VLSI LAB ASSIGNMENT QUESTIONS
1. Extract the Vt of NMOS and PMOS transistor from Vgs Vs Id characteristics and estimate the variation in Vt with variation in Vbs. The variation is between +Vdd and –Vdd and establish a relation between Vbs and Vt. Solution: Vt of NMOS CIRCUIT DIAGRAM
STEPS
(1) Choose nMOS nMOS and right click and pick New MOSFET MOSFET IRLML6346. (2) Select voltage sources for Vgs and Vdd. (3) Set DC value of 5V to Vdd. (4) From simulate tab, select edit simulation Cmd .
tab and select select
(5) On DC sweep tab select 1st source as V2 with start value :0, stop value :5 and increment 0.1. (6) Run the file. OUTPUT
(1) Right click on plot and select Id from add trace. (2) Place the cursor on the plot at the point where Id starts to increase from zero.
RESULT:
Vt of NMOS obtained is 1.053V
Vt of PMOS
The same procedure can be repeated for finding Vt of PMOS CIRCUIT DIAGRAM
RESULT :
Vt of PMOS obtained is -0.905V . Relation between Vt and Vbs of NMOS (1) Choose nMOS4. (2) Select voltage sources for Vgs, Vbs and Vdd. (3) Set DC value of 3V to Vdd. (4) From simulate tab, select edit simulation Cmd . st (5) On DC sweep tab select 1 source to sweep as V2 with start value :-3, stop nd value :5 and increment 0.1. Select 2 source to sweep as V1 with start value :-3, stop value :3 and increment 1. (6) Run the file. CIRCUIT DIAGRAM
OUTPUT
(1) Right click on plot and select Id from add trace. (2) Place the cursor on the plot at the point where Id starts to increase from zero.
RESULT:
Vt varies as -2.7, -1.7, -0.7, 0.25, 1.25, 2.25 V as Vbs is varied from -3 to 3 in steps of 1V.
Relation between Vt and Vbs of PMOS (1) Choose pMOS4. (2) Select voltage sources for Vgs, Vbs and Vdd. (3) Set DC value of -3V to Vdd. (4) From simulate tab, select edit simulation Cmd . st (5) On DC sweep tab select 1 source to sweep as V1 with start value :-3, stop nd value :5 and increment 0.01. Select 2 source to sweep as V2 with start value :-3, stop value :3 and increment 1. (6) Run the file. CIRCUIT DIAGRAM
OUTPUT (1) Right click on plot and select Id from add trace. (2) Place the cursor on the plot at the point where Id starts to increase from zero.
RESULT: Vt varies as -2.2, -1.2, -0.2, 0.6, 1.6, 2.6 V as Vbs is varied from -3 to 3 in steps of 1V.
2. Estimate and plot transconductance of nmos and pmos transistors for Vds and Vgs variations. Solution: Transconductance of nmos Circuit Diagram
Steps: (1) Choose nmos-IRLML6346. (2) Connect two voltage sources for gate and drain respectively. (3) Set Vds=3V and vary Vgs from 0 to 1.3V. (4) Obtain the plot-Id vs Vgs. (5) Plot transconductance using the same plot by using the formula () = () (6) Now set Vgs=1.3V and vary Vds. (7) Plot Id vs Vds graph and obtain transconductance.
Output:
gm_Vgs
gm_Vds
Transconductance of pmos Circuit Diagram
Steps: (1) Choose pmos-IRF7404.
(2) Connect two voltage sources for gate and drain respectively. (3) Set Vds=-20V and vary Vgs from -5 to 0V. (4) Obtain the plot-Id vs Vgs. (5) Plot transconductance using the same plot by using the formula
=
() ()
(6) Now set Vgs=-2.5V and vary Vds. Plot Id vs Vds graph and obtain transconductance.
Outputs:
gm_Vds
gm_Vgs
(3) Design a CMOS inverter and simulate the circuit. Solution : CIRCUIT DIAGRAM
STEPS (1) Choose nmos , pmos, voltage sources for Vin and Vdd and wire them as shown in the circuit diagram. (2) Set DC value of 5V to Vdd. (3) From simulate tab, select edit simulation Cmd. (4) On DC sweep tab select 1st source as V1 with start value :0, stop value :5 and increment 0.01. (5) Run the file to obtain transfer characteristics of the inverter. (6) Transfer characteristics can be obtained by placing the probe on the output node. (7) To obtain the transient response, from simulate tab, select edit simulation Cmd. (8) On transient tab give stop time 10, time to start saving data 0 and maximum time step 1. (9) Give a pulse input to V1 by right clicking on V1,selecting advanced options and selecting pulse input. Give Von as 5, Ton as 0.5, Tperiod as 1 and Ncycles as 7.
(10) Run the file. Add traces of V[n002] and V[n004] to observe transient characteristics.
TRANSFER CHARACTERISTICS
TRANSIENT RESPONSE
RESULT : A CMOS inverter circuit has been designed and simulated and the transfer characteristics and transient response are observed.
4. Change the βn/ βp ratio from 1,2…10 and study the CMOS characteristics (Vout/Vin). Solution : CIRCUIT DIAGRAM
STEPS (1) Choose nmos4 , pmos4, voltage sources for Vin and Vdd and wire them as shown in the circuit diagram. (2) Set DC value of 5V to Vdd. (3) From simulate tab, select edit simulation Cmd. (4) On DC sweep tab select 1st source as V1 with start value :0, stop value :5 and increment 0.01. (5) Right click on nmos4 and set length to 1μ and width as {X}. (6) Right click on pmos4 and set length to 1μ and width to 1μ. (7) In the spice directive type .step PARAM X 1u 10u 1u to swipe the width of nmos4 from 1μ to 10μ thereby changing the βn/ βp ratio from 1,2…10. (8) Run the file to obtain the transfer characteristics for various values of βn/ βp.
RESULT:
The transfer characteristics of inverter for values of βn/ βp ranging from 1,2….10 has been plotted and the switching threshold of the inverter which was approx. 2.5V is found to reduce as βn/ βp is increased from 1 to 10.
