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I'm trying to build an electric field receiver to detect VLF waves so my bandwidth will essentially operate within the range of 1 kHz to 100 kHz. I will have a short whip antenna (1-2 m long) as my antenna and I am trying to figure out the design for the preamplifier.

I'm a beginner at this so I may be approaching this wrong and I wanted to get some feedback over my design. I don't have a detailed circuit diagram yet but as a rough concept, I'm thinking of using a differential JFET common drain (source follower) as the input stage (which will have high input impedance and low output impedance). This stage will have really low noise and low gain since this stage affects the noise figure of the whole amplifier the most.

Then, for the gain stage, I am thinking of using a differential BJT common base pair which will be DC-coupled. This stage will bring the most gain and again needs to be low noise as well.

For the output stage, I am thinking of using a voltage follower op-amp with a step-down transformer. (I am not sure of this stage. Maybe using an emitter follower is a better idea?)

If I can optimize this design, would it work pretty well for my case in terms of gain, noise and bandwidth? Also would this be a better design over a conventional JFET input op-amp/instrumentation amplifier?

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  • \$\begingroup\$ vlf.it/cr/differential_ant.htm describes a differential preamp with dipole antenna. Have also seen a single JFET common-drain preamp. We live in a 1/f environment where noise sources at these low frequencies are dominant. \$\endgroup\$
    – glen_geek
    Commented Apr 20, 2017 at 3:54
  • \$\begingroup\$ What input impedance are you expecting? Feedback on your design - try adding a schematic, not words. \$\endgroup\$
    – Andy aka
    Commented Apr 20, 2017 at 7:21

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Low noise design starts with assessing the impedance level required. The best amplifier for a 1k source impedance (OPA211, 1.1nV) is not going to work well with higher impedance (2m whip antenna up to 100kHz) because of its large noise current. For your application you need a FET input amplifier as the front end.

You should start experimenting with something as simple as TL071, which although the noise is 18nV, has essentially no current noise, so suits your source well. A little bit of looking around will bag you FET input amps with lower noise voltage, but if you want the best, you should brew your own with discrete FETs, following the general ideas of Bob Pease in this article, using FETs instead of bipolars at the front end.

Then again, once you've consulted the atmospheric background noise spectrum in Kevin's post, do you need better than 18nV anyway?

Once your first stage has enough gain, the noise contribution of any subsequent stages becomes irrelevant. If your first stage has again of more than 3, and first and second stages have equal noise, the first stage noise will totally dominate the second stage.

Just a thought. At 100kHz, the wavelength is km, so a number of antennae is not a phased array, it's just a group of antennae, for any practical size. Use 4 antennae, into a quad amplifier, and add the outputs. Environmental noise will be correlated so will add as voltage, amplifier noise is uncorrelated, so will add as power. Every time you double the number of recievers, you improve the signal to noise ratio (SNR) by 3dB.

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Your biggest source of noise at low frequencies will probably be atmospheric and man-made rather than circuit generated. Your basic design sounds reasonable though

Atmospheric noise (Wikipedia)

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  • \$\begingroup\$ I'm basically trying to detect that atmospheric noise but yes, man-made noise will probably my problem. I still want to make the amplifier as noiseless as possible though. Do you think this design would work better than a conventional JFET input instrumentation amplifier or should I stick with that? And if not, do you think for the output stage I should use an emitter follower instead of a voltage follower+transformer? Thanks. \$\endgroup\$
    – Starior
    Commented Apr 20, 2017 at 4:04

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