Transistor bias calculator
Measure a JFET’s Idss and Vgs(off), or a transistor’s gain and leakage, and enter the voltage you want on the drain or collector. Get the resistor that sets it, rounded to stock values, and see how far a different part of the same type would move it.
Swipe sideways for the whole drawing. Tap a part to jump to its value. The part in blue is the one found. The red numbers are DC voltages to ground at the stock values.
The part
What you want
Held
Stock values
How biasing works
A transistor stage needs its output resting somewhere between ground and the supply, so the signal can swing both ways before it clips. About half the supply is the usual target. Set it lower or higher and one side of the wave runs out of room first. Builders tune that by ear on fuzzes and boosters.
The JFET. Its drain current flows through the source resistor and lifts the source above the gate, which sits at 0 V on its gate resistor. So the gate is negative to the source, and that throttles the current. The square law says how much: Id = Idss (1 − Vgs / Vgs(off))². Pick the current from the drain resistor and the drain voltage you want, find the gate voltage that passes it, and the source resistor is that voltage over the current. Idss and Vgs(off) vary several times over within one part number, which is why builders measure each JFET or fit a trimmer in place of the source resistor.
The BJT. The collector current is the base current times the gain, plus the leakage: Ic = hFE Ib + Iceo. With collector feedback, the base current comes through the resistor from the collector, so more current pulls the collector down and cuts its own base current. That holds the bias steadier than a fixed base resistor but still leaves it moving with the gain. A base divider with an emitter resistor holds it steadiest, because the emitter resistor sets the current more than the gain does.
Leakage. Germanium transistors pass some collector current with no base current at all. A tester reads it with the base open (Iceo). Gain on those testers is read as the extra current from a known base current, with the leakage taken out. Leakage acts like extra base current, so it pulls the collector toward the emitter, and it about doubles every 16 °F. Silicon parts leak a few nA, too little to matter here.
Stock values. The exact resistor almost never comes in a stock value. The table lists the stock values on either side of it and where each puts the output, so you can pick the one nearer what you want. Under the spread table, a fixed resistor and a trimmer in series that cover every part in it.
Warnings. Red flags a voltage the circuit can’t reach with any value of the resistor, a JFET pushed out of its saturation region, a transistor saturated or nearly off at the stock values, or the solver not settling.
How this calculator works. The answer comes from the same engine and part presets as the bench and the transistor and op-amp stage tool. The stage is built from engine parts and its DC operating point solved. The resistor is changed in small steps, halving the gap each time, until the output sits at the voltage you want. The textbook hand calculation is worked beside it. The two differ slightly because the engine’s part models include effects the hand formulas leave out, such as the collector or drain voltage nudging the current. The measured figures replace the preset values. Everything else about the part stays as the preset has it, so treat the answer as a starting point, typical of the part, not measured. The spread rows and the chart are DC solves with the stock values.
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