Output resistance (ro): definition

Beginner

How well a transistor keeps its current steady when the push across it changes. Higher means steadier.

Novice

The inverse of the slope of the $I_{\mathrm{D}}$ – $V_{\mathrm{DS}}$ curve in saturation: $r_{\mathrm{o}} = \Delta V_{\mathrm{DS}}/\Delta I_{\mathrm{D}}$ . An ideal current source has infinite $r_{\mathrm{o}}$ ; real transistors have a finite $r_{\mathrm{o}}$ because of channel-length modulation and DIBL.

Expert

$r_{\mathrm{o}} = (\partial I_{\mathrm{D}}/\partial V_{\mathrm{DS}})^{-1} \approx 1/(\lambda I_{\mathrm{D}})$ from CLM alone, lower again with DIBL. Combined with transconductance it gives the intrinsic gain $g_{\mathrm{m}} r_{\mathrm{o}}$ , which falls as channels get shorter.