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IF amps for a superhet AM receiver

  1. Aug 15, 2014 #1

    perplexabot

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    Hey all. So I am now trying to work on IF amps for a superhet AM receiver (medium wave) and found the following figure in the ARRL handbook:
    IF_AMP.png

    I have a couple of questions regarding these schematics.

    First and most importantly, do these work for IF signals (of 455KHz with BW of 18KHz)?

    Assuming the answer to the above is "Yes," these questions follow:
    For medium wave can I disregard VHF and UHF parasitics and go for the simplest schematic (schematic A)?
    Why is it that the collector is not connected to the center tap (but somewhere below/above)?
    What is the variable capacitor's job?
    Why are the inductors (of the output transformer) not the same size? Does this just mean that the transformer has a ratio of x:1?
    Why is there a capacitor connected to Vcc? Is it to prevent the signal from going to the source, Vcc?

    I am sorry for the long list of questions but I have tried to search for IF amps and this is the simplest of the circuits that I could find, yet I am not able to understand some of it. Thank you for reading.

    EDIT: Thank you for the title rename...
     
    Last edited: Aug 15, 2014
  2. jcsd
  3. Aug 16, 2014 #2

    vk6kro

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    No these won't work as IF amplifiers in a receiver. They don't have any bias resistors, so they would be very insensitive as IF amplifiers.
    Not difficult to arrange, but, as shown, they wouldn't work properly.

    The tapping positions and transformer ratios depend on the impedances that are connected to the transformer.

    The variable capacitor is used to tune a winding of the transformer to resonance. It is intended to pass a narrow range of frequencies and reject other frequencies.

    The capacitor at Vcc is a bypass capacitor. This does more than stop signal going along the power supply line.
    It is a return path to ground for the output of the amplifier and essential for the amplifier to work properly.
    It is just as important for an amplifier as a complete circuit is for a flashlight.
     
  4. Aug 16, 2014 #3

    perplexabot

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    That is disappointing, I was hoping my IF AMP search was close to an end. If I add bias resistors will it fix the problem? I am not sure what you mean by bias resistors? Do you mean biasing the common emitter with DC to setup the quiescent point?

    I see two weird things about this transformer, first the primary and secondary windings are not equal. Second, the tap isn't in the center. Why play with both these parameters to adjust impedance, instead of just altering one?

    I just recently learned about tuned transformers. I thought the capacitor would have to be in parallel with the transformer winding?

    I see, that makes sense.

    Thank you for your reply.
     
  5. Aug 16, 2014 #4

    vk6kro

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    Yes, just add resistors to apply a suitable base current to the transistors.

    The primary of the transformer is a tuned circuit. If you connect the fairly low impedance of a transistor collector across this, it will affect the resonance and cause reduced output and increased bandwidth.
    Tapping low on the coil reduces this effect.

    The variable capacitor is in parallel with the primary of the transformer. Just regard the bottom end of the coil as grounded because of the bypass capacitor.
     
  6. Aug 16, 2014 #5

    perplexabot

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    Thank you so much! Now that I know with the correct biasing these circuits work as IF amps, do you think circuit (A) will suffice for medium wave AM?
     
  7. Aug 16, 2014 #6

    vk6kro

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    IF amplifiers also have AGC or automatic gain control and these amplifiers have no provision for this.

    This is possible with bipolar junction transistors, but you might like to search for actual circuits as the way AGC is generated matters for it to work properly.

    Most circuits reduce the bias current for stronger signals. Unfortunately, this also makes the amplifier more liable to overload. So, it is a delicate balance.
     
  8. Aug 16, 2014 #7

    perplexabot

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    I was afraid of having to look into AGC : ( I have seen it around in IF AMP literature but have constantly swept it under the rug. That's what I get :rofl: . I guess it is time. I will read up on AGC and find a circuit that incorporates it. Thank you.
     
  9. Aug 16, 2014 #8

    sophiecentaur

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    AGC (historical note):
    I believe that the invention of AGC was originally due to Armand Dennis (famous and influential wildlife photographer and TV broadcaster from way back). He funded his filming expeditions on money he made from the invention. One of the first uses of feedback in electronics; where would we be without it?
     
  10. Aug 16, 2014 #9

    vk6kro

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    I love feedback loops until something goes faulty in the loop. Then you get screwy behaviour which is very difficult to locate.

    I have built receivers using mainly dual gate Mosfets.
    In the IF strips, I use transistor radio IF transformers backwards. That is, with the input fed to the low impedance winding normally used as an output.
    Surprising gain is available like this because the secondary is largely unloaded by the FET gate.

    AGC is achieved by reducing the voltage on Gate 2 or the drain.
     
  11. Aug 16, 2014 #10

    perplexabot

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    Interesting. Thanks for the info.

    Yes, I was looking into dual gate mosfets! So you flip the transformer around? I see, I will keep that in mind. I will post a circuit today with AGC (maybe even soon). THANK YOU!
     
  12. Aug 16, 2014 #11

    perplexabot

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    So here is my sad attempt at this AGC circuit along with an edited schematic (A), what do you think? I don't need something that is superb, I want something that will work. After I finish this project at the basic level, I will then start to upgrade it.
    IFAMP_AGC_sml.png
     
  13. Aug 17, 2014 #12

    perplexabot

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    EDIT: I forgot to include a capacitor in parallel with R4!
     
  14. Aug 17, 2014 #13

    Bobbywhy

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    O sophiecentaur! I found another historical note about the origin(s) of AGC:

    AM radio receivers
    In 1925, Harold Alden Wheeler invented automatic volume control (AVC) and obtained a patent. Karl Küpfmüller published an analysis of AGC systems in 1928.[1] By the early 1930s most new commercial broadcast receivers included automatic volume control.[2]
    http://en.wikipedia.org/wiki/Automatic_gain_control

    Cheers, Bobbywhy
     
  15. Aug 17, 2014 #14

    vk6kro

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    Mostly OK.

    Signal is normally connected to gate 1 of a dual gate mosfet.

    At this gate, you need a parallel tuned circuit to ground. This can use the larger of two windings on a transformer as its inductor. The IF transformers from transistor radios are ideal for this, used backwards as mentioned earlier.

    This produces a large voltage step-up which gives you good gain.

    You don't need R4.

    I would try reducing the voltage on gate 2. One way would be to put a resistor (4.7k ?) from G2 to Vcc. Then put a NPN transistor from G2 to ground. Put a bypass capacitor, 0.1uF, from G2 to ground.
    Turn the transistor on via a large resistor (100k ?) from the positive AGC to the base of the transistor, reducing the voltage on G2.

    I also used AGC to reduce the drain voltage. FETs give really poor gain at low drain voltages but no extra distortion, so this is useful for gain control.
     
  16. Aug 18, 2014 #15

    perplexabot

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    THANK YOU SO MUCH! I will start working on it as soon as possible.
     
  17. Aug 18, 2014 #16
    Is this project primarily educational? If not, there are a number of IF integrated circuits that will do the job with a lot less effort.
     
  18. Aug 18, 2014 #17

    perplexabot

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    I am working on this project in my spare time. It is "educational," in the sense that I am being educated by myself, and of course, the wonderful community here at PF.
     
    Last edited: Aug 18, 2014
  19. Aug 18, 2014 #18

    vk6kro

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    Sometimes projects like this are worth doing just because they are fun to do.

    Using a large IC may get you a result, but you won't learn much. It will either work or it won't and if it doesn't you won't be able to do much about it because the IC's operation is mostly hidden from you.

    They still need filters and minor components, so you end up with all this stuff crowded around one IC.

    At least using discrete components, you can spread it all out, see what is happening and fix it if something isn't working.

    It may not be economical though.
    I have an MP3 player that has an FM radio in it. Cost $7 delivered.
    If I had 300 songs, it could hold them and play them back at beautiful quality, and all on a single AA battery.
    How could you compete with that?
     
  20. Aug 19, 2014 #19

    perplexabot

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    So, I just finished editing the schematic as you have said in one of your previous posts. Looking at it now, I realize that the extra transistor you have told me to insert at G2 is in the common emitter configuration... Will that not in effect increase the voltage at G2, instead of decreasing it?
    I also used an nmos instead of a npn... is that ok?

    Here is an updated schematic:
    IF_AMP_v2.png
     
  21. Aug 19, 2014 #20

    vk6kro

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    No.

    If there is zero base current in the transistor, the will be no collector current.

    As you increase the base current, there will be a voltage drop across the 4.7k resistor, so there will be less voltage on G2 and less gain from the mosfet.

    Try to use an NPN. They will start to turn on as the base voltage gets higher than 0.6 volt.

    You can use any N-channel dual gate mosfet.
    I have used several types like MPF121, MPF131 and BF981 but you might have to check what is available on EBay.

    Check the voltage recommended for G2 for your mosfet. You can have a voltage divider and put an NPN transistor across the bottom resistor of the divider to get AGC control.

    It is also usual to put an source resistor in circuit. Maybe 150 ohms with 0.1 uF across it. These go between the source and ground.
     
    Last edited: Aug 19, 2014
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