Making a cheap microphone with a simple preamp circuit.

[Nigel Goodwin]

Sorry, but that's nothing like a radio - and 741's are completely unsuitable for RF use anyway.

I presume you're not actually building it?, but just simulating it - neither case would work, but simulation might fail even more badly, as you're doing such silly things with 741's.

Is this how the professor expects it to be done?, if so what is he a professor of? - English?.

 

[Fluffyboii]

Sorry, but that's nothing like a radio - and 741's are completely unsuitable for RF use anyway.

I presume you're not actually building it?, but just simulating it - neither case would work, but simulation might fail even more badly, as you're doing such silly things with 741's.

Is this how the professor expects it to be done?, if so what is he a professor of? - English?.

He made us build that as a radio and most of the students failed to make it work under 3 hours of lab time because the input op amps didn't worked as intended. Fortunately mine worked enough when the AM signal was directly fed into it. 175 kHz was the operating frequency so it is not like a full radio with an usable range.

Looks like silly me drew the AM envelope detector part because I forgot the RC resistor of 10K at the drawing. The correct schematic is above.

 

[Nigel Goodwin]

The first amp stage is an RF amplifier, it's not something you use an opamp for, and certainly not a 741.

What's the gain of a 741 at 175KHz?.

A 1N4148 is also a really crappy detector diode, you need a germanium one, like an OA91.

 

[Fluffyboii]

The first amp stage is an RF amplifier, it's not something you use an opamp for, and certainly not a 741.

What's the gain of a 741 at 175KHz?.

A 1N4148 is also a really crappy detector diode, you need a germanium one, like an OA91.

They supposed to have gain of 5 at each stage for total 125x gain at the output. I assume you prefer a germanium diode because those have less voltage drop. I am sure the point was just to proof the calculations and not to actually have a good performing radio. If that was the case I am sure he could gave us a better schematic. But he doesn't really make circuits himself and more like gives lectures about EE courses. Stuff like calculating currents on transistor circuits and such which I suck at.

 

[audioguru]

A 741 opamp is 53 years old and has high level slew rate trouble above 9kHz. At low levels its maximum gain is 8 times at 100khz but you have three 741 opamps trying to amplify up to 1.6MHz (AM radio band) which is impossible.
What radio band uses the 175kHz you mentioned? The poor slew rate of a 741 opamp at 175kHz produces a maximum output of only 1V peak and its maximum low level gain is 2 or 3 times.


You also have a very low capacitance (10uF) feeding 2khz and up to an 8 ohm speaker when no opamp can drive a speaker.

A TL071 opamp has slew rate trouble above 120kHz so it is fine for 20kHz audio and its maximum gain at 100kHz is 33 times.

Your envelope detector is shorting the output of the 3rd opamp.

 

[Fluffyboii]

A 741 opamp is 53 years old and has high level slew rate trouble above 9kHz. At low levels its maximum gain is 8 times at 100khz but you have three 741 opamps trying to amplify up to 1.6MHz (AM radio band) which is impossible.
What radio band uses the 175kHz you mentioned? The poor slew rate of a 741 opamp at 175kHz produces a maximum output of only 1V peak and its maximum low level gain is 2 or 3 times.


You also have a very low capacitance (10uF) feeding 2khz and up to an 8 ohm speaker when no opamp can drive a speaker.

A TL071 opamp has slew rate trouble above 120kHz so it is fine for 20kHz audio and its maximum gain at 100kHz is 33 times.

Your envelope detector is shorting the output of the 3rd opamp.

No radio uses 175Khz I think. I guess it is not proper to call it a radio. In the lab they used function generator with AM modulation option to create the signal. The output at the end gets connected to a speaker but that speaker has its own amplification circuit inside so it doesn't actually load the op amp. The envelope dropped 1.5V to around 30 milivolts which got amplified back around 3V at the last op amp. I am not the one who designed this very bad circuit and I have no idea the teacher made us do it. At least that explains the 741 chain having that odd 1.5V output clipping while it had a lot of headroom. I had to feed the first op amp 30 milivolts so that it wont clip at the output of that last last stage.

I don't know why slew rate affects gain. I thought gain bandwidth product / gain = bandwidth. I guess my lab report will not be good :/ No one teach us about Slew rate.

 

[audioguru]

Then you are not making an AM radio, instead you are receiving a signal generator set to 175kHz.
The 22nF envelope detector capacitor is 42 ohms at 175kHz which shorts the output of any opamp that has a minimum load of 2000 ohms. Add a 2k resistor in series with the diode.

Get rid of the antique 741 opamps because their maximum gain at 175kHz is about 3 times and their maximum output level is reduced by their poor slew rate to only 1V peak which is mostly used by the detector diode.

 

[audioguru]

Slew rate affects the maximum output level because the signal cannot slew fast enough then it must swing in the other direction back and forth.
The old 741 opamp has reduced maximum output signal level above 9kHz caused by its antique slew rate. Most modern opamps work at full output level up to 100kHz or higher.

Gain is at low levels.

 

[Nigel Goodwin]

No radio uses 175Khz I think.

Is it not in the LW band?, certainly 198KHz is, as it's used for Radio 4 (I think) in the UK - it used to be 200KHz, then they moved it. Had a quick google, 175KHz is definitely in the LW band.

Saw a programme the other night, about a LW transmitter in Poland - 2MW and the tallest structure in the world when built - then after 18 years they decided maintenance was required, so gave the job to someone completely unqualified, and with no idea what he was doing. He released a guy rope without providing extra support, and the entire mast came down - end of transmitter