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EXAMPLE:
DESIGN AND SIMULATION OF AN INVERTING AMPLIFIER
This example will help you familiarize with Design Framework II. You will design a
simple inverting amplifier, and then observe its operating point and frequency response
behavior. This will show the most important commands and steps to use when working
with schematics in DFII.
Before starting with the design example, there are some details that are worth
mentioning:
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The appendix contains some notes about using the on-line help feature, and using
the mouse. There's also a table with keyboard shortcuts.
Most of the commands in DFII can be accessed in multiple ways: pull-down
menus, shorcut keys, buttons in toolbars, etc. In the described example, all the
commands are referenced by their position in the pull-down menus.
The most used key in DFII is ESC. It is used to cancel on-going commands.
The following picture shows the inverting amplifier circuit, ready for netlisting. The next
section explains how to draw it in DFII.
Figure 1: Design example.
1. Create a library for your new design:
From the library manager window:
File->New->Library
Type a new name, such as TEST.
Under the heading "Technology File", choose "Compile a new techfile".
Then from the dropdown menu choose "TSMC 0.24u CMOS025/DEEP (5M, HV FET).
Click OK
2. Create a new cell, where you will design the inverter:
In Library Manager:
Highlight your new library (TEST if that is what you chose).
File->New->Cellview
Choose library TEST, cell name "inverter", view name "schematic", and Tool
"Composer-Schematic". Click OK.
A schematic window will open.
3. Design your circuit:
3.1 Placing components:
For this inverter, you will need an nmos transistor, a resistor, a capacitor, and power and
ground nodes.
From Schematic window:
Add->Instance
Add Instance and Component Browser windows will open.
Make sure the Library in the Component Browser is set to NCSU_Analog_Parts.
Use the Component Browser window and single click N_Transistors, then select
nmos4 and place it.
Use the same procedure to go under R_L_C and place a res and cap, from library
NCSU_Analog_Parts.
Also, from the NCSU_Analog_Parts, get the symbols for vdd and gnd from
Supply_Nets (they define the net names for the power and ground nodes).
If you make any mistake, you can always use:
Edit->Delete or
Edit->Rotate or
Edit->Move or
Edit->Stretch
When using editing commands, you can access additional options (for example, to flip a
component sideways), by pressing F3.
You need to change the properties of some components:
Edit->Properties->Objects
Select the transistor, and change the following parameters:
Width: 60u
Length: 1.2u
Make sure you change the Width and Length boxes to these values, not the
Width(grid units) and Length(grid units) boxes.
NOTE: For this design kit you can only set transistor lengths and widths in
multiples of grid units(.06uM). Keep this in mind when making your circuits.
Select the resistor. Edit the properties to set a resistance of 4k Ohms.
By default, the capacitor value should be 1 pF. You can leave it that way.
3.2 Connect components:
Connect the component terminals as shown in figure 1, using:
Add->wire (narrow)
4. Adding Pins:
By adding pins, you can identify the I/O ports of the schematic. At a later stage, you can
also use pins as connection points for hierarchical designs. To learn more about this, see
the section about creating symbols.
Add->Pin
Type the pin name, such as Vin, select the direction as "input", and place it in the
schematic. Do the same for Vout, selecting the direction as "output".
5. Simulate:
From analogLib, get your signal source. You can find anything you need for this project
(and much more), in there: vdc, idc, vpwl, vsin, vpulse, etc. For this example, let's get
vdc, and connect it to the gate of the transistor.
Edit the properties of vdc so that it supplies a DC voltage of 770 mV, and an AC
magnitude of 1 V.
Get another vdc from analogLib, and connect it to vdd! and gnd!. You can place more
vdd! and gnd! symbols since they are global (that's what the exclamation mark '!' means).
Edit the properties of vdc so that it supplies a DC voltage of 3.3 V.
Your schematic should now look something like this:
Figure 5.1: Schematic ready for simulation.
In Schematic Window:
Design->Check and Save
There should be no errors. Warnings such as "Solder dot on cross over" are okay.
Tools->Analog Environment
In Cadence Analog Design Environment:
The following two steps only need to be done for this first session.
Setup->Simulator/Directory/Host
Set the simulator to Spectre.
Setup->Model Libraries Use the model files in the directory (for VLSI lab):
/home/class/ee5333t1/models/spc/models.scs Section “c0”. Click Add and
OK.
Set Analysis:
Analyses->Choose
Select dc analysis
Click "dc operating point"
Select ac analysis
Choose sweep range, from 1 kHz to 1 GHz.
Choose sweep type "logarithmic, 10 points per decade.
Click OK.
Choose nodes to be saved:
Outputs->Save all, click 'OK'.
Start simulation:
Simulation->Run
The simulator status can be seen in the log (CIW) window. Ignore the warnings about the
model files, and wait until it finishes.
6. View Results:
To see the operating point, do the following in the Analog Artist Simulation Window:
Results->Annotate->DC Node Voltages.
The node voltages will be annotated in the schematic window. You can also annotate the
transistors operating points by choosing:
Results->Annotate->DC Operating Points.
Figure 2: DC operating point results.
To see the results of the ac analysis, do the following:
In the Analog Artist Simulation Window:
Tools->Calculator
Then, in the Calculator:
Click the "vf" button, click on output net in schematic.
Click "plot".
The Waveform Window opens. You can find, for example, the unity gain frequency
easily by choosing:
Markers->Horizontal Marker
Type 1 Volt in "Marker locations", and click "Apply". If you want to see the exact
value, click "Display Intercept Data".
You can also see the output voltage in dB. Just click the 'dB20' button in the calculator,
and then 'Plot'. To delete other plots, click directly on the curve and press Delete.
Figure 3: AC analysis results