IC power-supply pin

In circuit diagrams and circuit analysis, there are long-standing conventions regarding the naming of voltages, currents, and some components.[3] 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. In practice V_CC and V_EE then refer to the plus and minus supply lines respectively in common NPN circuits. Note that V_CC would be negative and V_EE would be positive in equivalent PNP circuits.

Exactly analogous conventions were applied to field-effect transistors with their drain, source and gate terminals.[3] 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.

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.[3]

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.[3]

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). ICs using bipolar transistors have V_CC (positive) and V_EE (negative) power supply pins - though V_CC is also often used for CMOS devices as well.

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 which are not in active use.

More advanced circuits will 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.

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