Diode & Transistor Circuit Builder
Pick a topology, set the supply and component values, and watch the operating point solve in real time. The tool walks the Shockley diode equation, simplified Ebers-Moll BJT, and square-law MOSFET models, then plots the I-V family curves with a live load-line operating point and a switching transfer characteristic.
Circuit
Controls
Operating Point
I-V Curve
Red dot marks the load-line operating point where the diode's exponential curve meets the resistor's load line.
Reference Guide
Shockley Diode Equation
A pn-junction diode conducts a current that grows exponentially with forward bias and saturates to a tiny reverse current.
Combined with a series resistor R and supply V_supply, the operating point is the intersection of this curve with the load line I = (V_supply - V_D) / R. The tool solves this with Newton iteration.
NPN BJT Regions
A bipolar junction transistor in common-emitter has three regions. Cut-off when V_in is below V_BE ~ 0.7 V. Active when V_in is above the threshold and V_CE stays above V_CE_sat ~ 0.2 V. Saturation when the load resistor cannot drop any more.
In saturation I_C is clamped to (V_supply - V_CE_sat) / R_C, and V_out collapses to a logic-low level near 0.2 V.
MOSFET Square-Law Model
A long-channel NMOS in saturation has a quadratic current. In triode region it depends on V_DS as well.
Saturation when V_DS is greater than or equal to V_GS minus V_th, else triode. Below V_th the device is in cut-off.
Switching and Inverters
A common-emitter BJT, an NMOS-with-drain-resistor, and a CMOS pair all invert their input: low V_in gives high V_out and vice versa. The transfer-characteristic plot shows the steep middle region where the device transitions between cut-off and saturation/triode.
CMOS only dissipates short-circuit current during the transition. At V_in = 0 the NMOS is off and V_out = V_supply. At V_in = V_supply the PMOS is off and V_out = 0. In between, both devices conduct briefly.
