OP AMP for AM amplifier

Discussion in 'Wireless & RF Design' started by jrdoner, May 28, 2013.

  1. jrdoner

    Thread Starter New Member

    Feb 21, 2012
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    I have volunteered to teach a (very) simple electronics course for a few interested teenagers in a summer program. I remember as a little kid that my aunt gave me a crystal radio kit. I was amazed that such a simple thing could actually receive radio stations, so I figured out why it worked, and that turned me on to electronics for the rest of my life (now 70).

    But although elecronics is my hobby, I've never played with RF before. What I want to add to the crystal radios that my students build is a little solar powered (5 v.) antenna amplifier. I'll put the antenna input though an RC high pass filter, then use an opamp to multiply the signal by about a factor of 10. Then I'll feed that to an RC low pass filter to chop the amplified signal range to AM plus some amount. I like this idea because it represents relatively simple technology that I can explain to the kids, without burdening them with a huge amount of theory.

    But being a complete novice at RF design, I have two questions.

    1. Can someone suggest an opamp is reasonably cheap (like $1.00), will operate single-sided and include ground in the input range, will operate on 5 v., and has adequate BW product ?

    2. In order to get good response over the AM band, where should I set my 3 db cutoff frequencies?

    Being naive about RF, I may have missed some other important aspects of this scheme, so feel free to critique my approach.

    Thanks in advance for any suggestions.
     
  2. MrChips

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    Before you try an opamp, try using a single transistor common emitter amplifier after the diode rectifier to amplify the audio signal.
     
  3. jrdoner

    Thread Starter New Member

    Feb 21, 2012
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    I want to amplify before the diode, because the diode requires a minimum voltage above which the signal must be to get any reception. Amplifiying the antenna signal means more signals can "jump over" the .34 v needed for diode conduction.
     
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    If I was doing this I'd use the MK484 TRF receiver. Usually easy to implement & works off supply voltage as low as 1.5V.
     
  5. jrdoner

    Thread Starter New Member

    Feb 21, 2012
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    Yes, that would be ever so simple, but you missed the point: I purposely picked a crystal radio for this project because it is only involves very elementary theory. I don't have time (nor am I qualified) to teach a graduate level course in integrated circuits.
     
  6. t_n_k

    AAC Fanatic!

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    An op amp is an integrated circuit.

    Plenty of folks without engineering degrees have most likely built the TRF receiver. A simple block diagram tells the story well enough.

    My suggestion was based on the overriding principle that young people like to get things done & dusted in short order. No doubt the higher cost of the MK484 IC would be a deterrent.

    How about a simple FET pre-amp with the crystal set option? There's plenty of info around on that approach.
     
  7. vk6zgo

    Active Member

    Jul 21, 2012
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    You are making a radio which doesn't look like other radios,so it doesn't really teach the students much,as Op Amps are not usually used in this manner.

    I'm not sure how well you will go with finding a Op Amp which will have enough gain over the frequency range required to be useful,but if you do,there are other problems associated with this idea.

    Substantial wideband gain prior to the detector of a simple receiver is not a good idea,for the following reasons:

    (1)Noise is amplified by the same amount as wanted signals,so that your noise level is not improved.

    (2)Unwanted signals are also amplified by the same amount as wanted signals,so local strong signals may have a high enough level to override the selectivity of the Crystal Set,& interfere with the signal to which you are tuned.

    There is a reason why early tube radios were called "TRFs"--Tuned Radio Frequency!

    It was not at all beyond the technology of the day to produce untuned amplifiers with reasonable gain & around 2MHz bandwidth,but Engineers discovered that,apart from being expensive,it produced a poor receiver,with the problems referred to above.

    A tuned preamplifier stage supplied more gain,more selectivity,& thus effectively less noise,& largely prevented the breakthrough of strong unwanted signals at the cost of an additional tuning control.
    ( TRFs had their own problems,but these are not pertinent to this discussion)

    Untuned,or bandlimited preamplifiers are used in some applications,but
    this is in conjunction with Superheterodyne receivers of reasonably sophisticated design.
    Even in these cases,untuned gain must be used with caution.

    My suggestion is to place any gain between the tuned circuit & the detector,or leave the Crystal set alone,& just amplify its audio output.
     
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  8. jrdoner

    Thread Starter New Member

    Feb 21, 2012
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    Thanks, That sounds like a good improvement on my original idea. I really haven't messed with radio before, so I welcome the suggestion.
     
  9. davebee

    Well-Known Member

    Oct 22, 2008
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    I'd suggest keeping the radio as simple you can make it, and have it actually work.

    Teenagers already have plenty of devices that are too complicated for them to understand that deliver clear, loud audio.

    What they don't have is a working radio that is simple enough that they can understand it.

    Don't worry about volume, or clarity, or tuning multiple stations; being able to receive a single station is so amazing, even if it's distorted and low volume. Explaining the basics of that would be a complete first course in radio. The concept of 3db cutoff frequencies is probably way too advanced for a beginner class.

    If you have a local AM station that is powerful enough to drive earphones with no amplification needed, I think that would be way more exciting than a complicated circuit with batteries and transistors.

    If you really need an amplifier, maybe you could get away with just a 9 volt battery and a MPF102 FET or something simple like that.
     
  10. MrChips

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    Just been playing around with the amplified crystal radio idea today.
    I took an MPF102 FET and amplified the RF signal straight out of the antenna LC tuned circuit before rectification. I will post the circuit later when I get a chance.
     
  11. bertus

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  12. MrChips

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    I finally found some time to play with my homemade crystal radio set.

    Here is my basic crystal radio:
    [​IMG]
    The antenna is about 30 feet (10m) of hook-up wire. Works well indoors and even better outdoors.
    The coil is made by winding about 40 turns of 24 AWG enameled wire on toilet paper roll.
    The headset is a high impedance piezo electric earphone.

    Here is the coil and tuning capacitor mounted on a piece of wood and using wood screws as connection points.

    [​IMG]


    Selectivity is quite good and I can pick up three local AM/MW radio stations.
    I am only using one section of the ganged variable capacitor.
    I also added a second coil which greatly improved signal strength and selectivity. I will post this a bit later.
     
    Last edited: Jun 9, 2013
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  13. MrChips

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    Here I have added a MPF102 FET in the front end to amplify the RF signal.

    [​IMG]


    The power source is 3V from two AA batteries. You could use two AAA batteries or one 3V coin cell instead.

    Now it is no longer a true "crystal radio" but the signal strength is much improved.
     
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  14. MrChips

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    Next, I added an AF amplifier using a regular 2N3904 NPN BJT after the diode rectifier. The big advantage here is you can now use a regular low impedance headphones instead of the hard to find high impedance headphones.

    The sound quality is absolutely amazing.

    [​IMG]
     
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  15. bertus

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  16. MrChips

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    Thanks. I have seen this RF bypass capacitor in other circuits.
    I have built many crystal radios and I have always found that it makes no difference with or without.
     
  17. WBahn

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    Mar 31, 2012
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    A variable capacitor along the lines of this one should be pretty easy to make from scratch. I've heard that it is fairly easy to find a cat-wisker diode using a new penny (which has a highly pure copper plating on it) and a rusty needle/nail. I don't know how "easy" it is to really get it to work. But let's assume that there is a way to make a detector using everyday components around the house.

    The last thing is an earphone. Is there any way to make a working (even if just barely) high-impedance earphone using everyday materials?

    If so, then despite today's wisbang, high-def gadgets, I still think you have a change of capturing a kid's attention when you take a box of junk (especially if you pulled the stuff out of a trash can sitting in the corner that just happen to be seeded with the right trash) and fashion a radio that works, no matter how poorly.
     
  18. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Last edited: Jun 10, 2013
  19. WBahn

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    The foxhole radio, in general, I've known about for some time. This reference does provide some information about the improvised headphones and the reference is draws upon,

    http://bizarrelabs.com/foxhole.htm

    provides some more.

    I don't know that there is really enough information to reconstruct what they did.

    Also, we must keep in mind that people such as POWs often have a lot of time on their hands and often focus and become obsessed with building something just to give their day purpose in order to keep from going insane. Who knows how many hours were spent over the course of how many months/years and how many failed attempts were involved.
     
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