Transistor Biasing

Why biasing comes first

A transistor amplifies an AC signal by riding it on top of a steady DC current, without that DC baseline (the Q-point, or operating point), a signal that dips negative would just cut the transistor off instead of being amplified. Biasing circuitry exists purely to set that DC baseline before any signal is applied.

Fixed-bias circuit equations

In the simplest bias circuit, a resistor $R_B$ feeds base current from the supply, and a resistor $R_C$ sits between the supply and the collector:

where $V_{BE}\approx0.7\text{V}$ for a silicon transistor and $\beta$ (also written $h_{FE}$) is the transistor's current gain.

Worked example

A fixed-bias circuit has $V_{CC}=12\text{V}$, $R_B=470\text{k}\Omega$, $R_C=2.2\text{k}\Omega$, and $\beta=100$. Find $I_B$, $I_C$, and $V_{CE}$.

Solution:

Since $V_{CE}$ lands comfortably between 0V and $V_{CC}$, the transistor is biased in its active region, ready to amplify a signal riding on top of this DC point.

Fixed bias like this is simple but sensitive to $\beta$, which varies between individual transistors and with temperature. Voltage-divider bias is the more common practical alternative specifically because it holds the Q-point stable even when $\beta$ drifts.