IC power-supply pins denote a voltage and current supply terminals in electric, electronics engineering, and in integrated circuit design.^[a] Integrated circuits (ICs) have at least two pins that connect to the power rails of the circuit in which they are installed. These are known as the power-supply pins. However, the labeling of the pins varies by IC family and manufacturer. The double subscript notation usually corresponds to a first letter in a given IC family (transistors) notation of the terminals (e.g. V_DD supply for a drain terminal in FETs etc.).

The simplest labels are V+ and V−, but internal design and historical traditions have led to a variety of other labels being used. V+ and V− may also refer to the non-inverting (+) and inverting (−) voltage inputs of ICs like op amps.

For power supplies, sometimes one of the supply rails is referred to as ground (abbreviated "GND") – positive and negative voltages are relative to the ground. In digital electronics, negative voltages are seldom present, and the ground nearly always is the lowest voltage level. In analog electronics (e.g. an audio power amplifier) the ground can be a voltage level between the most positive and most negative voltage level.

While double subscript notation, where subscripted letters denote the difference between two points, uses similar-looking placeholders with subscripts, the double-letter supply voltage subscript notation is not directly linked (though it may have been an influencing factor).^[3][4]

BJTs

ICs using bipolar junction transistors have V_CC (+, positive) and V_EE (-, negative) power-supply pins – though V_CC is also often used for CMOS devices as well.^[2]: 71

In circuit diagrams and circuit analysis, there are long-standing conventions regarding the naming of voltages, currents, and some components.^[5] In the analysis of a bipolar junction transistor, for example, in a common-emitter configuration, the DC voltage at the collector, emitter, and base (with respect to ground) may be written as V_C, V_E, and V_B respectively.

Resistors associated with these transistor terminals may be designated R_C, R_E, and R_B. In order to create the DC voltages, the furthest voltage, beyond these resistors or other components if present, was often referred to as V_CC, V_EE, and V_BB.^[1] In practice V_CC and V_EE then refer to the positive and negative supply lines respectively in common NPN circuits.^{[citation needed]} Note that V_CC would be negative, and V_EE would be positive in equivalent PNP circuits.

The V_BB specifies reference bias supply voltage in ECL logic.^[d]

FETs

Exactly analogous conventions were applied to field-effect transistors with their drain, source and gate terminals.^[5] This led to V_D and V_S being created by supply voltages designated V_DD and V_SS in the more common circuit configurations. In equivalence to the difference between NPN and PNP bipolars, V_DD is positive with regard to V_SS in the case of n-channel FETs and MOSFETs and negative for circuits based on p-channel FETs and MOSFETs.

CMOS

CMOS ICs have generally borrowed the NMOS convention of V_DD for positive and V_SS for negative, even though both positive and negative supply rails connect to source terminals (the positive supply goes to PMOS sources, the negative supply to NMOS sources).

In many single-supply digital and analog circuits the negative power supply is also called "GND". In "split-rail" supply systems there are multiple supply voltages. Examples of such systems include modern cell phones, with GND and voltages such as 1.2 V, 1.8 V, 2.4 V, 3.3 V, and PCs, with GND and voltages such as −5 V, 3.3 V, 5 V, 12 V. Power-sensitive designs often have multiple power rails at a given voltage, using them to conserve energy by switching off supplies to components that are not in active use.

More advanced circuits often have pins carrying voltage levels for more specialized functions, and these are generally labeled with some abbreviation of their purpose. For example, V_USB for the supply delivered to a USB device (nominally 5 V), V_BAT for a battery, or V_ref for the reference voltage for an analog-to-digital converter. Systems combining both digital and analog circuits often distinguish digital and analog grounds (GND and AGND), helping isolate digital noise from sensitive analog circuits. High-security cryptographic devices and other secure systems sometimes require separate power supplies for their unencrypted and encrypted (red/black) subsystems to prevent leakage of sensitive plaintext.

BJTs and FETs mixed

Although still in relatively common use, there is limited relevance of these device-specific power-supply designations in circuits that use a mixture of bipolar and FET elements, or in those that employ either both NPN and PNP transistors or both n- and p-channel FETs. This latter case is very common in modern chips, which are often based on CMOS technology, where the C stands for complementary, meaning that complementary pairs of n- and p-channel devices are common throughout.

These naming conventions were part of a bigger picture, where, to continue with bipolar-transistor examples, although the FET remains entirely analogous, DC or bias currents into or out of each terminal may be written I_C, I_E, and I_B. Apart from DC or bias conditions, many transistor circuits also process a smaller audio-, video-, or radio-frequency signal that is superimposed on the bias at the terminals. Lower-case letters and subscripts are used to refer to these signal levels at the terminals, either peak-to-peak or RMS as required. So we see v_c, v_e, and v_b, as well as i_c, i_e, and i_b. Using these conventions, in a common-emitter amplifier, the ratio v_c/v_b represents the small-signal voltage gain at the transistor, and v_c/i_b the small-signal trans-resistance, from which the name transistor is derived by contraction. In this convention, v_i and v_o usually refer to the external input and output voltages of the circuit or stage.^[5]

Similar conventions were applied to circuits involving vacuum tubes, or thermionic valves, as they were known outside of the U.S. Therefore, we see V_P, V_K, and V_G referring to plate (or anode outside of the U.S.), cathode (note K, not C) and grid voltages in analyses of vacuum triode, tetrode, and pentode circuits.^[5]