Antenna input becomes zero when connected to a op amp amplifier

Discussion in 'General Electronics Chat' started by Jayvin, Mar 25, 2015.

  1. Jayvin

    Thread Starter Member

    Mar 8, 2015
    47
    0
    Hello guys, i need some insight on a ciruit design. I have an antenna which is capturing a signal and when i try to amplify that signal using a non inverting amplifier. it becomes zero. Is it due to the high input impedance of the op amps? How should i amplify the signal?

    my goal is to design a gain 10 amplifier. Can anyone help me out with this design? How do i send the signal to the op amps?

    -------------------------------
    EDIT : Signal 1kHz along with 50 Hz noise coupled with it. I am not interested in the signal in itelf but only the amplitude.. As the antenna moves closer to the transmitter the signal amplitude increases, i just need to measure the difference.

    I was going to smooth the signal later using a low pass RC filter of 0.5Hz or 1Hz.
     
    Last edited: Mar 25, 2015
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    whats the frequency of the signal,and what amplitude?
     
  3. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    why not use an mmic? 10 db gain, and a minamum of parts. check out minicircuit labs for info.
     
  4. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    How bout show us what you have so far?
     
  5. Jayvin

    Thread Starter Member

    Mar 8, 2015
    47
    0
    the required signal is a 1kHz one. In the picture there is a 50Hz noise coupled with it which i have to remove. My goal is remove the 50Hz from the signal then amplify it. I was thinking of using active filters for it, but i believe i need a method to get the signal amplified using other means before it can be amplified by the op amp.
     
  6. Jayvin

    Thread Starter Member

    Mar 8, 2015
    47
    0
    [​IMG]

    Well the input is an antenna connected to an oscilloscope. thats it. I have a transmitter which is a square wave running on a piece of wire. I am working on the receiver. So i tried putting the antenna output to a non inverting gain amplifier.When the output of the antenna was connected to the op amp. The signal reduced to zero. Is there a way i can capture the signal from the antenna and use it for signal conditioning stages.

    I used CA3140E op amps.
     
  7. Papabravo

    Expert

    Feb 24, 2006
    10,144
    1,790
    Opamps are generally unsuitable for working with RF signals from an antenna. A high frequency input above say 1 MHz. will be so substantially attenuated that it will look like zero output. In the opamp data sheet there will be a parameter called GBW or gain bandwidth product. At a given frequency it will tell you how much gain to expect. For example a 741 runs out of gas in the middle of the audio range around 9 kHz.

    You will need to learn a great deal about components and RF signals before you can design a proper amplifier. A popular joke goes that the way to design an oscillator is to start with an amplifier, and vice versa.

    However, everybody has to start somewhere. Tell us what you know, or what you think you know, and we can direct your learning.

    Edit: 1kHz. is way too low for practical RF work. To radiate any significant amount of signal the antennas would have to be huge (300,000 Meters). Connecting the antenna to the receiver input is not the same thing as radiating a signal and picking it up. You have much to learn young Jedi. Where shall we begin?
     
    Last edited: Mar 25, 2015
  8. Jayvin

    Thread Starter Member

    Mar 8, 2015
    47
    0
    Actually i need to filter out the 50Hz using an active filter then amplifier the signal. I was using a CA3140 which have a GBW of 4.5MHz which should be enough for this amplification.

    I am thinking that since an antenna is only a small piece of wire that i am connecting to the op amp, it would become the voltage that the pin was on the op amp initially? does it work this way or i am not getting it right?

    I just saw something about preamps, is that what i am looking for?
     
  9. Papabravo

    Expert

    Feb 24, 2006
    10,144
    1,790
    An antenna is one thing and a piece of wire connecting two things together is something else. If you don't need the connection between the transmitter and receiver to be wireless, and at 1 kHz. there is no reason why you should, then just call it a piece of wire and be done with it. You can design a 50 Hz. active filter, and then have a gain stage. It will help you to treat them as separate problems. First you need a filter specification.
    1. How much attenuation do you want in the stopband? For example 35 dB
    2. How wide is the stopband? For example 20 Hz., so the stopband is 40-60 Hz
    3. How wide are the transition bands on either side?
    4. How much attenuation in the passbands? For example 6 dB
    Now you can design a gain stage that brings the passband gain up to where it needs to be, compensating for the insertion loss and the desired gain of 10.

    If you want to go back and design some additional gain into the active you can of course do that so you can lower the gain of the following stage.

    For a complete introduction to filter design, I recommend Van Valkenberg, Analog Filter Design

    BTW, how are your math skills?
     
  10. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    Last edited: Mar 25, 2015
    Papabravo likes this.
  11. crutschow

    Expert

    Mar 14, 2008
    13,014
    3,234
    Is the signal shown in your original post measured directly from the antenna?
    If so what it look like when you connect the op amp?
    At 1kHz your circuit should work. Does it amplify a test signal?
     
  12. Jayvin

    Thread Starter Member

    Mar 8, 2015
    47
    0
    Yeah. directly from the antenna. I did not take the picture as i didnt even receive any signal. it was about zero.
    If i use a 1kHz signal from a signal generator, the amplifier works.
     
  13. Papabravo

    Expert

    Feb 24, 2006
    10,144
    1,790
    That's an interesting website, with some practical circuit designs that appear useful. What puzzles me is why you would be interested in listening for natural phenomena in this frequency range which is "low" but not explicitly specified. I'm guessing the upper limit is near the lower part of the AM broadcast band at 500 kHz.
     
    Dodgydave likes this.
  14. Jayvin

    Thread Starter Member

    Mar 8, 2015
    47
    0
    I need to design something like this.

    [​IMG]

    I need to personalise this preamp for my purpose. how do i do that? i seriously lack the knowledge.
     
  15. Jayvin

    Thread Starter Member

    Mar 8, 2015
    47
    0
    The first part is a high pass filter, to remove the dc in the signal. Now how do i select the JFET?
     
  16. Papabravo

    Expert

    Feb 24, 2006
    10,144
    1,790
    Since the particular JFET is not specified, the implication is that any JFET you select will work to some degree or another. Order a handful of each of three different devices and try them out.
    OR
    Use the one the author used in the earlier circuit; it was a 2SK117 IIRC. If you decide to build the more complicated circuit you'll have some on hand.
     
  17. Jayvin

    Thread Starter Member

    Mar 8, 2015
    47
    0
    Actually the system is a receiver that will acknowledge that it is near a 1kHz source. To do that i need to amplify the signal first. My goal is to amplify the signal i am getting from the antenna.

    I think it might be due to the antenna lacking a ground. thats why opamps are not working. I have to condition it before sending it to the op amp,
     
  18. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    How about you tell us what you are trying to receive?
     
  19. Papabravo

    Expert

    Feb 24, 2006
    10,144
    1,790
    I don't think your analysis is correct. I think the problem is that it is extraordinarily difficult for a 1 kHz source to radiate much energy from a random length wire. If you look on the website you'll see that antennas are AC coupled to the high impedance inputs by means of a capacitor. You haven't really shown us a schematic of your setup yet. Why don't we start there?
     
  20. Jayvin

    Thread Starter Member

    Mar 8, 2015
    47
    0
    There is no information on the signal. It is a plain 1kHz signal that is being captured. EMF. I will probably be using an RC Low pass filter of 0.5-1Hz to make it a smooth line on its average.
     
Loading...