A standard cascode kills the Miller effect by holding the common-emitter transistor's collector at a fixed DC voltage, so the effective load impedance the input transistor sees is just 1/g_m of the cascode device. But there's still a leftover parasitic: the collector-substrate capacitance (C_CS) of the lower transistor, and any stray wiring capacitance at the cascode node, still get charged and discharged by the signal current. In discrete RF and high-Z instrumentation front-ends, this residual capacitance can dominate above 100 MHz or push input impedance below what you need.

The bootstrapped cascode fixes this by AC-driving the cascode bias node so it moves with the input signal, not the fixed rail. If the bias node tracks V_in exactly, then the voltage across every parasitic capacitance from that node to the input side stays constant — and a capacitor with no dV/dt draws no current. The parasitic effectively disappears.

Topology: Standard cascode with Q1 (common-emitter, input) below Q2 (common-base, cascode). Normally Q2's base sits at a fixed DC voltage from a divider. In the bootstrapped version, you AC-couple the input signal (or better, a buffered replica from Q1's emitter) into Q2's base through a capacitor, while still setting the DC bias with the divider. Q2's base now wiggles in phase with the input.

Real-world example: Charge-sensitive preamps for semiconductor radiation detectors (Si(Li), HPGe) need input capacitance under 1 pF to preserve energy resolution — the detector's own capacitance plus the amp's input C sets the noise slope (ENC ~ C_tot·√(1/τ)). A JFET cascode front-end typically has 2-4 pF of residual Miller and stray. Bootstrapping the cascode gate drops this to ~0.5 pF, improving resolution at short shaping times by a factor of 3-5×.

Rule of thumb: The bootstrap is only as good as the tracking. If the buffer driving the cascode node has gain A_b instead of unity, the effective capacitance is reduced by (1 − A_b). So a bootstrap buffer with A_b = 0.98 shrinks parasitic C by 50×; A_b = 0.95 gives only 20×. Use an emitter follower with heavy local feedback, or a source follower biased at high current to push A_b above 0.99.

Watch out for: Bootstrapping adds a positive feedback path. If the buffer's phase shift crosses unity gain at the wrong frequency, the whole stage oscillates. Always include a small series resistor (10-100 Ω) in the bootstrap path and check loop stability with a Bode plot.