Standard op-amp voltage followers already have high input impedance — often 10¹² Ω looking into the amplifier itself. But the moment you add a bias resistor to ground (which you need for AC-coupled inputs to give the input bias current somewhere to flow), that resistor becomes your input impedance. A 1 MΩ bias resistor gives you 1 MΩ input impedance, period. For measuring high-source-impedance sensors like piezoelectric pickups, pH probes, or electret microphones, that's often not enough.

The bootstrap trick: instead of tying the bias resistor to ground, tie it to the op-amp's output through a capacitor. Since the follower's output tracks its input almost perfectly (gain ≈ 1), both ends of the bias resistor move together. Almost no signal current flows through it. The effective AC impedance becomes enormous.

Classic topology: AC-couple the input through C1 to the op-amp's non-inverting pin. Add R1 from that pin to a midpoint node. Add R2 from midpoint to ground (DC bias path). Add C_boot from the op-amp output back to the midpoint node. At DC, R1+R2 sets bias. At AC, the output drives the midpoint through C_boot, "lifting" the bottom of R1 to follow the input.

The math: If the follower has voltage gain A (typically 0.999+ for a good op-amp at audio frequencies), the effective input impedance becomes:

Z_in(effective) = R1 / (1 − A)

With A = 0.9995 and R1 = 1 MΩ, you get 2 GΩ. That's a 2000× improvement from one capacitor and one wire. The catch: as A rolls off with frequency (op-amp bandwidth), the bootstrap effect degrades. And any parasitic capacitance from the input node to ground bypasses your bootstrap — you must guard the input trace with a driven shield.

Real-world example: A guitar preamp for a piezo pickup. Piezos are essentially small capacitors (~1 nF) — they behave as voltage sources with high impedance. If your input impedance is 1 MΩ, the −3 dB rolloff sits at 160 Hz, killing your bass response. Bootstrap to 100 MΩ and the rolloff drops to 1.6 Hz — the whole audio band is flat. This is why boutique piezo buffers cost real money: they get the bootstrap topology, guard rings, and low-bias-current op-amps (JFET or CMOS input) right.

Rule of thumb: Size C_boot so its impedance is under R1/10 at your lowest frequency of interest. For R1 = 1 MΩ and f_low = 20 Hz, C_boot ≥ 80 nF — use 220 nF film for margin.