Lecture 20 Today we will Look at why our NMOS and PMOS inverters might not be the best inverter designs Introduce the CMOS inverter Analyze how the CMOS inverter works
NMOS Inverter 5 V 5 V When VIN is logic 1, VOUT is logic 0. Constant nonzero current flows through transistor. Power is used even though no new computation is being performed. R R VOUT D ID = 5/R VOUT D ID = 0 VIN 0 V VIN + VDS _ 5 V + VDS _ 5 V 0 V When VIN changes to logic 0, transistor gets cutoff. ID goes to 0. Resistor voltage goes to zero. VOUT “pulled up” to 5 V.
PMOS Inverter 5 V 5 V When VIN is logic 0, VOUT is logic 1. Constant nonzero current flows through transistor. Power is used even though no new computation is being performed. VIN VIN - VDS + - VDS + VOUT VOUT 0 V 5 V ID = -5/R 5 V ID = 0 0 V R R When VIN changes to logic 1, transistor gets cutoff. ID goes to 0. Resistor voltage goes to zero. VOUT “pulled down” to 0 V.
Analysis of CMOS Inverter We can follow the same procedure to solve for currents and voltages in the CMOS inverter as we did for the single NMOS and PMOS circuits. Remember, now we have two transistors so we write two I-V relationships and have twice the number of variables. We can roughly analyze the CMOS inverter graphically. VDD (Logic 1) S D VOUT D VIN S NMOS is “pull-down device” PMOS is “pull-up device” Each shuts off when not pulling
NMOS Inverter ID VGS = 3 V X Linear ID vs VDS given by surrounding circuit X VGS = 1 V VDS
Linear KVL and KCL Equations VDD (Logic 1) VGS(n) = VIN VGS(p) = VIN – VDD VGS(p) = VDS(n) - VDD ID(p) = -ID(n) VDS(n) = VOUT VDS(p) = VOUT – VDD VDS(p) = VDS(n) - VDD + VGS(p) - S D VOUT D + VDS(n) _ VIN + VGS(n) - S Use these equations to write both I-V equations in terms of VDS(n) and ID(n)
CMOS Analysis ID(n) As VIN goes up, VGS(n) gets bigger and VGS(p) gets less negative. NMOS I-V curve PMOS I-V curve (written in terms of NMOS variables) VIN = VGS(n) = 0.9 V VDS(n) VDD
CMOS Analysis ID(n) As VIN goes up, VGS(n) gets bigger and VGS(p) gets less negative. NMOS I-V curve PMOS I-V curve (written in terms of NMOS variables) VIN = VGS(n) = 1.5 V VDS(n) VDD
CMOS Analysis ID(n) As VIN goes up, VGS(n) gets bigger and VGS(p) gets less negative. NMOS I-V curve PMOS I-V curve (written in terms of NMOS variables) VIN = VGS(n) = 2.0 V VDS(n) VDD
CMOS Analysis ID(n) As VIN goes up, VGS(n) gets bigger and VGS(p) gets less negative. NMOS I-V curve PMOS I-V curve (written in terms of NMOS variables) VIN = VGS(n) = 2.5 V VDS(n) VDD
CMOS Analysis ID(n) As VIN goes up, VGS(n) gets bigger and VGS(p) gets less negative. NMOS I-V curve PMOS I-V curve (written in terms of NMOS variables) VIN = VGS(n) = 3.0 V VDS(n) VDD
CMOS Analysis ID(n) As VIN goes up, VGS(n) gets bigger and VGS(p) gets less negative. NMOS I-V curve PMOS I-V curve (written in terms of NMOS variables) VIN = VGS(n) = 3.5 V VDS(n) VDD
CMOS Analysis ID(n) As VIN goes up, VGS(n) gets bigger and VGS(p) gets less negative. NMOS I-V curve PMOS I-V curve (written in terms of NMOS variables) VIN = VGS(n) = 4.1 V VDS(n) VDD
CMOS Inverter VOUT vs. VIN both sat. curve very steep here; only in “C” for small interval of VIN VDD NMOS: cutoff PMOS: triode NMOS: triode PMOS: cutoff NMOS: triode PMOS: saturation NMOS: saturation PMOS: triode VIN D A B C E VDD
CMOS Inverter ID ID VIN D A B C E VDD
Important Points No ID current flow in Regions A and E if nothing attached to output; current flows only during logic transition If another inverter (or other CMOS logic) attached to output, transistor gate terminals of attached stage do not permit current: current still flows only during logic transition D S VDD VOUT1 D S VDD VOUT2 VIN