FAQ on voltage and current sources: part 1

Voltage and the lesser-known current sources provide critical IC, circuit, and system functions.

Voltage sources and their complement to current sources are essential elements of many systems, circuits, and IC designs. Experimenters and novice engineers are familiar with voltage sources of various types — even if not by that name — in the form of batteries, AC/DC supplies, and DC/DC converters and regulators. At the same time, however, they are unfamiliar with the concept and utility of current sources.

While the current source is less well-known than the voltage source, it has many vital electrical roles and functions. This FAQ will examine ideal and actual voltage and current sources, their principles, applications, and real-world considerations. It will not discuss how to design one from scratch or select an available one, as that is a “deeper dive” with many subtleties.

Voltage and current sources are widely used to excite sensors, establish operating points, and more. They are also used within ICs to create balance or cancellation sub-circuits and implement other IC-designer “tricks” to compensate for unavoidable circuit deficiencies, process weaknesses, or thermal considerations.

Voltage sources

Q: What is a voltage source?
A: That’s a relatively easy question. An ideal voltage source maintains a constant voltage at its two output terminals, regardless of the load value. It delivers the amount of current “demanded” by the load at this voltage (Figure 1).

Figure 1. The ideal voltage source (circle at the left) has zero internal resistance and delivers a constant voltage at all current levels. (Image: Electrical Technology)

Q: What characterizes an ideal voltage source in addition to the ability to deliver a constant voltage?
A: An ideal voltage source has zero internal resistance, so there is no voltage drop across it.

Q: Are ideal voltage sources available?
A: No, that’s why they are called ideal. A real, practical voltage source has some internal resistance in series with the source, usually on an ohm or less.

Q: What are the implications of that internal resistance?
A: This means that as the load draws current, the voltage source output will “droop,” as Ohm’s law dictates (Figure 2). The source’s terminal voltage is less than the actual voltage generated by the source due to internal resistance.

Figure 2. A non-ideal natural voltage source has a resistance in series with the load, so the delivered voltage decreases as the current increases. (Image: Electrical Technology)

Q: Since no voltage source is ideal, what is a good criterion for how much internal resistance is acceptable?
A: The answer depends on the application and how much voltage droop it can tolerate. As a good starting point, the internal resistance should be one-hundredth or less than the equivalent circuit resistance. If the circuit resistance is not explicitly known, it can easily be determined from Ohm’s Law.

Q: Can a voltage source run out of “energy”?
A: Absolutely. At some point, the voltage source can no longer maintain the intended voltage while also delivering the current demanded by the circuit. At that point, the voltage will decline rapidly, and the source will no longer be available. That’s why the top-tier specification for a voltage source is its fixed output voltage, followed by its maximum current rating.

Q: How do you make a helpful voltage source with low internal resistance?
A: Chemical batteries and power supplies, designed to have very low output resistance, are usually good options.

Q: What’s a Thévenin source and its relation to a voltage source?
A: Thévenin’s theorem states that any linear circuit containing several voltage sources and resistors can be simplified for current/voltage analysis by a Thévenin-equivalent circuit with a single voltage source and resistance connected in series with a load. (Léon Charles Thévenin, 1857 to 1926, was a French telegraph engineer who extended Ohm’s law to analyze complex electrical circuits.)

Q: What’s the difference between a voltage source, voltage reference, or power supply?
A: In concept, there’s very little difference. In reality, it’s a matter of scale and priorities. Both power supplies and voltage references are voltage sources but at different levels and priorities. A power supply may deliver anywhere from a few volts to hundreds of volts at currents ranging from a few hundred milliamps to many amps. In contrast, a voltage reference is usually a more precise source that provides a highly accurate voltage with low drift, but usually only a few volts and on the order of tens of milliamps.

Q: What parameters characterize a voltage source?
A: In addition to output voltage and maximum current, there are factors such as voltage stability versus load changes (load regulation), temperature drift, protection, and efficiency. The relative importance of these depends on the application.

Q: Are all voltage sources DC?
A: No. Although there are both DC and AC sources, DC sources are much more common in electronic designs.

Q: How do you build a voltage source?
A: There are many ways to do this, but the most common is to use a voltage reference (basic diode, buried Zener, bandgap, or other) and then boost the output voltage and current to the needed levels using an op-amp output driver.

The next part looks at the basics of current sources.