Frequency: Miller, cascode, cascade

Từ vựng

English	Ý nghĩa
coupling capacitor	tụ ghép
Miller capacitance	điện dung Miller
dominant pole	cực chi phối

English

The low corner: each coupling capacitor against the resistance it sees with sources zeroed, $f_L = 1/(2\pi R_{eq}C)$. The high corner: Miller capacitance — a capacitor $C$ across a stage of gain $-A$ behaves at the input as $C(1+A)$ — under the dominant-pole rule $f_H = 1/(2\pi R_{eq}C_{eq})$.
The cascode kills the gain across the capacitor and reclaims the bandwidth; the cascade (stages in series) shrinks corners by $\sqrt{2^{1/n} - 1}$ per stage added.

Example

The CE stage: coupling sees $R_B \parallel r_\pi = 2.08$ kΩ, so 10 µF gives $f_L = 7.7$ Hz. Miller: 5 pF across a −80 stage becomes $405$ pF; with 20 pF stray, $f_H = 187$ kHz. Cascode (Miller killed): $1/(2\pi \times 2\mathrm{k} \times 25\mathrm{p}) = 3.18$ MHz — 17×. Two cascaded 187 kHz stages: −3 dB at $187 \times \sqrt{2^{1/2} - 1} = 120$ kHz.