Hello, I have built the op-amp radio described on this site and am surprised that something so simple can deliver such good results. However I have come across a problem that is it only seems to work with the 741 op amp. I have tried the LF 353 JFET and the LM358 which work well as audio amps but not the radio. Anyone any ideas why? Also, would it be possible to adapt the circuit to make a FM receiver? Or alternatively does anyone know of a fairly basic circuit to make a FM receiver?
The Opamp Radio circuit is a mess and should not work: 1) The reference input (pin 3) is at 0V but the input of an old 741 opamp does not work when its voltage is less than +3V. 2) The opamp is inverting which has an extremely low input impedance which destroys the very weak signal and ruins the tuning. 3) The negative feedback resistor value is 10M which is much too high for an old 741 opamp because its input current is too high.
It is supposed to be a very simple crystal radio with an opamp to drive the earphone. But a crystal radio does not have the radio frequency and IF frequency very high gain of a real radio for good sensitivity (without an external antenna and external earth ground)and does not have the many tuned circuits (for good selectivity) of a real radio and does not have the automatic volume control of a real radio.
You can look at the datasheets of the very old opamps you found yourself to see why they don't work.
With reference to my original Op Amp radio circuit post.
Thanks for the reply. I include a description of the circuit that works very well with the AM signals here in the UK. It is an IN 1458 dual op amp using the first element as the radio the second as an audio amp. With a short arial and good earth connection it drives standard earphones. Substituting the other op amps as described before the circuit does not work but both do work as additional audio amps to drive a small speaker. As my electronics knowledge is pretty basic don’t know how to look at a data sheet and decide why one works and the others don’t I just thought there may be a simple reason why.
A 1458 has two lousy old 741 opamps in it. In the circuit you posted, the first opamp is in exactly the same opamp radio circuit as the project here. The second opamp is an audio amplifier with a voltage gain of 47k/22k= 2.14 times. It can drive an earphone but not a speaker.
I'm just being curious, i just don't get why anyone would want to build an op-amp driven crystal radio? Why not build a regenerative type or a TRF which is more selective and sensitive enough if you preferred using external power anyway. Like i said i'm just being curious and i'm not stopping anyone to build the op-amp version...
OP AMP Radio Question
Posted - Mar 04 2011 : 2:49:49 PM
Op Amp Radio.JPG