I studied electronics a long time ago (and forgot almost everything) but now I would like to create a FET amplifier up to 100 MHz of extremely small signals (e.g. 1nV or even smaller). Signals are so small that an opamp (I believe) is not possible to use. I would like to simulate it first (ngspice or LTspice) and then actually build it.
I know the noise level will also be around some nV...but please lets assume I work with extremely low temperatures and this is not an issue.
Power efficiency is not an issue (could be 1% if needed) and power supply could be at 20-30 V if needed.
I though about using the cascade amplifier with one FET and one BJT NPN with a current source driven by 2x BJT PNP transistors (using a current mirror). This works well until ~1 MHz, but not to 100 MHz. I believe my problem is finding a good FET transistor (maybe a GaAsFET?)
A good starting place was this circuit (not using a current source...but close enough): https://www.designworldonline.com/cascode-amplifiers-2/
However the FET (https://pt.mouser.com/ProductDetail/Broadcom-Avago/ATF-34143-BLKG?qs=RuhU64sK2%252Bt35SXiAk9qwg%3D%3D) is no longer for sale.
I actually don't understand this circuit in the R5 resistance: since the transistor is a FET, isn't the gate current zero? Why is R5 there? I simulated this circuit (using the linear spice model) and I see that R5 does make quite a difference in the spectrum...hence I can only "conclude" that its because we are using the linear spice model and not an accurate model (doesn't seem to have V_th for example). But I think I'm missing something.
So I guess my main problem is how to find a good FET transistor that:
- works from 100 kHz to 100 MHz (with a good gain). If this is not possible, then at least from 1 MHz to 80 MHz would be nice.
- has a "full" spice model (not just the linear model since I want to see the transient response)
- Is currently being sold (not deprecated)
But of course, if there is a better way to design this amplifier please let me know.
Many thanks in advance for your help!