Q1Analog Circuits

The Fixed Biasing circuit for an NPN transistor consists of a base resistor $R_B$ and a collector resistor $R_C$ connected to a common DC supply $V_{CC}$.

Applying Kirchhoff's Voltage Law (KVL) from $V_{CC}$ through $R_B$ to the Base-Emitter junction to ground:

Solving for the base current $I_B$:

Since $V_{CC}$ and $R_B$ are fixed constants, $I_B$ remains fixed, hence the name 'Fixed Bias'.

The collector current $I_C$ is determined by the transistor's current gain $\beta$:

Applying KVL to the collector side from $V_{CC}$ through $R_C$ to the Collector-Emitter junction to ground:

Solving for the operating voltage $V_{CE}$:

The operating point (Q-point) is $(V_{CE},I_C)$. However, because $\beta$ varies heavily with temperature, $I_C$ fluctuates, making fixed bias thermally unstable.