Where should the quiescent point be placed for maximum symmetric output swing in a common-emitter amplifier?

• A. At mid-supply
• B. Near cutoff
• C. Near saturation
• D. At the supply rail

Answer: (A)

The central idea is to maximize undistorted output range by centering the operating point so the signal can swing equally in both directions. In a common-emitter stage with a collector resistor to the supply, the output at the collector can move from near the supply voltage (when the transistor is off) down to roughly the saturation voltage (a few tenths of a volt above ground) when the transistor is fully on. To get the largest symmetric swing, bias the transistor so the collector voltage sits about halfway between those two limits. If the bias is near the top, there’s lots of room to swing downward but little room upward, causing distortion on one side. If it’s near the bottom, the opposite happens. Centering the quiescent point around half the supply voltage provides roughly equal headroom in both directions, yielding the maximum symmetric output swing.