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ADS Fundamentals 2003

LAB 3: DC Simulations and Circuit Modeling

Overview - This chapter introduces parametric subnetworks: how to create and usethem in hierarchical designs. Beginning with a device model, the lowest levelsubnetwork will also contain packaging parasitics to better model the devicebehavior. Also, a test template will be used to simulate curve tracer responses fromwhich a bias network can be computed, built, and checked. The circuit in this lab

exercise will be the foundation of the amplifier that will be used for the other labexercises in this course.

OBJECTIVES

Model a generic BJT with parasitics and save it as a sub circuit.

Set up and run numerous DC simulations to determine performance.

Calculate bias resistor values in the data display.

Build a biased network based on the DC simulations.

Test the biased network.

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Lab 3: DC Simulations

Table of Contents

1. Create a new project: amp_1900....................................................................... 3

2. Set up a generic BJT symbol and model card. .................................................. 3

3. Add parasitics and connectors to the circuit. ..................................................... 5

4. View the default symbol.....................................................................................6

5. Set up the Design Parameters and built-in symbol............................................6

6. Use a curve tracer template to test the bjt_pkg sub-circuit. ...............................8

7. Modify the template Parameter Sweep............................................................ 10

8. Simulate at beta=100 and 160......................................................................... 10

9. Open a new design and check all your files in the Main window. .................... 12

10. Set up and simulate the dc bias parameter sweep....................................... 13

11. Calculate bias values Rb and Rc for a grounded-emitter circuit. ..................14

12. Set up the biased network............................................................................ 15

13. Simulate and annotate the DC solution. ....................................................... 16

14. OPTIONAL: Sweep Temperature................................................................17

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PROCEDURE

The circuit you build for this lab exercise will be used as the lower level sub-circuitfor all of the amplifier labs to follow.

1. Create a new project: amp_1900.

a. If you have not already created this project, do it now. Then, in thisnew project, amp_1900, open a new schematic window and save itwith the name: bjt_pkg. Also, set any desired Options > Preferences.

2. Set up a generic BJT symbol and model card.

a. In the schematic window, select the palette: DevicesBJT. Select theBJT-NPN device shown here and insert it onto the schematic.

b. Insert the BJT_Model (model card) shown here.

NOTE: The BJT_NPN symbol shows

Model = BJTM1. This means the

symbol will use that specific model

(model card) for simulation.

c. Double click on the BJT_Model card. When the dialog appears, clickComponent Options andin the next dialog, click Clear All forparameter visibility then clickApply. This will remove the Gummel-Poon parameter list from the schematic. Keep this dialog open.

NOTE on Binning: You can insert multiple model cards and use the Binningcomponent to vary the model device variations. These variations can be parameterssuch as temperature, length or width. The Binning component allows you to create amatrix that references the desired models.

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d. Next, in BJT_Model dialog, select the Bfparameter and type in theword beta as shown here. Also, click the small box: Displayparameter on schematic forBfonly and then clickApply. Beta isnow a parameter of this circuit later on you will tune it like a variable.

Use Component

Options to clear all the

displayed parameters.

Click here todisplay an

individual value.

e. SetVaf(Forward Early Voltage) =50 and display it.

f. Set Ise (E-B leakage) = 0.02e-12, and display it also.Then close the dialog with OK. The device now hassome more realistic parameters.

g. For the BJT device, remove the unwanted displayparameters (Area, Region, Temp and Mode) byunchecking the box. This will make the schematic lesscrowded with parameters that you are not using.

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3. Add parasitics and connectors to the circuit.

The picture shown here is the completed sub-circuit with connectors and parasitics.Remember to use the rotate icon for orientation of the components as you insert

them. Here are the steps:

a. Insert lumped L and C components: Insert three lead inductors of320pH each and two junction capacitors of120 fF each. Be sure to usethe correct units (pico and femto) or your circuit will not have thecorrect response. Tip: type L or C in component history to get thecomponents onto your cursor without using the palette.

b. Add some resistance R= 0.01 ohms to the base lead inductor anddisplay the desired component values as shown.

c. Insert port connectors: Click the port connector icon (shown here)

and insert the connectors exactly in this order: 1) collector, 2)base, 3) emitter. You must do this so that the connectors have theexact same pin configuration as the ADS BJT symbol.

d. Edit the port names as show here: change P1 to C, change P2 to B, andchange P3 to E.

e. Clean up the schematic: Position the components so that the schematiclooks organized this is good practice. To move component text, pressthe F5 key, select the component, use the cursor to position the text.

NOTE: You must

number the ports

(num=) exactly as

shown or the device

will not have the

correct orientationfor the symbol that

will be used.

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4. View the default symbol.

There are three ways to create a symbol for a circuit: 1) use the defaultsymbol, 2) draw a symbol, or 3) use a built-in ADS symbol. For this lab you

will use a built-in BJT symbol. The following steps show how to do this:

a. To see the default symbol, click: View > Create/EditSchematic Symbol. When the dialog appears, click OKand the default symbol will appear.

b. Next, a box or rectangle withthree ports is generated. Thisis the default symbol.However, delete this symbolusing the commands: Select> Select All. Then click the

trash can icon or delete key.

Default symbol

c. Return to the schematic - click:View > Create/EditSchematic.

5. Set up the Design Parameters and built-in symbol.

a. Click File> Design Parameters and the dialog appears

b. In the General tab, make theses changes: 1) change the ComponentInstance Name to Q, 2) change the Symbol Name to SYM_BJT_NPNby clicking the arrow and selecting it (this is the built-in symbol), 3) in

the Artwork field, select Fixed and SOT23 as shown here.

Built-in symbol:

SYM_BJT_NPN

SOT23 fixed artwork

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c. Click Save AEL File to write these changes but do not close this dialogyet because you still need to set other parameters.

d. Go to the Parameters tab. In the Parameter Name area, type in beta

and assign a default value of100 by clicking theAdd button. Be surethe box to Displaythe parameter is checked as shown here. Click theOK button to save the new definitions and dismiss the dialog.

NOTES: multiplicity _M

You can define multiplecomponents in parallel.You can also copyparameters from anotherdevice or file.

The parameterbeta is now

recognized as a variable of

this circuit. Its value can

now be passed (assigned)

from an upper level

hierarchy when you use

bjt_pkg as a sub-circuit.

e. In the schematic window, Save the design (click theicon shown here) to make sure all your work creatingthis sub-circuit will not be lost. In the next steps, youwill see how the Design Parameters will be used.

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6. Use a curve tracer template to test the bjt_pkg sub-circuit.

a. In the current schematic of the bjt_pkg, click File > New Design.When the dialog appears, type in the name: dc_curves and select the

BJT_curve_tracer template as shown. Click OK and a new schematicwill be created with the template, ready to insert bjt_pkg.

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This is where you

will connect the

device (bjt_pkg) in

the next few steps.

This is a data display

template it willautomatically plot the

curves.

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b. Save the design and then click the ComponentLibraryicon (shown here).

c. When the dialog opens, select the amp_1900

project as shown and click on the bjt_pkg sub-circuit. Insert it into the schematic as shown here. Every circuit thatyou build will be available in the project library as a sub-circuit.

d. Connect the bjt_pkg componentas shown. You may have toadjust the wires and text (F5) tomake it look good. Also, you can

now close the library windowand save the dc_curves designagain - it is good practice to saveoften.

NOTE on templates: You can also insert templatesusing the schematic window command: Insert >Templates. Many templates have pre-defined values,

node names (wire labels), and variables. Therefore, youmay have to make modifications to fit your circuit. Also,many of these templates have data display templates thatautomatically plot the data in a pre-defined format.These same data display templates are available in thedata display window. In general, using templates is veryefficient and timesaving if you know how to use ADS.

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7. Modify the template Parameter Sweep.

a. Change the Parameter Sweep IBB values to: 0 uA to 100 uA in 10 uAsteps as shown here. Do not change the DC simulation controller

default settings for sweeping VCE they are OK. Notice that the VAR1variables (VCE=0 and IBB=0) do not require modification because theyare only required to initialize (declare) the variable for the simulator.

VarEqn is required

to initialize

Only one variable

can be swept in the

controller.

Parameter sweep used for

multiple variable sweeps. Note

that DC1 is the name

(SimInstance Name) of the

simulation controller.

8. Simulate at beta=100 and 160.DC Curves at

beta = 100a. Simulate (F7) with Beta = 100. Afterthe simulation is finished, the datadisplay will appear with the curve tracerresults (data display template). Trymoving the marker and watch theupdated values appear.

b. Simulate againwith Beta = 160bychanging the value directly on theschematic. You should see

the updated values.

c. Verify the values for beta =160 and VCE = 3V, whereIBB=40 uA and IC=3 mAwith about 10mW ofconsumed power If not,

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check the design.

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9. Open a new design and check all your files in the Main window.

a. Save the current schematic: dc_curves. In the same window,create a new design (without a template) named: dc_bias.

Then save the design by clicking on the Save CurrentDesign icon (shown here) so that it is written into the ADSdatabase.

b. Now, check the ADS Main Window: you should have 3 designs in thenetworks directory: bjt_pkg, dc_curves, and dc_bias. You may have todouble-click on the file browsernetworks to refresh the browser.

Click here to refresh

or reread the

database.

c. In the File Browser area, click on the plus / minus boxes and the uparrow (or two dots). This allows you to see the files you have created inthis project. Remember: you can only work in one project at atime, but you can copy files from other projects and bring them in.

Project and directory

path are shown here.

Try clicking the plus

(expand) and minus

(contract).boxes.

Try the up arrow.

d. Finally, try the Show / Hide all windows feature. This isused for security or to find other open windows that arenot ADS. In this case, only the Main window remains.

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10. Set up and simulate the dc bias parameter sweep.

NOTE on parameter sweeps: If only one variable is swept, you can use theSimulation controller (Sweep tab). However, if more than one parameter is swept, a

Parameter sweep component is required as in the templates you have just used. Ingeneral, all simulation controllers allow you to sweep only one parameter (variable).

BUILD the circuit without a template - the steps follow:

a. Insert the bjt_pkg using library icon or the component history. Nowpush into the bjt_pkg and click File > Design Parameters. Reset thebeta parameter default to 160, pop out and delete the bjt_pkg andreinsert it beta is now 160 whenever you use the modeled circuit.

b. From the Probe components palette or component history, insert anI_Probe and rename it IC instead of I_Probe1 as shown here.

c. From the Sources-Frequency domain palette or using componenthistory, insert a dc supply and current source and set their values asshown: V_DC = 3 Vand I_DC = IBB as shown here.

d. Wire the components together and add the ground (ground icon).

e. Insert aDC simulation controller or DC. Edit (double click) thecontroller and go to the Sweep tab and assign: IBB: 10 uAto 100 uAin 10 uAsteps. Then go to the Display tab and check the settings tobe displayed as shown. Then clickApplyand OK.

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f. Insert aVAR (click icon) variable equation. Use the cursor on thescreen to set IBB=0 Ato initialize (declare) the variable to be swept.

g. Insert a wire labelVBE at the base. The voltages at that node will

appear in the dataset for use in calculating bias resistor values.

h. Simulate and plot the data. When the data display opens,insert a list ofVBE and IC.i only. Because you swept IBB toget these values, IBB will automatically be included.

NOTE on results: As you can see, with 3 volts across the device, 40 uA of basecurrent results in about 799mV across the base-emitter junction, with about 3.3 mA ofcollector current. If you want, draw a box around the values at 40 uA IBB.

i. Save the design and data display.

11. Calculate bias values Rb and Rc for a grounded-emitter circuit.

a. In the data display, insert an equation and type: Rb = (3 VBE) / IBB.

b. Select the Rb Eqn and use the keyboard Ctrl C and Ctrl Vtocopy/paste it it will become Rb1.

c. Highlight the Rb1 equation as shown and change itto become Rc: Rc = 2 / IC.i. The total DC supplywill be 5 volts. Therefore, with 3 volts VCE, 2 voltsremain for the collector resistor.

d. Insert a new List and scroll down to the Equations menu (shownhere), and add Rb and Rc. Then edit both column headings on the list

with a bracketed [3] as shown. This references the 40uA IBB using itsindex value [3]. Index values begin at zero: 0, 1, 2, 3, etc. You can alsouse Plot Options to add a label to the list as shown:

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12. Set up the biased network.

Now that you have the calculated bias resistor values, you can test the bias network.

a.Save

the design (dc_bias) with a new name:dc_net

. By this time, youshould know how to do this (File > Save Design As). Also, save andclose the dc_bias data display.

b. Modify the design as shown. Begin by deleting the current source IBB,the I_Probe, and the Var Eqn.

c. Go to the Lumped Components palette or use component history orHot Key Ctrl R to insert resistors for the base (56 kOhm) andcollector (590 Ohm) as shown. You may need to use the rotate icon toinsert it correctly.

d. Change the instance R names to RC and RB as shown.

Insert the cursor

and type over to

rename it to RC.

NOTE on components with artwork: Later on (after the last lab), you can easilyand quickly change to lumped components with artwork by changing the componentname for example, change R to R_Pad1, C to C_Pad1, L to L_Pad1, etc. Then youcan create a layout of the schematic. For now, use lumped without artwork.

e. Set the V_DC supply:Vdc = 5 V. Wire the circuit and organize it.

f. Delete the DC simulation controller and put a new one in its place thisis faster and more efficient than removing the sweep settings. Becausethere is no sweep, you do not have to set anything to check DC values.

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13. Simulate and annotate the DC solution.

a. Use the Simulate > Simulation Setup(or Hot Key S if you have set it) and

uncheck the automatic open datadisplay feature.

b. Press the F7 keyboard key and the simulation will be launched with thedataset name that is the same as the schematic this is the default. Youcan verify this by reading the status window:

c. Annotate the current and voltage by clicking on the menu command:Simulate > Annotate DC Solution. If necessary, move componentsor component text (F5 key) to clearly see the values of voltage andcurrent. Be sure that you have the same values shown here. If not,check your work, including the sub-circuit.

NOTE: To move

pin/node names,

point-click-drag,

d. Clear the annotation, click: Simulate > Clear DC Annotation andthen Save all you work. Close all windows if nor doing the optionalsteps that follow.

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14. OPTIONAL: Sweep Temperature

a. Edit the DC controller select it and click the edit icon.

b. In theSweep tab

, enter the ADS global variabletemp

(default isCelsius) as shown here and enter the sweep range: -55 to 125 with stepsize = 5. Also, in the Display tab, click the boxes to display theannotation on the controller clickApplyto see it and OK to dismissthe dialog.

c. InsertVC andVBE node / pinlabels.

d. Set the simulation dataset nameto dc_temp, and check the box toopen the data display dc_net.ClickApplyand then Simulate.

NOTE: Trise can be set for an individual

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e. Plot the results in a rectangular plot asVC vs temp andVBE vs temp -you should be able to do this as shown:

f. Put two markers on

each trace in deltamode to see the changein voltage astemperature changes.The plot should looklike the one shownhere: collector voltagedecreases at almostone-half the rate ofVBE as thetemperature increases.You can use this

temperature sweepmethod (sweeping theglobal variable temp)for any ADS simulation.

EXTRA EXERCISES:

1. Use the template SP_NWA_T to generate S-parameters for bjt_pkg atall the bias points. This is a very useful template!

2. Plot current (probe: IC.i) vs. temperature using a probe. Or, try settingup a passed parameter for temperature (Temp = 25 in the optionscontroller). The Options controller is in every simulation palette andcan be used to set convergence for DC and constant simulationtemperature.

3. Replace the Gummel-Poon model card with another model (Mextram)and resimulate. Afterward, compare the results.

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