Analog can Scale. Here's how. - by Sango's TA

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Analog can Scale. Here's how.<br>From a transistor to an MNIST accelerator, let's build up to see how an analog computing system would borrow the right abstractions to push the frontier of possible.

Sango's TA<br>Jul 04, 2026

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When I posted my last article, it got both positive reception and lots of classic confusion/questions about the things I was saying. Paraphrasing roughly, they were “what do you mean analog programming is novel, don’t we have trimming?” Or “that analog stuff is cool and all, but it really doesn’t scale” or “what about device variation? (mismatch)”. It was then I understandably realized I had a lot more explaining to do and it’s on me to be clearer about what’s going on at the lowest levels of our computing devices. Even members of niche forums dedicated to adjacent topics may not be aware of how the claims I was making about analog being so great mapped to the research papers that were linked. So, in this article, I’m going to walk through the stack. Starting from a simple transistor and its physics all the way to a system benchmark of an analog vs digital implementation.

Sneak preview of the digital vs analog benchmark we will be working up to. Read on to understand what the cards mean.<br>What is a transistor?

It is a device with three or four terminals whose basic operation is to act like a valve for the flow of current between two terminals. More technically, using a “gate” or “base” pin, the amount of charge carriers (electrons or holes) moving between two other pins are modulated. A very simple analogy (which has flaws we will get to) is a tap. When you turn it “on” water flows and you can adjust the rate of water till the maximum throughput is flowing. There are two main types of transistors to consider: Bipolar Junction Transistors (BJTs) and Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) but far more types exist. There is so much interesting history about how they stemmed from vacuum tubes and Field Effect Transistors were invented first but the BJT gained adoption before MOSFETs came to dominate most use cases today. But we have stuff to get through so maybe we’ll save history for another time.<br>Thanks for reading Sango's's Substack! Subscribe for free to receive new posts and support my work.

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Figure 1: (a) Cross section of a BJT (b) Symbol of a BJT (c) Cross section of a MOSFET (d) Symbol of a MOSFET<br>While the BJT still has many uses in production chips today, the MOSFET will be the focus of our discussions due to its wider availability in process nodes from semiconductor manufacturers and the fundamental fact that floating gates can exist in a MOSFET because charge is stored at the gate. A BJT would see that stored charge flow through one of its terminals (see Figure 1 for BJT vs MOSFET cross section). Quick aside on the MOSFET symbol: Even though it’s a 4-terminal device, the substrate voltage is usually held constant and is often not shown in symbols.<br>Digital vs Analog View

Focusing on the MOSFET we can circle back to one flaw of the valve analogy. With a water valve, the water can be completely turned off when the valve is closed but in MOSFETs you could still have some electrons flowing through the channel even when you have applied the proper voltage to turn the device “off”. In fact, this binary view of a transistor is one of the interesting fundamental divides between a digital and analog view of the same transistor current.<br>Take one of the most basic curves for understanding a transistor: a gate voltage sweep as caricatured in Figure 2. This is a circuit experiment for say an N-type MOSFET where you apply a fixed higher potential to the drain, apply ground to the substrate terminal, slowly raise the gate voltage from ground to the fixed potential and measure the current coming out of the source.

Figure 2:(a) Gate sweep circuit setup (b) Linear view of current (c) Logarithmic view of current<br>A digital or binary view of the transistor is to say up till the mid-point (aka threshold voltage) the transistor is off and when the current is maximum the FET is on (Fig 2b). An analog or continuous view would be to take the logarithm of the current (Fig 2c) and we see even at the lowest voltage there is a corresponding change in the output current with far more intermediate states (10-12 to 10-3 is 9 orders of magnitude or 1,000,000,000 levels). It is important to pause to acknowledge the major tradeoffs between these two views. Neither is wrong but with a binary view there are only two states to keep track of so signals are easily recovered. With a continuous view, far more information can be encoded across those levels, but more effort is paid to processing, storing and recovering signals. However, this basic distinction starts to underscore how much more efficient analog processing can be. In the previous article, we showed how many fewer transistors analog...