GoKawiilElectronic circuit theory is a frequent theme on this blog. As a part of this sorta-curriculum, I published a number of articles about operational amplifiers. I keep coming back to this topic for two reasons. First, I think these components are usually explained poorly, making them a major stumbling block for folks trying to learn the craft. Second, op-amps have gotten really good, inexpensive, and small, so I think they should be used more.
If the component is still a mystery to you, this article is probably not the best place to start; we’ll cover the very basics, but I recommend carving out some time to go over two other write-ups from 2023:
I also cover op-amp theory (and a lot more!) in The Secret Life of Circuits, a lovingly-crafted book that’s available in early access and will be hitting the shelves in about two months.
Today, I’d like to talk about a circuit that didn’t make the cut for the book. It’s not nearly as useful as a transimpedance amplifier, an integrator, or a Sallen-Key filter, all of which get in-depth treatments from first principles. At the same time, it’s just too cool not to share.
But first, a word from our sponsor
I know that most readers don’t click links, so before we dive in, let’s recap what an op-amp does. If you have it pegged as some sort of a variable-gain amplifier that “reads” the value of a pair of external resistors to configure internal gain, it’s bet to forget all that and start afresh.
An ideal op-amp does one thing and one thing only: it calculates the difference between the voltages on its two input pins (Vin- and Vin+), multiplies it by a humongous constant factor (A OL , typically 1,000,000 or more), and then outputs the resulting voltage in relation to the midpoint of the supply (Vmid). We can write this as:
\(V_{out} = V_{mid} + (V_{in+} - V_{in-}) \cdot A_{OL}\)
In practical terms, it means that if Vin- is noticably less than Vin+, the output voltage swings toward the positive supply of the chip; conversely, if Vin- exceeds Vin+, the output swings dives toward the negative rail. Intermediate output voltages are possible only in a very narrow, microvolt-range linear region of Vin- ≈ Vin+.
The simplest op-amp circuit — and the only one we need today — is the voltage follower:
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