Thevenin's theorem: A fundamental theorem that says any circuit driving a load can be converted to a single generator and series resistance.

Thermistor: A device whose resistance experiences large changes with temperature.

Thermal Shutdown: A feature found in modern three-terminal IC regulators. When the regulator exceeds a safe operating temperature, the pass transistor is cut off and the output voltage goes to zero. When the device cools, the pass transistor is again turned on. If the original cause of the excessive temperature is still present, the device again shuts off. If the cause has been removed, the device works normally. This feature makes the regulator almost indestructible.

Thermal Runaway: As a transistor heats, its junction temperature increases. This increases the collector current, which forces the junction temperature to increase further, producing more collector current, etc., until the transistor is destroyed.

Temperature Coefficient: The rate of change of a quantity with respect to the temperature.

Switching Regulator: A linear regulator uses a transistor that operates in the linear region. A switching regulator uses a transistor that switches between saturation and cutoff. Because of this, the transistor operates in the active region only during the short time that it is switching states. This implies that power dissipation of the pass transistor is much smaller than in a linear regulator.

Swamped Amplifier: A CE stage with a feedback resistor in the emitter circuit. This feedback resistor is much larger than the ac resistance of the emitter diode.

Surge Current: The large initial current that flows through the diodes of a rectifier. It is the direct result of charging the filter capacitor, which initially is uncharged.

Surface-Leakage Current: A reverse current that flows along the surface of a diode. It increases when you increase the reverse voltage.

Superposition: When you have several sources, you can determine the effect produced by each source acting alone and then add the individual effects, to get the total effect of all sources acting simultaneously.

Summer Circuit: An op-amp circuit whose output voltage is the sum of the two or more input voltages.

Substrate: A region in a depletion mode MOSFET located opposite from the gate, forming a channel through which electrons flowing from source to drain must pass.

Stray Wiring Capacitance: The unwanted capacitance between connecting wires and ground.

Stiff Voltage Source: A voltage source whose internal resistance is at least 100 times smaller than the load resistance.

Stiff Voltage Divider: A voltage divider whose loaded output voltage is within 1 percent of its unloaded output voltage.