BJT biasing: the Q-point and the β shield

Từ vựng

English	Ý nghĩa
active region	active region: vùng khuếch đại
bias point	bias point: điểm làm việc

English

In the active region $I_C = \beta I_B$ with $V_{BE} \approx 0.7$ V; the bias must land the Q-point (the bias point) firmly active ($V_{CE} \gg 0.2$ V) and survive β varying 2:1 between devices.
Thévenin the divider, solve the base loop exactly ($I_E = (\beta+1)I_B$), check the region — and let $R_E$ turn any current rise into a $V_{BE}$ reduction: negative feedback inside the bias.

Example

$V_{CC} = 12$ V, $R_1 = 68$ kΩ, $R_2 = 12$ kΩ, $R_C = 2.2$ kΩ, $R_E = 1$ kΩ, $\beta = 100$: $V_B = 1.8$ V, $R_B = 10.2$ kΩ. Exact: $I_B = 1.1/(10.2\mathrm{k} + 101\mathrm{k}) = 9.9$ µA → $I_C = 0.99$ mA, $V_{CE} = 8.8$ V ✓ active. Double β to 200: $I_C = 1.04$ mA — a 5% move. The quick estimate (1.1 mA) was 11% high from divider loading; both error sources now have numbers.