IF-amplifier, signal becomes unstable past a certain amplitude?

Discussion in 'Wireless & RF Design' started by yoamocuy, Nov 27, 2010.

  1. yoamocuy

    Thread Starter Active Member

    Oct 7, 2009
    80
    0
    Hello, I'm working on building an intermediate frequency amplifier and it seems to be doing something that I don't quite understand. As you can see by the outputs I've attached, when the output signal is below about 1.6 V both the input and output signals are perfectly fine. Whenever the output reaches 1.6V, the output is a little bit sketchy but the input is horrible. Whenever the output reaches goes past 1.6 V, both it and the input look very unstable and it stops increasing even if the input is increased. The signal does not appear to be clipping, but rather deforming. Along with some circuit outputs, I also attached a diagram of how my circuit is hooked up. In saying all of that, I am mainly looking for either an explanation or a link to some information that would explain why my input and output become so deformed. Regards
     
  2. PRS

    Well-Known Member

    Aug 24, 2008
    989
    35
    I may be wrong, but your transformer seems to be connected in a step up mode. What is this step up value? I noticed you're getting a gain of 10 volts per volt as long as the output is small.

    You're driving a 470 ohm resistor. This is actually a heavy load for this design. For such a design I'm not sure even 1:1 would work; I would put the center tap at Vcc rather than the way you have it. There should be no unconnected lead at the primary. The voltage at the collector should be stepped up via the primary and down via the secondary windings. The 470 load is in this way reflected to the primary as a much larger resistance and maintains your voltage gain.

    The output is always going to be a sinusoid though distorted due to the fact of the tank circuit. But the input distortion is caused by harmonics that are due to impedance mismatch. And since the input is not tuned like the output is, it adds all of those harmonics and projects then via the voltage signal there.

    What is the turns ratio between the primary and secondary, and is the primary actually center tapped?
     
  3. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Try putting a 100 microfarad cap in parallel with a 0.1 microfarad cap across the 9 volt power supply.

    hgmjr
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    I don't know if your circuit is correct or not, but do not trust Simulink for simulations. Sometimes it behaves stupid, use PSpice or LTSpice.
     
  5. PRS

    Well-Known Member

    Aug 24, 2008
    989
    35
    I'll show you the "correct way" of doing this if you reveal the specific variables with respect to your transformer. Also, you need a schematic for doing it right. ;)
     
    Last edited: Nov 29, 2010
  6. yoamocuy

    Thread Starter Active Member

    Oct 7, 2009
    80
    0
    I attached a schematic of the if transformer. The impedance of the primary is 50k, while the impedance of the secondary is 500. The turns ratio from 1-2 is 77, the turns ratio from 2-3 is 66, and the turns ratio from 4-6 is 7. If I have it hooked from 1 to 3 ill have a 143:7 turns ratio from primary to secondary won't I?
     
  7. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Yes.

    66+77=143 on Primary
    And 7 on Secondary
     
  8. PRS

    Well-Known Member

    Aug 24, 2008
    989
    35
    I'm pretty sure you expected too much from your transformer. Driving a signal across a 500 ohm load is not trivial. You need to compromise voltage gain for power. God willing, tomorrow, I shall post the schematic of an IF stage such that you can understand and then generalize into whatever it is you wish to do. But now I need to sleep. God bless you.
     
  9. steinar96

    Active Member

    Apr 18, 2009
    239
    4
    I have to agree with mik3. Simulink has never been trustworthy when it comes to simulating circuits in my experience. Propably a configuration thing in the simulation but nevertheless i would try this in ltspice.
     
  10. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    A example is when you build an op amp amplifier and the output does not saturate at the battery voltage but it goes beyond it. !!!
     
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