Vceq

Vceq, commonly written VCEQ, is the DC quiescent collector-emitter voltage of a bipolar junction transistor (BJT) at the operating point (Q-point) in a biasing circuit. It represents the voltage across the collector-emitter junction when no signal is applied and the transistor is in its active region. VCEQ is a central parameter in bias stability and load-line analysis, helping to predict swing and linearity of a transistor amplifier.

In a typical common-emitter amplifier with supply voltage Vcc, collector resistor Rc, and emitter resistor Re, VCEQ can be found from the DC bias conditions. VCEQ = Vc − Ve, where Vc = Vcc − Icq Rc and Ve = Ie q Re. Since Ie ≈ Icq for large transistor beta, Ve ≈ Icq Re and VCEQ ≈ Vcc − Icq(Rc + Re). A more exact expression uses Ie = (β+1)/β · Icq. VCEQ is also related to the quiescent current Icq and the quiescent power Pq = VCEQ · Icq.

Design considerations: the goal is to place the Q-point in the transistor’s active region with adequate headroom for the desired signal swing, often choosing VCEQ near half of Vcc for symmetric output swing. Ensure VCEQ remains well above the saturation voltage (typically around 0.2 to 0.3 V) to avoid clipping, and account for variations in beta and temperature. The Q-point stability can be improved with feedback biasing or emitter degeneration. Understanding VCEQ helps in plotting the DC load line and predicting amplifier performance.

Example: with Vcc = 12 V, Rc = 4.7 kΩ, Re = 1 kΩ, and Icq ≈ 1 mA, Vc ≈ 7.3 V and Ve ≈ 1 V, giving VCEQ ≈ 6.3 V.