A cascode stacks a common-emitter (CE) amplifier underneath a common-base (CB) stage. The bottom transistor does the voltage-to-current conversion. The top transistor passes that current to a high-impedance load while clamping the bottom collector to a nearly fixed voltage. That clamping is the whole trick.

Why it exists: the Miller effect. A CE stage's collector-base capacitance C_cb (a few pF in a 2N3904) gets multiplied by the voltage gain when reflected back to the input:

• C_Miller = C_cb × (1 + |A_v|)

For a CE amp with A_v = −100 and C_cb = 4 pF, the input sees an effective 404 pF. Drive it from a 1 kΩ source and your −3 dB bandwidth collapses to:

• f_c = 1 / (2π × 1 kΩ × 404 pF) ≈ 394 kHz

Now cascode it. The bottom transistor's collector swings only into the emitter of the upper transistor — an impedance of roughly 1/g_m, maybe 25 Ω. The local gain at that node is about −1. Miller multiplication essentially vanishes:

• C_Miller ≈ 4 pF × 2 = 8 pF
• f_c ≈ 1 / (2π × 1 kΩ × 8 pF) ≈ 20 MHz

A 50× bandwidth improvement, and the overall stage gain is unchanged — the upper transistor passes current essentially 1:1, so all the voltage gain still develops across the load resistor at the top.

Bonus: output impedance. The cascode pair has output impedance approximately g_m·r_o·r_o instead of just r_o. That's why every modern op-amp's gain stage is a folded cascode — it lets you build 100 dB of voltage gain in a single stage.

Concrete example: cable TV tuner front-end. A JFET (low noise) cascoded with a BJT (high f_T) gives you a low-noise input device and bandwidth up to several hundred MHz, without the input capacitance choking the antenna source.

Bias the upper transistor's base from a stiff reference — a zener, a divider with a bypass cap, or a bandgap. Any noise on that node modulates the bottom collector voltage and shows up at the output. Allow about 1 V of headroom for each transistor's V_CE, so the cascode needs roughly 2 V more supply than a plain CE stage to stay out of saturation.

Rule of thumb: if your CE amp's gain-bandwidth is disappointing and you have an extra volt or two of headroom, cascode it before reaching for a faster (more expensive) transistor.