intermediate frequency amplifier circuit design

Discussion in 'The Projects Forum' started by yoamocuy, Oct 9, 2010.

  1. yoamocuy

    Thread Starter Active Member

    Oct 7, 2009
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    I'm supposed to design and build the intermediate frequency amplifier portion of an am receiver.

    I've been looking around online and correct me if I'm wrong but the intermediate frequency amplifier takes an ac signal, filters it, amplifies it, and then goes through that whole process 1 to 2 more times. After looking at some sample circuit designs online it seems that a lot of them use transformers. Is it really necessary to use a transformer? Couldn't I just build a band pass filter and hook that up with a transistor acting as an amplifier?
     
  2. Wendy

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    Mar 24, 2008
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    I suspect what you think are transformers are in fact filters. They come as little silver boxes, very modular.
     
  3. yoamocuy

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    Oct 7, 2009
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    Oh ok thanks, that clears up a lot. I think I'll just design my own filter then.
     
  4. bertus

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  5. PRS

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    Aug 24, 2008
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    As Bill Marsden said use cans. These include a capacitor and a tuning slug to vary the inductance of a coil such that you can tune in the desired frequency of 455 KHz. Otherwise you have to wind your own toroids and design a tank circuit. This is much harder to do than using cans. The slug is turned by a screwdriver (usually) and makes it possible to vary the center frequency such as to tune out the stray capacitance that actually exists between amplifier stages. Each stage will be identical to the next if you use 3 cans of the same type. Don't worry about 1st, 2nd, and 3rd stage cans, just buy one type of can and make 3 identical stages, taking into consideration the input resistance and capacitance of the next stage. If you were given an impedance for the source and an impedance for the load, these, too, will have to be considered.
     
    Last edited: Oct 9, 2010
  6. Wendy

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    Art Collins, founder and former owner of Collins Radio, invented a mechanical filter that was very profitable. We used the devil out of them for IF in his telephony systems. They were very precision on their frequency response, but a bit big and cumbersome, and as the name implies, weren't that electrical in construction (though in nature they were very electronic).

    I have spent 1½ days tuning a bandpass M derived filter to it's specs, tuning filters is not as easy as the OP thinks. This is the reason the little canned filters were created, as a major time saver.
     
  7. yoamocuy

    Thread Starter Active Member

    Oct 7, 2009
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    Ok, I found some online. I'll do some more research and see what I can come up with.
     
    Last edited: Oct 9, 2010
  8. yoamocuy

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    Oct 7, 2009
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    Ok, so apparently my prof doesn't want me to use IF filters. I did some calculations to come up with a decent gain for a bjt transistor, and attached a copy of that circuit along with it's corresponding H parameter diagram. Could I build a circuit like the one I attached, calculate the cutoff frequencies for each capacitor, and then just put a couple of those in series using different rated transistors to get the desired gain?
     
  9. Wendy

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    He doesn't want you to use IF filters for an IF amp. By definition an IF amp has filters, unless this is for something other than audio. Are you sure he doesn't want you to use premade filters?

    Generally an IF amplifier is used after a major frequency conversion, such as superheterodyne. This way on a tuned filter section works over the entire RF band.

    What is your definition of a IF amplifier? It might get us on the same page.

    Superheterodyne receiver
     
  10. yoamocuy

    Thread Starter Active Member

    Oct 7, 2009
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    I'm building an IF amplifier for a superheterodyne receiver so we are talking about the same thing I believe. In our notes he said we can only use BJT's and integrated circuits. I'm going to go meet him during office hours tomorrow though and clarify.
     
  11. marshallf3

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    Jul 26, 2010
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    You'll need some sort of tuning method.

    At some point before turning AM radios into just one IC they did make the IF amp as a building block but I wouldn't have a a clue what IC they used.
     
  12. marshallf3

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    Jul 26, 2010
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    There were some PLL based IF amp ICs at one time, the ham radio people seemed to like them. Might want to search around for those.

    Aside from that you're going to need to get a good amount of gain for a specific frequency only and reject anything above or below that frequency - that's the function of an IF amp.
     
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