The marvelous disappearing capacitor

Have used that JFET trick on large area photodiode amplifiers in a satellite based radiation detector. Works quite well.<p>Also quite similar is actively driving the shield used in many high impedance sensor circuits (most notably capacitive touch sensing) to reduce the parasitic capacitance you would get when ground is used for the E-field shield:<p><a href="https:&#x2F;&#x2F;www.ti.com&#x2F;lit&#x2F;an&#x2F;snoa926a&#x2F;snoa926a.pdf" rel="nofollow">https:&#x2F;&#x2F;www.ti.com&#x2F;lit&#x2F;an&#x2F;snoa926a&#x2F;snoa926a.pdf</a>

This is called a bootstrap circuit. It is basically positive feedback that changes the input impedance. You can see this sort of jfet circuit not only in trans-impedance amps, but also in a lot of audio circuits.

Digikey has a useful article as well. Note how in Figure 5 the PD is reverse-biased to reduce the capacitance without any extra components.<p><a href="https:&#x2F;&#x2F;www.digikey.co.uk&#x2F;en&#x2F;articles&#x2F;how-to-use-photodiodes-and-phototransistors-most-effectively" rel="nofollow">https:&#x2F;&#x2F;www.digikey.co.uk&#x2F;en&#x2F;articles&#x2F;how-to-use-photodiodes...</a>

AoE and AoE X has a bit on TIAs as well, their &quot;final form&quot; circuit idea enhances the basic JFET follower idea with an additional emitter follower to reduce source impedance and by AC-coupling the compound follower&#x27;s output they get to set favorable DC working points.<p>The tricky bit is going to find a suitable replacement for the BF862.

This reminds me of a somewhat similar circuit, the capacitance amplifier:<p><a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Capacitance_multiplier" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Capacitance_multiplier</a><p>Which leads me to think about planned obsolescence in electronic circuits. In many electronic circuits the electrolytic capacitors are the first to degrade, limiting electronics lifetime to 10+ years. What if those capacitors were replaced with amplified capacitors where the capacitor was not electrolytic, say a film capacitor. Maybe the circuit would work the same and last 20-30 years, eliminating a lot of electronic waste and reducing environmental carbon due to manufacturing? Part of the solution to climate change has to be manufacturing products that last longer. Electrolytic capacitors, built in batteries, fans, and parts made of rubber instead of silicone are some of the prime components that cause failures. I have a suspicion car companies have tuned steel formulations and paint to cause rust after a specific number of years as well.

Very cool! Can this trick be used for piezo sensors as well? My understanding is that those too are essentially magic capacitors that sometimes move charges on their own