The Historical Origins And Technical Meanings Of Vcc, Vdd, Vee, And Vss

Definition and Purpose of Supply Voltages and Ground

In electronics, supply voltages provide power to components and integrated circuits. The main supply voltage is denoted as VCC or VDD, indicating the higher, positive voltage rail. Another voltage called VSS, also referred to as ground or 0 volts, provides the reference point and return path for current to flow back to the source.

Additional negative or positive supply voltages may be present, such as VEE for negative voltage supplies used in operational amplifiers and other analog circuits. These supply voltages power the circuits and allow current to flow, enabling the logic functions and internal operation.

VCC and VDD – Positive Supply Voltages

VCC and VDD both refer to the higher, positive supply voltage in a circuit. They provide power to the components and ICs so they can operate.

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Historical Use of VCC in TTL Logic Circuits

VCC originated from early Transistor-Transistor Logic (TTL) integrated circuits as a symbol representing the positive collector supply voltage. In TTL logic gates like 7400 series ICs, VCC was the higher voltage connected to the collectors of the bipolar junction transistors (BJTs) inside the chips. The TTL circuits used 5V for their VCC in most applications.

VDD as Positive Supply in CMOS Circuits

With the development of Complementary Metal-Oxide Semiconductor (CMOS) logic technology, VDD became more commonly used to indicate the positive supply rail. Unlike TTL, CMOS uses field effect transistors (FETs) rather than BJTs inside the logic gates. VDD connects to the drains of PMOS transistors and allows current to flow through the internal circuitry.

The use of CMOS allowed reduced power consumption compared to TTL, since the inputs draw negligible leakage current. CMOS ICs can therefore operate at lower VDD voltages like 3.3V or 1.8V, offering further power savings. So VDD evolved to represent the positive supply for all types of digital logic families while VCC remains tied historically to TTL circuits.

VEE – Negative Supply Voltage

While most logic circuits just use VDD and ground, some require a negative supply voltage typically labeled VEE. This is most common in operational amplifiers (op-amps) and other analog ICs designed for audio, voltage regulation, instrumentation and other applications.

Role of VEE in Op-Amps and Analog Circuits

In an operational amplifier IC, VEE refers to the negative supply pin, while VCC or VDD is the positive supply. Many op-amps can work with either single supply where VEE is connected to ground, or dual supplies with both positive and negative voltages.

Using symmetrical positive and negative supplies allows op-amps to swing both above and below ground, useful for AC signals. So VEE provides extra headroom and range for the output. Analog circuits like audio equipment, sensors and feedback loops utilize these dual +/- supply arrangements with VEE and VCC/VDD to accommodate their input and output signal characteristics.

VSS – Ground Reference Voltage

VSS represents the circuit ground, serving as the 0V return path for current flow. It may also be labeled GND. Every circuit requires a ground connection via VSS to function.

Providing a Return Path for Current

For current to flow through a circuit, it needs a complete loop. So the power from VCC/VDD branches out through the components and logic gates. Then VSS gives current a path to return back and allow continuity of flow. Without VSS, no current would be able to actually pass through the devices because the loop would be incomplete.

VSS/GND serves as a reference point for voltage measurements. Zero volts or ground is the baseline from which all other voltages in a circuit derive meaning. Signal voltages and logic levels relate back to the ground reference provided by VSS.

Example Circuits Using VCC, VDD, VEE, and VSS

Here are some examples of how these supply voltages appear in real circuits:

A simple CMOS NOR gate IC has VDD providing +5V, VSS/GND serving as 0V ground, with no VEE or VCC present.
A dual-supply op-amp circuit has VCC=+15V, VEE=-15V as symmetrical +/- supplies, along with 0V VSS ground reference.
A TTL NAND gate chip from the 7400 series would show VCC=+5V as the TTL positive voltage, and VSS=GND completing the flow loop.

So in all cases, expect VDD and/or VCC providing positive voltage levels, VEE in analog dual-supply ICs, and universally VSS/GND serving as the critical circuit ground reference 0V connection.

How Supply Voltages Impact Circuit Performance

The values of the supply voltages impact current flow and power dissipation in circuits. Higher VDD levels allow faster switching speeds but also increase power consumption proportional to the square of VDD. Reducing VDD saves energy but can reduce speed and noise margins in digital logic circuits.

Minimum VDD levels are dependent on the manufacturing process and transistors used within the integrated circuits. Older logic families like TTL require 5V VCC for proper operation. More modern CMOS can work down to 1.8V or lower VDD.

Dual supplies also affect analog amplifier circuits. Wider spaced +/- supplies using lower |VEE| and higher VCC/VDD allow greater voltage swings and signal headroom. But higher supply values increase power and heat generation which require management.

So the designer chooses supply voltage parameters tailored for application requirements of speed, power, and accuracy tradeoffs.