There’s an old electronics joke that if you want to build an oscillator, you should try building an amplifier. One of the fundamental criteria for oscillation is the presence of signal gain; without it, any oscillation is bound to decay, just like a swing that’s no longer being pushed must eventually come to a stop.
In reality, circuits with gain can occasionally oscillate by accident, but it’s rather difficult to build a good analog oscillator from scratch. The most common category of oscillators you can find on the internet are circuits that simply don’t work. This is followed by approaches that require exotic components, such as center-tapped inductors or incandescent lightbulbs. The final group are the layouts you can copy, but probably won’t be able to explain to a friend who doesn’t have an EE degree.
In today’s article, I wanted to approach the problem in a different way. I’ll assume that you’re up-to-date on some of the key lessons from earlier articles: that you can tell the difference between voltage and current, have a basic grasp of transistors, and know what happens when a capacitor is charged through a resistor. With this in mind, let’s try to construct an oscillator that’s easy to understand, runs well, and has a predictable operating frequency. Further, let’s do it without peeking at someone else’s homework.
Swing… and miss
The simplest form of an oscillator is a device that uses negative feedback to cycle back and forth between two unstable states. To illustrate, think of a machine equipped with a light sensor and a robotic arm. In the dark, the machine is compelled to stroll over to the wall switch and flip it on. If it detects light, another part of its programming takes over and toggles the switch off. The machine is doomed to an endless cycle of switch-flipping at a frequency dictated by how quickly it can process information and react.
At first blush, we should be able to replicate this operating principle with a single n-channel MOSFET. After all, a transistor can be used as an electronically-operated switch:
A wannabe oscillator.
The transistor turns on when the voltage between its gate terminal and the source leg (Vgs) exceeds a certain threshold, usually around 2 V. When the power supply first ramps up, the transistor is not conducting. With no current flowing through, there’s no voltage drop across the resistor, so Vgs is pulled toward the positive supply rail. Once this voltage crosses about 2 V, the transistor begins to admit current. It stands to reason that the process shorts the bottom terminal of the resistor to the ground and causes Vgs will plunge to 0 V. If so, that would restart the cycle and produce a square wave on the output leg.
In practice, this is not the behavior you’ll see. For a MOSFET, the relationship between Vgs and the admitted current (Id) is steep, but the device is not a binary switch:
BS170 Vgs-Id curve for Vds = 1 V. Captured by author.
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