Stiff Versus Weak Source - Electronic Circuits II & III: Decision Networks and Interfaces Step by Step

This stiff-versus-weak-source lesson compares how much load voltage survives when source resistance changes. The 12 V stiff case and 6 V weak case are exact only for the stated source/load resistances in an ideal DC divider. Real sources have output-impedance variation, current limits, heating, bandwidth effects, parasitics, noise, calibration limits, and measurement uncertainty.

Changing source resistance changes the same load voltage by exact divider arithmetic. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

highlighted = computed this step

A lower source resistance holds more load voltage

With source resistance 2 ohm, the load gets 12 volts.

V_{L,\text{stiff}} = 12\ \text{V}

A higher source resistance loses more voltage

With source resistance 10 ohm, the same load gets 6 volts. Reactive impedance and frequency response are deferred.

V_{L,\text{weak}} = 6\ \text{V}