The operating point (also called the Q-point, quiescent point, or bias point) is the set of DC voltages and currents a device sits at when no signal is applied. For a diode it is the pair ; for a MOSFET it is ; for a BJT it is . It is the anchor point on the device’s – curve about which all signal behaviour happens.
How it is found
The operating point is found by DC analysis: zero every signal source and solve the circuit using the device’s large-signal (non-linear) model — the Constant-voltage-drop model or Exponential diode model for a diode, the square-law equation for a MOSFET. This is Step 1 of Small-signal analysis. For the worked diode example with V, k, the CVD model gives the operating point from a single KVL equation.
The operating point matters because the small-signal parameters are evaluated at it. The Diode small-signal resistance is — it depends entirely on where Q is. Move the bias current and you change the gain of every amplifier built around the device. The bias point is not incidental; it is a design choice.
Why Q must sit in the active region with room to swing
An amplifier only amplifies linearly if the device stays in its active region (saturation for a MOSFET, forward-active for a BJT, the steep forward part of the curve for a diode) throughout the entire signal swing. The signal rides up and down around Q. If Q is placed too close to an edge of the active region, one side of the swing pushes the device out of the active region — into triode, cutoff, or saturation for a BJT — and the output clips: it flattens off instead of following the input.
Choosing in saturation so the small-signal swing stays linear; placing near an edge clips the signal.
So a good operating point is centred in the active region with enough headroom on both sides for the largest expected signal swing — and biased stably, so component tolerances and temperature do not drift Q into a bad spot. Choosing it well is the job of MOSFET biasing. The locus of all possible operating points as the input DC level is varied traces out the Voltage-transfer characteristic; the useful, linear region of that curve is exactly where Q must live.