I'm working on freq counter pre-amp. Freq range is HF and maybe VHF.
I want it to be like an active probe with JFET input. The idea is straightforward - the signal goes to JFET as a source follower and then immediately into an amplifier.
Clearly this has been implemented many times over, so I'm studying prior art. I found a few similar schematics where the signal is fed through some sort of compensation network? I've highlighted it on the images.
I'm guessing this is the frequency compensation to improve linearity? Does not seem to be relevant here.
Or is it meant to form capacitive divider and matching resistive divider with JFET gate capacitance to source? Attenuation? Why do you need attenuation here? Component values do not match in the top picture then. 10pF/gate-capacitance is not like 39k/1M Ω
DC blocking capacitor on the input 470nF in one case and 100nF in the second case would have SRF somewhere within 0-50MHz band for sure, are they compensating for the SRF of the input cap?
I often see caps thrown at input. Are they operating them as low value inductors on the other side of SRF? Getting 100nF cap that work to VHF will be challenging.
Two level limiting diodes would add to input capacitance, why it's not a concern?