Figure 1 shows a basic operational amplifier with a gain of 5. You will use Noise Analysis to obtain results for noise voltage for R1 and R2 and display a graph of the noise spectrum across a frequency range between 1 Hz and 10 GHz.
Figure 1. Inverting amplifier circuit.
Configure and Run a Noise Analysis:
- Open circuit file amplifier_circuit.ms11 located in the Downloads section.
- Select Simulate»Analyses»Noise Analysis. The Noise Analysis window opens. Table 1 describes the Analysis Parameters tab in detail.
Table 1. Parameters used in Noise Analysis.
Input noise reference source
|Specifies the name of the independent voltage or current source that is to be the input reference source to which equivalent input noise is referred.|
|Specifies the node at which all noise contributions will be summed.|
|Specifies the reference node for the output noise voltage.|
Displays nodes contained within subcircuits or hierarchical blocks. There are three options:
- Display internal nodes. Displays nodes within hierarchical blocks and subcircuits.
- Display submodules. Displays components within semiconductor devices determined by the SPICE model of the semiconductor device.
- Display open pins. Displays all unconnected nodes of the circuit.
Calculate power spectral density curves
|Generates a graph of the power spectral density.|
Points per summary
|Specifies how often the noise contributions of each noise generating device are reported. The recommended value is 1.|
Calculate total noise values
|Generates a table with total noise data.|
Note: In SPICE, the command that performs a Noise Analysis has the following general form (the complete statement is more complex): .NOISE <OUTPUT_VOLTAGE> <INPUT_SOURCE> <OUTPUT_INTERVAL>
Note that these parameters are similar to those defined in Table 1, however, in Multisim you do not have to worry about the SPICE syntax.
- Configure the Analysis Parameters tab as shown in the following figure:
Figure 2. Analysis parameters.
- Select the Frequency Parameters tab. Leave the default settings as shown in Figure 3 (these settings are appropriate for most cases).
Figure 3. Frequency parameters.
You can reset all the parameters to their default values by clicking the Reset to default button. To copy the settings from the current AC Analysis to this analysis, click Reset to main AC values.
Refer to the AC Analysis tutorial for more details on how to configure the Frequency Parameters tab.
- Select the Output tab.
- Select the Variables in circuit list, select All variables from the drop-down list, and then highlight inoise_total_rr1 from the list.
- Click the Add button to move the variable to the right side under Selected variables for analysis.
- Repeat this process for the inoise_total_rr2 variable. The Output tab will look as shown in Figure 4.
Figure 4. Output variables for the Noise Analysis.
- Click Simulate. The Grapher View opens and displays the noise contribution for each resistor (Figure 5).
Figure 5. Noise Analysis results.
Plot the power spectral density.
- Close the Grapher View.
- Select Simulate»Analyses»Noise Analysis. In the Analysis Parameters tab enable Calculate power spectral density curves.
- Enter 1 in the Points per summary field.
- Select the Output tab.
- Add the variables onoise_rr1 and onoise_ rr2 to the Selected variables for analysis list as shown in the following figure:
Figure 6. Output variables for the Noise Analysis.
- Click Simulate. The Grapher View shows the noise spectral density curves (Figure 7).
Figure 7. Noise spectral density curves.
As you can see, the graph shows that the noise voltage is constant for lower frequencies. For higher frequencies, the noise voltage drops considerably.