Challenge: Square a Voltage

Wait 5 sec.

Your design task, should you decide to accept it: given an input voltage, square it. Ok, that’s too hard since squaring 8 volts would give you 64 volts, so let’s say the output should be 10% of the square, so 8 volts in would result in 6.4V. How do you do it? [Engineering Prof.] knows how and will show you what you can do in the video below.The circuit uses two op amps and some transistors. However, the transistors are used in a way that depends on the temperature, so it is important to use a transistor array so they are matched and will all be at the same temperature.The math depends on the fact that the transistor response has a natural log term in it, and the property that the sum of two logs is the same as the log of the product of the numbers.Because of the matching transistors, many of the terms in the equation cancel out. Because the transistors are current devices, the transistor circuit’s output current is the input current squared divided by the output transistor’s collector current. Then it is just a matter of converting the voltage to a current and back again using the right scaling.There’s more to it, of course, but that’s the gist of it. You can dig into the math by watching the video. If the KCL references are fuzzy for you, here’s a refresher. Squaring a voltage would be pretty important for an analog computer.