To kick off our adventure with analog computing systems, let’s start with a bit of history.

It all began in 1947 when John Bardeen and Walter Brattain built the first working transistor prototype using germanium at Bell Labs. Sidenote: in reality, many labs around the world were working on the concept, so it’s a collective achievement.

The jump from bulky vacuum tubes to tiny transistors was a game-changer - it triggered explosive growth in electronics. The next big milestone came quickly: in 1967 Karl D. Swartzel developed the first operational amplifier - an electronic building block that could perform mathematical operations:

  • summing
  • differentiating
  • integrating
  • comparing
  • multiplying
  • function generation

Op-amps became the cornerstone of early analog computers, enabling real signal-processing algorithms.

Learning by Example

Let’s take the simplest practical case: monitoring temperature and triggering an alert if it exceeds a threshold.

flowchart TD A[Temperature input] --> B[Operational amplifier
Math algorithm] C[Setpoint] --> B B -->D[OUTPUT
HIGH Temp > Setp
LOW: Temp < Setp]

We start with a PTC sensor whose resistance increases with temperature. From its datasheet we pick the resistance at our target temperature. Assume we want switching at 100°C.

Analog algorithm for signal processing

The analog algorithm of a temperature signal processing

Here’s a basic op-amp circuit that does exactly that. With a 1:1 voltage divider at that point and $%R_3$% = 1385 Ω, the circuit looks like this:

A simple cicuit diagram implemening signal processing on Op-Amp

Circuit diagram that implements our signal processing algorithm

The op-amp $%U_{1A}$% acts as a comparator, comparing the voltage on both inputs:

  • Setpoint (non-inverting input +): fixed 6 V ($%R_1/R_2$% = 1:1 divider from 12 V)
  • Temperature signal (inverting input -): decreases as temperature rises (PTC behavior)

When temperature:

  • is above 100°C, the inverting input drops below 6 V → output switches high (12 V)
  • is below 100°C, the inverting input goes above 6V → output switches low

Hysteresis

…but what if the temperature is exactly at 100°C? That’s a good question: without feedback, the comparator can chatter (multiple switches) at the exact threshold. To fix that, add a feedback resistor $%R_4$% (several MΩ) from output to non-inverting input.

Modified circuit diagram that adds ther hystheresis feature

Circuit diagram with hystheresis upgrade

When output goes high, it slightly raises the + input → the threshold becomes a bit higher for the reverse switch. This creates a dead zone (hysteresis window) and eliminates oscillation.

Build the circuit, experiment!

I chose this simple setup on purpose - so you can build it on your bench and play with it. Try these experiments:

  • remove $%R_4$% (feedback) and watch what happens at the switching point
  • change $%R_4$% value and measure how it affects the hysteresis window width
  • reverse the logic: trigger when temperature drops below 30°C
  • bonus challenge (if you’re feeling ambitious): adapt the circuit to support a different temperature sensor type (NTC, thermistor, etc.).

aftermath

This is a good moment to pause the story. Before we go further, I encourage you to spend some time experimenting with analog devices and getting comfy with. Even though today’s world is mostly digital and many people feel uncomfortable with analog techniques - operational amplifiers are still widely used.

Stay tuned for Part 2!