com bi national mos logic circuits

8/3/2019 Com Bi National MOS Logic Circuits

1/39

Combinational MOS LogicCombinational MOS Logic

CircuitsCircuits

By:

May (CMOS Logic Circuits)

Joshua (Complex Logic Circuits)

Hernan (CMOS Transmission Gates)


2/39

Combinational MOS Logic CircuitsCombinational MOS Logic Circuits

Generic combinational logic circuit (gate)

may


3/39


MOS Logic Circuits with Depletion nMOS Loads

a. Two-Input NOR Gate

VA VB Vout

low low high

low high low

high low low

high high lower

may


4/39


Calculation of VOH

which will result to:

may

-When VA and VB are lower than the VTH,driver, driver

transistors are turned off and ID,driver is zero.


5/39


Calculation of VOL

a. VA = VOH and VB = VOL

b. VA = VOL and VB = VOH

may


6/39


The output low voltage level VOL in both cases (a) and (b) is

If (W/L)A = (W/L)B , then VOL for (a) and (b) are identical

may


7/39


c. VA = VOH and VB = VOH

since

may


8/39


The output voltage level is

Thus, to guarantee the required value of VOL in the worst case, the design choice

should yield two identical driver transistors by

a. assuming either VA or VB is logic-high and

b. setting

may


9/39


Generalized NOR Structure with Multiple Inputs

may


10/39


The combined pull-down current can be expressed by

a. at linear

b. at saturation

may


11/39


Assuming that the input voltages of all the driver transistors are

identical,

a. at linear

b. at saturation

may


12/39


Equivalent inverter circuit corresponding to the n-input

NOR gate

may


13/39


Transient Analysis of NOR Gate

may


14/39


Equivalent NOR Gate

where:

may


15/39


b. Two-Input NAND Gate

VA VB Vout

low low high

low high high

high low high

high high low

may


16/39


The NAND2 gate with both of its inputs al logic high level

may


17/39


Thus,

may


18/39


Assuming

and

may


19/39


If

then

To simplify, assume

Thus, the drain currents in the linear region are

and

may


20/39


Since

then

Also, using

may


21/39


Generalized NAND Structure with Multiple Inputs

Assuming the threshold voltages of all transistors are

equal

a. In linear region

b. At saturation

may


22/39


Equivalent NAND Structure Gate

if

then

may


23/39


TransientAnalysis

Of NAND Gate

may


24/39


If VB = VOH and VA switches from VOH to VOL ,the lumped output

capacitance is

may


25/39


CMOS LOGIC CIRCUITS

a. CMOS NOR2 (Two-Input NOR)Gate

may


26/39


Complementary nature operation

When either one or both inputs are high, p-net is cut-off , when the net

creates a conducting path between the output node and the ground

If both input voltages are low, the n-net is cut-off, the p-net creates a

conducting path between the output node and the supply voltage VDD

For any given input combination, the output is either grounded or

connected to VDD

Thus, Vout attains logic-low of VOL = 0 and logic-high voltage of VOH = VDD

may


27/39


Design Considerations

Also,

If the devices are the same, then

Thus,(nMOS transistors are saturated, VGS=VDS )

Therefore,

(combined drain currents of nMOS transistors)

Finally,

(switching threshold voltage)

may


28/39


For Vin = Vout , M3 operates in the linear region while M4 in saturation,

thus

and

also

thus

may


29/39


Combining the two threshold voltages yields the threshold voltage of

the CMOS inverter

Note:

To achieve a switching threshold voltage of VDD/2 for simultaneous switching,

may


30/39


Other representation

And the switching threshold voltage is

may


31/39


Parasiticdevice

capacitance

of the

CMOS

NOR2circuit

may


32/39


EquivalentCMOS

NOR2

circuit with

lumped

outputcapacitance

may


33/39


b. CMOS NAND2 (Two-Input NAND)Gate

With

may


34/39


LAYOUT OF SIMPLE CMOS LOGIC GATES

a. CMOS NOR2 Gate

may


35/39


LAYOUT OF SIMPLE CMOS LOGIC GATES

b. CMOS NAND2 Gate

may


36/39


Stick Diagram

a. CMOS NOR2 Gate

may


37/39


Stick Diagram

b. CMOS NAND2 Gate

may


38/39

SAMPLE LAYOUTSAMPLE LAYOUT

may


39/39


Thank You!

may

com bi national mos logic circuits

Documents