Electronics > Beginners
RF Active probe input impedance
Nitrousoxide:
I'm currently designing an active RF probe (1MHz - 1GHz). I have a prototype designed, however, I have run into an issue.
Im able to impedance match all the components (doesn't really matter since the distance between nodes is very minimal) fine. It's the input impedance that Im having issues with.
The input impedance is meant to be in the order of megaohms (1-10 Mohm), but due to the nature of the internal capacitances (most likely body or cgs), the input forms an (almost) direct short to ground. Its not possible to place a high value resistor in series with the gate as that just forms a low pass filter with the capacitance which is causing the impedance to drop.
It seems like there's a massive tradeoff between bandwidth and input impedance. I know that there will always be a compromise between specifications, but is there a way to alleviate the issues? Is this something thats impossible to combat since you'd always have some form of junction capacitance? Could this just be an over-conservative model of the bf998 that Im using?
I've tried experimenting with both common source and common drain topologies, both present the same issue.
Schematics:
awallin:
the owls are not what they seem...
2.5kOhm @ 70 MHz is par for the course if you believe e.g. this:
https://www.electronicdesign.com/test-amp-measurement/choose-best-passive-and-active-oscilloscope-probes-your-tasks
can you simulate an input impedance curve vs. frequency, like in the article above?
David Hess:
At the high impedance input, impedance matching transmission line sections loses meaning and reducing capacitance is what is required. It becomes more of a mechanical problem than an electrical problem. Materials with a low dielectric constant can be used and holes cut to remove as much material as possible. A ground plane in this area is contraindicated and if present should be removed.
Bob Pease showed an example in his book shown below.
gcewing:
Instead of a resistor in series with the input, use a capacitor. This will form a capacitive voltage divider with the input capacitance, so you don't end up with a low pass filter. This is how the "x10" setting on a scope probe works -- it switches in a series capacitor with 1/9 the capacitance of the cable and scope input, giving 10 times the impedance at the expense of 1/10 the sensitivity.
Nitrousoxide:
Thanks for the tips.
I added a nominal 1pF capacitance in series with the gate which improved the input impedance. I'm getting around 500 ohms at 500 MHz and 3.8 Kohms at 70 MHz, however, as expected the signal is attenuated (can be fixed). This seems to agree with the literature that was linked in the thread.
I understand that the model presented in SPICE is only a model, and that the final implementation will probably vary by a significant margin.
Screenshot of performance for completion sake:
Browsing literature for buffer circuitry, I see the dual gate NMOS being used as a source follower (common drain). However, I would have expected a cascode configuration to preserve the bandwidth the best.
Also, Should I add an inductor (to reject RF noise) between the bias and the gate (G1) of the nmos device?
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