Rules Of Thumb - SubThreshold.com

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Above Threshold » Rules Of Thumb

Member of: DesignPrinciples & Equations

This page is for Rules of Thumb and other handy shortcuts.

Above threshold operation usually occurs for bias currents > 1μA in a 0.5μ process.

The transistor as an amplifier

nMOS

Gate voltage increases: I_ds increases by a large amount.
Drain voltage increases: I_ds increases by a small amount.
Source voltage increases: I_ds decreases by a large amount.

pMOS

Gate voltage increases: I_sd increases by a large amount.
Drain voltage increases: I_sd increases by a small amount.
Source voltage increases: I_sd decreases by a large amount.

This can be restated as:

Gate is a low impedance (high conductance) node.
Drain is a high impedance node.
Source is a low impedance (high conductance) node.

or in equation form: $\Delta I_{ds} = g_m \Delta V_g + g_d \Delta V_d - (g_m + g_d) \Delta V_s$ (for the small signal model)

Relative Values

Knowing these can help simplify lots of equations:

r_ds > 1/g_s> 1/g_m Approximately one order of magnitude between each step.
$g_m \approx 10^{-6} \frac1{\Omega}$ , $g_d = \frac1{r_{ds}} \approx 10^{-8} \frac1{\Omega}$

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Last edited by DrLock. Originally by DrLock.