Voltage-Controlled Voltage Source (VCVS): The ideal op amp, having infinite voltage gain, infinite unity-gain frequency, infinite input impedance, and infinite CMRR, as well as zero output resistance, zero bias, and zero offsets.
Voltage-Controlled Oscillator (VCO): An oscillator circuit in which the output frequency is a function of a dc control voltage; also called a voltage-to-frequency converter.
Voltage-Controlled Device: A device like a JFET or MOSFET whose output is controlled by an input voltage.
Voltage-Controlled Current Source (VCIS): Sometimes called a transconductance amplifier, this type of negative feedback amplifier has input current controlling output voltage.
Voltage Amplifier: An amplifier that has its circuit values selected to produce a maximum voltage gain.
Virtual Ground: A type of ground that appears at the inverting input of an op amp that uses negative feedback. It's called virtual ground because it has some of, but not all, the effects of a mechanical ground. Specifically, it is ground for voltage but not for current. A node that is a virtual ground has 0 V with respect to ground, but the node has no path for current to ground.
Very-Large-Scale Integration (VLSI): The placing of thousands or hundreds of thousands of components on a single chip.
Varistor: A device that acts like two back-to-back zener diodes. Used across the primary winding of a power transformer to prevent line spikes from entering the equipment.
Varactor: A diode optimized for a reverse capacitance. The larger the reverse voltage, the smaller the capacitance.
Upside-Down PNP Bias: When you have a positive power supply and a pnp transistor, it is customary to draw the transistor upside-down. This is especially helpful when the circuit uses both npn and pnp transistors.
Upper Trip Point (UTP): One of the two input voltages in which the output voltage changes states. UTP = BVsat.
Unwanted Bypass Circuit: A circuit that appears in the base or collector sides of a transistor because of internal transistor capacitances and stray wiring capacitances.
Universal Curve: A solution in the form of a graph that solves a problem for a whole class of circuits. The universal curve for self-biased JFETs is an example. In this universal curve ID/IDSS is graphed for RD/RDS.
Unity-Gain Frequency: The frequency where the voltage gain of an op amp is 1. It indicates the highest usable frequency. It is important because it equals the gain-bandwidth product.
Uninterruptible Power Supply (UPS): A device containing a battery and a dc-to-ac converter to be used during a power failure.