn't work...
 

wayneh

Joined Sep 9, 2010
16,102
Many of the replies to that post cite good results, so yes it can work. It's basically the old crystal radio set updated to use an op-amp. Many others report bad results like your's. The devil is in the details.

I suggest you post more details about your build and maybe someone here will spot a problem.
 

Thread Starter

tsukaranz

Joined Aug 23, 2011
24
well i guess my problem is with the coil and with the trimcap,... i don't know the specific values for them...
i am hoping for at max range of 108Mhz...
hope you can help me
 

steveb

Joined Jul 3, 2008
2,431
Um, well this is only going to receive AM stations. FM receivers require a more sophisticated detection method.
 

Adjuster

Joined Dec 26, 2010
2,147
Even an AM detector normally requires a DC load and filter capacitor! Try adding a resistor of about 10kΩ to ground from the junction of D1 and C2. Also add a capacitor of 10nF in parallel with the resistor. Finally, change the coil to about 300μH with a variable capacitor of a few hundred pF to tune the AM broadcast band and it might work, in very a limited way.

Basically this is an elementary AM detector plus AF amplifier circuit which has lost a few bits along the way. Confusingly, you might even hear something with the circuit as shown if some RF got coupled via the diode's self capacitance into the input of the op-amp, where some non-linearity might crudely demodulate the signal.
 

Audioguru

Joined Dec 20, 2007
11,251
The circuit is supposed to be an AM radio, not an FM radio.

The opamp has one input (pin 3) at the same voltage as its negative supply pin (pin 4) so it will not work. Pin 3 must be at least 3V more positive than pin 4.

If the opamp is biased properly then the radio still might not work because parts are missing.

I fixed it 4 years ago:
 

Attachments

KJ6EAD

Joined Apr 30, 2011
1,568
Very nice AG, two different op amp versions no less. What would it take to make it work with ordinary headphones or earbuds, another amplifier stage?
 

Audioguru

Joined Dec 20, 2007
11,251
Very nice AG, two different op amp versions no less. What would it take to make it work with ordinary headphones or earbuds, another amplifier stage?
Ordinary modern headphones are 32 ohms for each ear.
An opamp is overloaded when its load is 1k ohms or less.
The LM386 is not an opamp, it is a power amp that can drive a speaker or headphones. But an LM386 has hiss that can be heard through headphones so you should make your own little power amp with a low noise opamp driving output transistors.

Don't use a 741 opamp (it is 43 years old) because it is extremely noisy and limits its high frequencies to 9kHz.
 

#12

Joined Nov 30, 2010
18,076
Well done AG. When I saw that suggestion last night, I broke out in laughter from expecting you to blow a fuse. Only a beginner or a troll would suggest a 741 in a reply to you.

Troll: an internet poster trying to cause a reaction by posting incredible or outrageous comments.
 
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