Demystifying Negative Voltage: A Beginner’s Guide

Unless you’re an electrician or have experience working with or around voltages, the term “negative voltage” may be a confusing one to parse. After all, voltage is typically used to describe an electromotive force; the addition of the term “negative” therefore implies that the force is less than zero. How is that even possible? 

The truth is that the concept of a negative voltage cannot be understood simply by deconstructing the words, but rather by exploring the nature of voltage from a scientific point of view. Here’s what you need to know about negative voltage, how it’s generated, and how it is particularly useful when it comes to circuit design.

What is Negative Voltage? 

First and foremost, let’s begin by dispelling the interesting, albeit false notion that negative voltage represents some unique, untapped form of electricity. Fundamentally speaking, positive and negative voltages are not as different from one another as you might think. Both positive and negative voltages represent potential energy relative to a reference point (e.g. ground) at which the electrical potential is considered to be zero. When discussing electrical circuits, the earth itself is considered to have zero potential. Here’s an example to help you visualize the distinction. 

Let’s say we have a circuit with two nodes that uses conventional current (current in which charge flows from the positive terminal to the negative terminal). One node contains a positive voltage and the other is a reference node with zero potential. If you were to connect the nodes to one another using a conductor (e.g. wire), current will flow from the node with positive voltage to the reference node. If the circuit node in this example contained a negative voltage instead of a positive voltage, the current would “travel” in the opposite direction; from the reference node to the circuit node. 

To put it more simply, let’s describe this concept using a skyscraper as an analogy. Suppose you enter an elevator in a building’s lobby and note that there are 30 floors. The ground floor, labeled “G” (is equivalent to floor “0”), and is our reference point. The number of each floor increments the higher you go up. Upon arriving at the top floor, you confidently say that you are on the 30th floor of the building because you have just ascended 30 levels from your initial point of reference, i.e. the lobby. Imagine however that this building also has five levels below ground, numbered P1 to P5. If your reference point is the lobby, floors 1-30 represent the “positive” floors, while P1 to P5 represent the “negative” floors.  If you were to begin your upward journey from the lowest level, P5, to the penthouse on the 30th floor, you will have essentially traveled 35 levels through the building. The only difference between the “positive” and “negative” levels of the building corresponds to their relative position to the reference point, which in this case, is the lobby.
 

How Negative Voltage is Generated 

Basic circuits are generally powered using an unregulated power supply, like a battery or wall transformer. The term “unregulated” in this context refers to a power supply whose voltage output will vary based on the load. Linear regulators stabilize the voltage to maintain a steady output from the power supply. While linear regulators are used to effectively dissipate energy, they cannot be used to convert positive voltage into negative voltage - this can instead be achieved through the use of regulator circuitry that has the capacity to store energy. 

Both capacitors and inductors are good examples of simple components that store energy and can therefore be used in generating negative voltages. Negative voltage generators leveraging capacitors to store energy are often called charge-pump power supplies. These are so named because they utilize switches to occasionally pump a charge into a capacitor. As long as a charge is pumped at a steady interval, the capacitor can effectively hold and supply a current indefinitely. Switching the charge at the correct time can add charge to different rails. A “rail” is a term used to refer to different paths the electrical charge can take. For example, a digital circuit can be comprised of a 5V rail and a 0V (ground) rail. “Switching the charge” means that we can exert control over the electrical charge, effectively forcing the charge onto a given rail. This process is utilized by voltage doublers, which double applied voltages, and voltage inverters, which are capable of changing the polarity of voltage.

. 

How Negative Voltage is Leveraged in Circuit Design 

Negative voltage plays a crucial role when it comes to circuit design. Let’s consider these examples: 

Switching Devices - Some switching devices are not easy to turn “off” when ground voltage is being applied to their gate. Applying a negative voltage slightly below the ground voltage facilitates a reliable “switch off”.

Audio Amplifiers - Some audio amplification devices utilize negative voltage to efficiently amplify negative and positive audio signals. 

Operational Amplifiers - Operational amplifiers, or Op-amps,  are a type of circuit that is used to amplify electric signals. Dual supplies of positive and negative voltages are vital to their operation. 

Signal Biasing - In some audio circuits, a negative voltage is needed to alter the range of an analog signal. For example, if the signal is oscillating around 0V, negative voltage can be used to bias the voltage to match the range of the digital circuit. 

Telecommunications - Negative voltages are used as a safeguard against corrosion of the copper lines used in telecommunication systems.  

Negative Voltage is Not What You May Have Thought 

The next time that you hear the term, “negative voltage” you’ll know that it’s not referring to voltage that is “less than zero” but rather that it represents potential energy relative to a reference point. While many electronics do not require negative voltages to complete their function, some applications do in fact benefit from having a power supply with both positive and negative voltages. More importantly, if you happen to be considering a career as an electronics technician, your knowledge of voltages, both positive and negative, will form the foundation of your professional knowledge. 

If you would like to further your knowledge of negative circuits, how they work, and their importance, or if you’re ready to kickstart your career as an electronics technician, reach out to George Brown College and inquire about the Electronics Technician program.