Building an amplifier

Discussion in 'General Electronics Chat' started by Jayvin, Mar 8, 2015.

  1. Jayvin

    Thread Starter Member

    Mar 8, 2015
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    Hello guys, can you please help me out in building an amplifier with about 5mV pk to pk to 5 volts. the frequency is of 1kHz to 10kHz. There is a dc offset on the 5mV signal, which is from an antenna. I have tried using op amps it doesnt seem to work as predicted, can i use BJTs or FETs?

    i need to actually compare two same type of signals which should have different amplitudes with same waveform as the antenna are side by side. However the amplitude is too small.
     
  2. #12

    Expert

    Nov 30, 2010
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    Have you tried using 2 stages with a gain of 31.6 each?
    A gain of 1000 invites all kinds of interference and requires 100% shielding, no matter what you use for an amplifier.
    But yes, you can use single transistors to do this. It just won't happen in one stage (that I know about).
     
  3. Jayvin

    Thread Starter Member

    Mar 8, 2015
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    I assume you are talking about opamps, when i connected the opamp, the signal got lost. I tried adding a unity buffer and resulted in the same.
     
  4. #12

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    It has been my experience that op-amps are spectacularly immune to, "losing" their input.
    Can you post your schematic so we can see how you wired it?
     
  5. Jayvin

    Thread Starter Member

    Mar 8, 2015
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    There is a notch filter before the amplifier.
     
  6. #12

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    In theory, that looks like it will work, but that's theory.
    You did not show how you built the circuit. You showed how you planned to build the circuit.
    You left out the part about what kind of antenna picks up audio frequencies and how you addressed its input wiring.
    A gain of 1000 will make the input offset of 5 mv become 5 volts. You need to trim the offset out and AC couple the second stage or make the first stage with low gain (about 20) and then AC couple the second stage.
     
  7. Jayvin

    Thread Starter Member

    Mar 8, 2015
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    Let me explain the project, i am building a cable locator, that will locate a particular frequency, in this case 10kHz. Now there is 50Hz noise being captured by the antenna, assume the antenna being a piece of wire right now.

    I decided to filter out the 50Hz using a passive notch filter. Should i have filtered the dc before the notch filter?

    Ok, if i add a lets say 47uF electrolytic capacitor before the notch filter stage, it would remove the dc completely. Adding this signal to a unity gain feedback, give a zero. (simulated on livewire)
     
  8. BR-549

    Well-Known Member

    Sep 22, 2013
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    This is very low. The lowest I ever played with was 200 khz years ago.

    Is that 5 mv from the antenna just noise or is it a signal?

    What kind of antenna are you using? What is the coupling technique?

    Are you using filtering below and above the bandpass?

    Sorry for being so selfish. Your post gave me a work around to another problem I have been studying.

    From your post, I would quickly guess a bias or power rail reference problem.

    If you supply a schematic, I'm sure #12 can solve this better than I.

    Although a DC component on a long conductor would not surprise me. Especially a loop. That's a lot of wire at those frequencies.

    What are you expecting to find there?

    I have heard tell of earth signals, solar signals, galactic signals, cosmic signals and submarine signals.

    What's your flavor?

    Pardon me.....I did not see the intervening posts.
     
  9. #12

    Expert

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    The amplifiers you chose have an offset of up to 5 mv. You have to address that, too. That's why I said to, "trim it out". Some op-amps come with adjustment terminals for their offset...or you can build an offset circuit.

    If you just design this for the neighborhood of 10KHz, you won't need a notch filter. 10KHz is so far from 50 Hz that it gets a lot easier to use a high pass filter on the input.

    ps, I am still baffled by the idea of a long wire picking up audio frequencies and what its impedance would be. Probably just, "really high". At that range, you are going to pick up AM radio broadcasts, too.
     
  10. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    A circuit gain of 1000 at a bandwidth of 10 KHz is a bare minimum gain-bandwidth product (GBP) 10 MHz. That's a lot, especially for a part designed in the 70s. If you go with the minimum rule-of-thumb that there needs to be at least 20 dB of negative feedback to get you a reasonably high input impedance and low output impedance, now the required GBP is 1 GHz. If you are committed to the CA3140, splitting your gain into two successive stages is the only way to get there.

    Separate from that, I think the overall plan has issues. First, your simulations might not be accurate. You show two signal generator outputs tied directly together. If the output impedances are zero, the indicated combined signal is incorrect. Also, two identical filter sections in series rarely give you twice the filtering as intended because the input and output impedances are incorrect. So, a few questions:

    1. What is the intended notch frequency of each filter section?

    2. What is the output impedance of the two signal sources?

    3. What is the source of the input DC offset? It looks like over 30 mV, too much for the input of a voltage follower stage.

    4. Have you measured the DC offset in a real circuit, or is it just in the simulation?

    5. Is there any reason not to AC couple the stages?

    6. Is there a reason not to use an active filter stage?

    7. Are you sure the 50 Hz interfering signal is not common mode? A well-balanced differential amp or instrumentation amp might solve you problem with much less filtering.

    ak
     
  11. Jayvin

    Thread Starter Member

    Mar 8, 2015
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    It is the minimum signal that i am getting after rejecting power cables EMF. The signal itself should be about 50mV of 10kHz from the antenna with a 100mV of 50Hz. After notch filter, it attenuated to a pk to pk of 5mV with a dc of 30mV.
     
  12. Jayvin

    Thread Starter Member

    Mar 8, 2015
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  13. #12

    Expert

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    I just remembered...if your Vin is oscillating around zero volts, an amplifier with no negative power supply will lose half the wave form.

    I also think picking up audio frequencies with a gigahertz antenna is absurd, so I'm going to step out of this before I make a fool of myself.
     
  14. Jayvin

    Thread Starter Member

    Mar 8, 2015
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    everytime i connect it to the Op amp it loses the signal even if i use an ideal op amp.
    can i amplify it using a BJT or FET first, then move to op amp
     
  15. Jayvin

    Thread Starter Member

    Mar 8, 2015
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    1. Notch centre frequency 50Hz.
    2. the signal is source is an antenna. In this circuit it is being simulated on livewire

    I am new to circuit design, i have only studied the basics of it. I don't really have in depth knowledge of circuits.

    I tried Active filters and the results attenuated to a much greater extent.
     
  16. #12

    Expert

    Nov 30, 2010
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    Here is a sweet jfet amplifier. Designed for a millivolt level input, built, tested, sold, and works just fine.
     
  17. Jayvin

    Thread Starter Member

    Mar 8, 2015
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    Thank you i will try simulating that and see what it gives. If the signal is enough for my purpose then no more op amps.
     
  18. BR-549

    Well-Known Member

    Sep 22, 2013
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    With a Tayloe detector and a laptop, you can visually see the whole spectrum of interest. This should be easy for someone who can solder and has good eyes. A very accurate and high quality unit can be built for less than $50. You connect an antenna to the detector. Then connect detector to stereo mic input or aux. input to laptop. Then connect detector to usb for frequency and bandwidth control. The control and display software is free. The software can modulate OR demodulate EVERYTHING. You can make and adjust any type of filter you want.

    Then you can determine where is problem is.

    Neater than a skeeter's peter.
     
  19. Jayvin

    Thread Starter Member

    Mar 8, 2015
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    Using two of these stages did give me a better output. How do i shift the voltage back to dc. I mean how do i add an 2V dc offset to it? and is there a way i can linearise the peak, such a a rectified signal? I am only interested in the magnitude of it for comparison.
     
  20. Jayvin

    Thread Starter Member

    Mar 8, 2015
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    I know, but for my project, i only need to compare the magnitude of the signal. If it can pick up 50Hz then kHz should not be a problem as i am not particularly interested in the signal itself but only its magnitude which would be varying according to my transmitter.

    My goal is to capture the frequency, then compare its magnitudes. Is it increasing or decreasing? Thats it.
     
    Last edited: Mar 9, 2015
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