Differences Between VCC, VEE, VDD, and VSS in Electronics Circuits

In electronics, the terms VCC, VEE, VDD, and VSS are frequently used to describe different power supply voltages in circuits. These designations are essential for understanding the supply roles and the correct functioning of components like bipolar junction transistors (BJTs) and field-effect transistors (FETs). Each of these terms has its specific origin and application, often linked to the type of transistor being used—whether it's BJT or FET—and the configuration of the circuit. In this article, we'll break down the differences between these power supply designations and explain their relevance in circuit design.

What is VCC?

VCC stands for "Voltage at the Common Collector." This term is used to describe the positive supply voltage in circuits that use Bipolar Junction Transistors (BJT). The "C" in VCC refers to the collector terminal of the transistor. VCC is essential for powering BJTs, as the collector is typically connected to the positive voltage rail.

In NPN transistors, which are more commonly used because of their faster switching speed, VCC represents the positive supply at the collector terminal. For instance, in an NPN transistor circuit, the collector is usually connected to the VCC through a resistor or directly.

• Use in TTL and Op-Amps: In Transistor-Transistor Logic (TTL) circuits, VCC is widely used as the main positive voltage supply. Similarly, in operational amplifiers that primarily consist of BJTs, VCC plays a crucial role in driving the positive power rail.

V+ vs. VCC: In many cases, V+ can be used interchangeably with VCC. Both represent the positive supply voltage, although VCC is specifically tied to circuits that use BJTs. In circuits with multiple positive voltage levels, these may be labeled VCC1, VCC2, etc., to distinguish between different supply lines.

What is VEE?

VEE represents the negative supply voltage in BJT circuits, and the "E" in VEE stands for the emitter terminal of the transistor. VEE is often used in circuits where there is a need for both positive and negative supply voltages, such as in dual-supply systems. In these cases, VEE acts as the negative power rail.

In an NPN transistor circuit, the emitter is connected to VEE, which can either be ground or a negative voltage. In single-supply systems, VEE usually shares the same potential as ground.

• Use in Op-Amps: Like VCC, VEE is also used in operational amplifiers built with BJTs. It provides the necessary negative voltage for the emitter terminal.

Similar to how V+ can be used interchangeably with VCC, V- can be used in place of VEE in many circuits. When referencing the negative supply in BJT circuits, V- and VEE both signify the same concept.

What is VDD?

VDD refers to the positive supply voltage in circuits that use Field-Effect Transistors (FETs). The "D" in VDD stands for the drain terminal of the FET. VDD is crucial for powering N-channel FETs (NchFETs), which are widely used in Complementary Metal-Oxide-Semiconductor (CMOS) circuits.

In an N-channel FET, the drain terminal is typically connected to the VDD through a resistor or directly. VDD serves as the positive voltage rail in circuits that use FETs.

• Use in CMOS and Op-Amps: In CMOS technology, both N-channel and P-channel devices are used. While the naming convention might not perfectly match the polarity in CMOS circuits (because the roles of the drain and source are complementary), the use of VDD to describe the positive power rail persists due to historical reasons. VDD is also employed in operational amplifiers built with FETs.

In FET circuits, V+ can be used interchangeably with VDD to refer to the positive supply voltage.

What is VSS?

VSS stands for the negative supply voltage in circuits that use FETs, with the "S" referring to the source terminal of the transistor. In N-channel FETs, the source is connected to VSS, which often serves as the ground or negative voltage in the circuit.

• Use in CMOS: In CMOS circuits, VSS is typically the ground or 0V reference. Like VDD, VSS applies to both N-channel and P-channel devices in CMOS logic circuits, even though the source may not always be the negative terminal in a complementary setup.

Just like V- is synonymous with VEE in BJT circuits, it is also used interchangeably with VSS in FET circuits.

Comparing VCC, VEE, VDD, and VSS

The table below summarizes the key differences between VCC, VEE, VDD, and VSS in terms of transistor type, terminal, and polarity.

Origins of VCC, VEE, VDD, and VSS Naming Conventions

The terms VCC, VEE, VDD, and VSS are derived from a general naming convention used to describe voltages in relation to a reference node, often ground (GND). For instance, a voltage like Va refers to the voltage of a point relative to ground, while Vab indicates the voltage difference between two nodes, A and B.

A voltage with repeated subscripts (e.g., Vxx) results in a value of 0V because it's the voltage difference at the same node. Over time, these repeated subscripts (CC, EE, DD, SS) were adopted to represent DC supply voltages in electronic circuits.

In BJT circuits, VCC refers to the collector voltage and is usually positive, while VEE refers to the emitter voltage, often negative or ground. These terms are specifically tied to NPN transistors, although the usage can extend to PNP transistors as well.

In MOSFET circuits, the terms VDD and VSS refer to the drain and source voltages respectively. Similar to BJTs, VDD is typically positive, while VSS can be negative or serve as the ground node.

Though originally tied to specific transistors, the usage of VCC/VDD and VEE/VSS has evolved, and these terms are now often used interchangeably across different circuit types, such as CMOS and BJT circuits.

Common Misconceptions and Modern Usage

Over time, the strict definitions of VCC, VEE, VDD, and VSS have blurred. You may see VCC used in CMOS circuits or VDD applied in BJT circuits. The key takeaway is that these terms were originally meant to designate power supply voltages in BJTs and FETs, but are now often used more broadly to indicate positive and negative voltages in many types of circuits.

Additionally, in some circuits, VSS or VEE may serve as the ground node, while in others, a different reference point might be chosen, leading to variable voltage polarities.

Conclusion

In summary, VCC, VEE, VDD, and VSS are power supply designations that are essential for understanding the electrical characteristics of circuits built with BJTs and FETs. While these terms originated with specific transistor types, they are often used interchangeably in modern circuit design. Understanding their roles is critical for circuit design, ensuring proper power distribution and functionality.

If you’re designing or troubleshooting circuits with BJTs or FETs, it’s important to correctly identify and apply the appropriate power supply designations, whether you are working with TTL, CMOS, or op-amp circuits.