Voltage Dividers

Created at: 2025-01-28

Also called "de-amplifiers", voltage dividers are one of the most widespread circuit fragments, they are everywhere.

Definition

A circuit that given a certain voltage input, produces a predictable fraction of the input voltage as the output voltage. The output voltage is always lower than the input voltage.

img_voltage_divider.png

Vout/Vin = R2 / (R1 + R2)

or:

Vout = (R2 * Vin) / (R1 + R2)

Gotchas

  1. An adjustable voltage divider can be created from using a potentiometer to control Vout. However, in modern circuits you'll instead see a long series chain of equal-value resistors, with an arrangement of electronic switches that lets you choose any on the junctions as the output. This sounds more complicated but allows you to adjust the voltage ratio electrically rather than mechanically.

2. Attaching a load to the voltage divider makes the voltage on the divider to drop. This is often not desirable. One solution to the problem of making a stiff voltage source (“stiff” is used in this context to describe something that doesn’t bend under load) might be to use much smaller resistors in a voltage divider. Occasionally this brute-force approach is useful. However, it is usually best to construct a voltage source, or power supply, as it’s commonly called, using active com- ponents like transistors or operational amplifiers. In this way you can easily make a voltage source with internal (Thévenin equivalent) resis- tance as small as milliohms (thousandths of an ohm), with- out the large currents and dissipation of power characteris- tic of a low-resistance voltage divider delivering the same performance. In addition, with an active power supply it is easy to make the output voltage adjustable.