1. Example Circuit
This example investigates the dependency of the transistor parameter on the collector bias current. Use a discrete BJT to investigate the relationship (Figure 1).
Figure 1. Example Circuit
For this exercise, apply a constant voltage source and a DC current source to the base. By performing a DC analysis simulation, for which you sweep the current source to the base, you can see the relationship of plotted against. Observe this relationship with Multisim simulation.
2. Step 1: Open Multisim
Begin by drawing your schematic in the Multisim environment.
- Select Start»All Programs»National Instruments»Circuit Design Suite 11.0»Multisim 11.0 to open Multisim.
- Multisim opens showcasing the default capture and simulation environment.
Figure 2. Multisim Environment
There are two fundamental tasks in the design of a circuit: the placement of components and the wiring of these components to create a complete design.
- To select a component, go to Place»Component …
- The Select a Component dialog appears (also known as the Component Browser).
Figure 3. Select a Component Dialog
The component browser organizes the database components into three logical levels. The Master Database contains all shipping components in a read-only format. The Corporate Database is where you save custom components to be shared with colleagues (via a network collection and so on). Finally, the User Database is where you save custom components that can be used only by the specific designer.
- The components (or parts) are organized into Groups and Families to intuitively and logically group common parts together and make searching easier and more effective.
- The component selection box shows the Component name, Symbol, the functional description, Model, and Footprint all in a single pop-up.
3. Step 2: Place Components
To invoke simulation, you need a power source and a ground somewhere in your circuit to correctly reference voltages and currents in your circuit simulation.
- To place a Ground, go to the Sources group and highlight the POWER_SOURCES family.
- Highlight the GROUND component (as shown in Figure 4).
- Click OK.
- The component selection window temporarily closes and the ground symbol is "ghosted" to the mouse pointer.
- Move the mouse to the appropriate place on the schematic and left-click once to place the component.
- Now place two more GROUND components on the schematic.
Figure 4. Placing a Ground Symbol
To place a DC power supply:
- Go to the Sources group again and highlight the POWER_SOURCES family (if not already highlighted from the previous selection).
- Select the DC_POWER symbol.
- Place the DC Power source on the schematic.
- Double-click on the DC_POWER source and type in 2 in the Voltage (V): value box and click OK. You have just set the voltage value for .
- Without a power and ground, your simulation cannot run.
- If you need multiple components you can repeat the placement steps as shown or place one component and use copy <Ctrl-C> and paste <Ctrl-V> to place additional components as needed.
- By default, the component selection box keeps returning as a pop-up until you have completed placing your components. Close the window to return to the schematic entry window (Close button). You can change this in the global preferences dialog box.
Now place the remaining circuit components using the techniques discussed in the previous steps.
- Select Place»Component.
- To place 2N3904, select the Transistors group and BJT_NPN family. In the component box, type 2N3904 to filter the list of components in the family view.
At this point, your schematic should look something like Figure 5:
Figure 5. Components Placed
4. Step 3: Wire Components
Multisim is a modeless wiring environment. This means that Multisim determines the functionality of the mouse tool by the position of the mouse. You do not have to return to the menu to select between placement, wiring, and editing tools.
- To begin wiring, move the mouse close to a pin of a component.
- The mouse appears as a crosshair rather than the default Windows mouse.
- Place an initial wire junction by clicking on the pin/terminal of the component (see Figure 6 for an example).
Figure 6. Wiring the BJT
- Complete the wire by moving the mouse to another terminal or just double-click to anchor the termination point of the wire to a floating location somewhere in the schematic window.
- Continue with wiring the rest of the circuit as shown in Figure 7. Do not worry about the labeled numbers on the wires, which can be referred to as nets.
Figure 7. Wire the Components as Seen Above
5. Step 4: Place a Simulation Source
To use simulation in a Multisim schematic, you need to place and wire a signal source to the circuit to act as a stimulus.
You can choose from several signal sources including an LVM source based on LabVIEW. Place a basic DC current source for this demonstration.
- To place the DC_CURRENT source, select the Source group and SIGNAL_CURRENT_SOURCES family. In the component dialog box, select the DC_CURRENT component and place.
- Double-click on the DC_CURRENT source and type in 10 in the Current (I): value box and use the up/down arrows to select µA. Click OK when you have configured your DC_CURRENT source to output a 10 µA current.
- Now wire the DC_CURRENT source to the base of the BJT and Ground. It should look like Figure 8 when you are complete.
Figure 8. Complete Capture of the BJT Circuit
6. Step 5: Place Measurement Instruments
You are now ready to run an interactive Multisim simulation; however, you need a way to visualize the data. Multisim provides simulation-driven instruments such as oscilloscopes, wattmeters, and more to help you visualize the simulated measurements.
You can find instruments on the right menu bar. They are indicated by the following icons.
Figure 9. Instrument Toolbar
In this example, do not use the instruments. Instead, use the measurement probes.
- Click on the black arrow below the Measurement Probe icon. This opens a menu of the different types of measurement probes available.
- Select the AC Current probe and click on the wire connecting the current source to the BJT.
- Repeat the above step and place an AC Current probe to monitor the (as shown in Figure 10).
Figure 10. AC Current Probes Connected to Monitor the Current in the Net
7. Step 6: Run a Simulation
It is simple to begin running and visualizing an interactive simulation in Multisim.
- Press the green run button on the simulation toolbar as seen in Figure 11.
- Observe the current flowing in the nets.
- You can now stop the simulation by pressing the red stop button in the simulation toolbar (Figure 11).
Figure 11. Simulation Toolbar
8. Step 6: Run an Analysis
Now that you have confirmed a DC current source, is applied to the base and a constant voltage source is connected to the BJT. Observe the dependency of by performing a DC analysis simulation in which you sweep the current source. You can then plot the measurement versus to get the relationship between the ratio of the collector current to the base current .
- From the Multisim menu, select Simulate»Analysis»DC Sweep.
- This opens a window (see Figure 12).
- Now enter the following into the fields: you will be sweeping from 0A to Stop value of 0.0002A by 1e-006 increments.
- Click on the Output tab and verify that the settings are similar to those in Figure 13.
- Click on the Simulate button to begin the DC sweep.
- When the simulation completes, it opens up the grapher to display the data, as seen in Figure 14.
Figure 12. DC Sweep Analysis Parameter Settings
Figure 13. Output Settings for the DC Sweep Analysis
Figure 14. Dependance of on at =2V
By looking at the DC analysis of the BJT circuit, you can see the relationship between on . As you increase the bias current of a transistor, you see a drop on the current gain.
With this example, you have successfully placed components, wired a circuit, placed measurement probes, run a simulation, and observed the results of a DC analysis.