ECE 546 HOMEWORK No 9 Due Wednesday, April 22, 2020

In this homework you will exercise the DC analysis feature of Virtuoso and use them to properly

size the inverter transistors in order to achieve a 50-ohm match.

1. Transistor model

Recall that MOSFETs can be used as a voltage-controlled resistor when in diode-connected

mode as shown in Figure 1.

Figure 1. CMOS diode-connected mode

The transistor in the inverter-based transmitter you did in the previous homework can be

modeled as an ideal switch and a series resistor as shown in Figure 2.

Draw a similar model for the differential transmitter, channel and termination from the last

step of previous homework. Comment on the target on-resistance of each transistor.

Figure 2. CMOS inverter and switch equivalent-circuit.

2. DC analysis in Virtuoso

To enable DC analysis, simply choose ‘dc’ from the analysis setup in the ADE environment

(see Figure 3) after you finish setting up a circuit ready for DC analysis.

Figure 3. ADE L Analysis dialog box

In order to measure the on-resistance of the transistor, the voltage drop across the transistor, V

and the current through it, I will also need to be measured. The resistance is simply the ratio V/I.

This can be evaluated using the built-in calculator. You can enter the calculator mode by clicking

the ‘open’ button from the “output setup” dialog box within the ADE environment.

Figure 4. ADE L – Outputs settings dialog box

Next, click ‘vdc’ from the upper row of the new window, the schematic will automatically

come to the foreground. Click the node for which you want to measure the dc voltage. An

expression VDC(“/NODE_NAME”) will appear in the text box. This will measure the DC

voltage of the node. Similarly, clicking on ‘idc’ will measure the DC current, ‘vf’ and ‘if’ will

measure the frequency-domain voltage and current respectively, ‘vt’ and ‘it’ will measure the time

domain voltage and current respectively. You can also select functions from the ‘Function Panel’

button to perform advanced mathematical manipulations of the signal.

Figure 5. ADE L – Visualization and analysis calculator.

The expression for calculating the on-resistance of the transistor will be ‘abs((VDC(source) –

VDC(drain))/IDC(drain))’. Once you finished editing the expression, go back to the output setup

window and click ‘get expression’, the expression will be automatically copied. A proper name

should be given to the expression and then click ‘add’.

Figure 6. ADE L – Implementing mathematical expressions

3. Sizing the transistor

Note that the on-resistance of the transistor not only depends on the voltage drop across it but

also on the absolute voltage of both the source and the drain.

Make a schematic with an NMOS and PMOS in diode-connected configuration, like Figure 7.

Perform a DC simulation while sweeping the widths of each transistor and calculate Ron,N and Ron,P

for each width. You will then find out what value of the width should be for NMOS and PMOS to

achieve the desired matching impedance of the channel. Once DC analysis is selected in ADE L,

go to ‘Variables > Copy from Cellview’ to import variables from schematic. You can either enter

a value in Design Variables windows to run a single simulation or run a parametric sweep of any

variables.

Figure 7. DC setup for on-resistance measurement

To sweep variables, go to ‘Tools > Parametric Analysis’, right-click to add new row, for each

row, select a variable, set up how you want to sweep it and run the simulation from the Parametric

Analysis window.

Figure 8. Parametric analysis

Report the width of NMOS and PMOS that produces 50 ohm on-resistance. With those widths,

rerun the last part in the previous homework (the differential driver with TYCO.s4p channel).

Comment on the result compared to that in previous homework.