wien bridge

Circuit VR: Oscillating Bridges

April 27, 2018 by 21 Comments

Circuit VR is where we talk about a circuit and examine how it works in simulation with LT Spice. This time we are looking at a common low-frequency oscillator known as the Wien bridge oscillator.

What makes an oscillator oscillate? A circuit with amplification that gets the same amount of the output signal fed back into its input, in phase, will oscillate. This is the Barkhausen criterion. Here, we’re going to look into what makes an oscillator work in simulation, and gain some insight into what happens when there’s too much feedback and too little.

In particular, we’ll look at the Wien bridge oscillator, a very simple design that originated as a way to measure impedance back in 1891. Modern versions add some additional features, but let’s start with the most simple implementation and work our way up.

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Model Sputnik Finds its Voice After Decades of Silence

April 25, 2017 by 18 Comments

As we approach the 60th anniversary of the human race becoming a spacefaring species, Sputnik nostalgia will no doubt be on the rise. And rightly so — even though Sputnik was remarkably primitive compared to today’s satellites, its 1957 launch was an inflection point in history and a huge achievement for humanity.

The Soviets, understandably proud of their accomplishment, created a series of commemorative models of Earth’s first artificial moon as gifts to other countries. How one came into possession of the Royal Society isn’t clear, but [Fran Blanche] found out about it through a circuitous route detailed in the video below, and undertook to reproduce the original electronics from the model that made the distinctive Sputnik beeps.

The Royal Society’s version of the model no longer works, but luckily it came with a schematic of the solid-state circuit used to emulate the original’s vacuum-tube guts. Intent on building the circuit as close to vintage as possible and armed with a bag of germanium transistors from the 60s, [Fran] worked through the schematic, correcting a few issues here and there, and eventually brought the voice of Sputnik back to life.

If you think we’ve covered Sputnik’s rebirth before, you may be thinking about our article on how some hams rebuilt Sputnik’s guts from a recently uncovered Soviet-era schematic. [Fran]’s project just reproduces the sound of Sputnik — no license required!

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[Jeri] uses light bulbs in an oscillator

November 18, 2012 by 36 Comments

Way back when [Ms Ellsworth] was a kid, she kept seeing the same circuit over and over again in her various op-amp books. It was a Wien bridge oscillator, a small circuit that outputs a sine wave with the help of a light bulb. Now that [Jeri] is much wiser, she decided to play around with this strange oscillator and found it’s actually pretty impressive for, you know, a light bulb.

The interesting portion of the Wien bridge is the gain portion of the circuit. It’s just a simple resistor divider, with a light bulb thrown in on one of its legs. When the current increases, this causes the light bulb to warm up (not enough to glow, though). When the temperature increases, the resistance in the light bulb increases, making the oscillator reach an equilibrium.

It’s a clever setup, but what about swapping out a resistor in place of the light bulb? In the video, [Jeri] tries just that, and it’s a mess. Where the light bulb circuit is amazingly stable with very, very low distortion, the resistor circuit looks like a disaster on the scope with harmonics everywhere.

A very cool build that would be perfect for an audio synth, but as [Jeri] says in her YouTube comments, “This doesn’t have enough distortion for indie bands.”

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