Logical effort

Sunday, 25 August 2013

Logical effort

Logical effort is the ratio of the input capacitance of a gate to the input capacitance of an
inverter delivering the same output current.
Logical Effort indicates how much worse a gate is at producing output current as
compared to an inverter.
Cin can be calculated by estimating how much gate-width each input is seeing.
Logical Effort G = Cin/ Cref
Cref is the input capacitance of a symmetrical inverter, where fall-time = rise-time. This
was realized by increasing PMOS gate width to twice the gate width of the NMOS.
So, input of this inverter will see 3 gate-widths (2 for PMOS and 1 for NMOS).
A symmetrical inverter sees 3 gate-widths in its input. So, Logical Effort for this inverter
G = Cin/Cref = 3C/3C = 1

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