My First Oscillator: The Astable Multivibrator


I recently compiled and posted a project on describing how a Theremin (an electronic musical instrument) can be built with the Analog Discovery 2, the Analog Parts Kit, and a beverage can. I wanted to be able to break down the circuit into smaller stages so I could understand what was going on and so I could help readers understand as well. However, what I considered to be the “first stage” turned out to be quite a conceptual challenge for me, and it is still putting me through the ringer.

Deep thinking. Picture from:

This stage was presented to me as a “Variable-Frequency Oscillator.” A small op amp circuit with 3 resistors and a capacitor, this generated an oscillating output. The fixed capacitor was placed in parallel with a beverage can and a hand-held wire for the Theremin effect. I asked myself “what is the input to this stage, how does this op amp generate an oscillating output with only DC voltage supplies, and what’s the function of this capacitor?” This is when I fell down the rabbit hole.

Follow the white rabbit. Picture from:

The parallel capacitance formed by the beverage can and the handheld wire changed the frequency of the output. That was noticeable using an oscilloscope and supported the variable-frequency title. But why was there an oscillating output to begin with? Here is a schematic for the circuit I was using:

What I have gathered in attempt to understand the cause of this oscillation is that it still seems like magic and really does reflect the fact that every circuit component is imperfect in its own little special way (just like all of us). In theory, or ideally, the plates of the capacitor would be the exact same in every way, the resistor values would be the exact same in every way, and the transistors in the integrated circuit would be exactly the same in every way. The positive and negative voltages applied to the op amp would be exactly equal and opposite and applied at the same exact instantaneous moment and the environment would have no bearing on functionality. However, as you may be expecting, this is not reality.

Hashing it out before reality puts you in your place. Picture from:

Tiny imperfections and variations among like components, in conjunction with environmental electrical and magnetic noise, are the driving forces to this oscillator. Since instantaneous voltage changes are impossible, and the signal path to bias the circuit will vary between the two supplies, one plate of the fixed capacitor will begin to favor one of the voltage supplies and start charging towards that value.

Now what happens is a magical juggling act. The charging capacitor biases BJT transistors that are in the op amp that passes either a positive or negative voltage to the output. However, when the capacitor charge gets close to the value of one of the power supplies, this causes a switch-like behavior in some of the op amps transistors, rebiasing, and causes the capacitor to begin discharging and recharging towards the other power supply value. This continues to flip-flop on it’s own, hence the astable title. The oscillating output is characteristic of a square wave. Here is a screen shot of the oscillation in Waveforms:

There are also monostable and bistable multivibrators where one or two of these states are externally controlled. I can not fully explain this phenomenon with the op amp circuit, but some easier to understand examples of discrete multivibrator circuits exist online with better explanations. I will need much more time to think about this before I develop a confident understanding. This is some crazy cool physics! Check out these pages for tutorials that helped me begin to wrap my mind around this phenomenon, they use the same discrete circuit:

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  1. The pair of resistors providing positive feedback turn the op amp into a Schmitt trigger (you could, more cheaply, use a 74HC14N chip). The resistor and capacitor on the negative feedback provide charging and discharging of the capacitor towards the output of the Schmitt trigger, but as soon as the capacitor charges or discharges to the point where the thresholds of the Schmitt trigger are crossed, the output changes and the charging or discharging proceeds in the opposite direction.

    This oscillator is explained in many places (including my book and my blog
    —I would provide links, but then the comment is rejected as spam. Look for “touch sensor” posts on gasstationwithoutpumps from 2012)

    It is usually called a relaxation oscillator, though the term hysteresis oscillator is also used.

    • Ian Etheridge on

      Hello gasstationwithoutpumps! Thank you for connecting some dots for me and providing some more resources! Oscillator circuits are new to me but super interesting. I’m going to look into the Schmidt trigger and spend some more time with your blog. I think what is really tripping me out is the physics behind why the capacitor begins to charge in the first place. Thanks again for the feedback, I really appreciate it!

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