Typically for a small loop antenna you want a very low input impedance. So you probably should consider both input noise voltage and input noise current in a common base or gate configuration.
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On 20200326 05:58:02, prog wrote:
OK. Let's play. Here's a simple preamp I was modelling during the last few days. The goal is to design a preamp for the YouLoop antenna that can be easily replicated and tested. At the very end, I noticed this can also be used as a "mini-whip" preamp. That's a desirable side-effect.
I started from a Piezo Preamp I built for my ouds <https://en.wikipedia.org/wiki/Oud> and guitars a few years ago. Basically, it's a high impedance JFET front-end with a low impedance output and a negative feedback loop to improve the linearity. The whole thing uses around 18mA at 4.5v (usable from 3v to 9v) and has a noise figure just over 1 dB. The natural frequency response is just what we need for HF. Max noise of the device is around 13.5 nV/sqrt(Hz) for ~15 dB of voltage gain. That's much better than your regular high-end opamp. The choice of the transistors is not very critical, but it's hard to find better alternatives than what is presented, considering the performance to cost ratio. I have put the LTspice circuit in the files section <https://groups.io/g/airspy/files/YouLoop/Preamp/HF_Loop_Preamp_2SK3557_MMBTH81_base.asc>.
Want to help improving it? Let's discuss.