Noise figure is used to specify exactly how noisy a device is. For a transistor, noise figure is simply a measure of how much noise the transistor adds to the signal during the amplification process. In a circuit network, the noise figure is used as a figure of merit to compare the noise in a network with the noise in an ideal or noiseless network. It is a measure of the degradation in signal-to-noise ratio (SNR) between the input and output ports of a network.
Noise figure for a particular component (for instance, a transistor) is provided on the part datasheet. In a SPICE model, the noise figure is specified by the parameter NF. Below is an extract of a SPICE model for a transistor:
.MODEL BF517 NPN (IS=0.480F NF=1.008 BF=99.655 VAF=90.000 IKF=0.190
+ ISE=7.490F NE=1.762 NR=1.010 BR=38.400 VAR=7.000 IKR=93.200M
+ ISC=0.200F NC=1.042
Although noise figure in a model is achieved using a combination of several model parameters, the parameter NF is the most significant value (when simulating noise figure in a transistor model). When selecting a SPICE model to use for Noise Figure Analysis, you must ensure that this parameter exists. If not, the part will operate without noise parameters and therefore will generate no noise.
Multisim calculates the noise figure using the equation:
F = NO / GNS
Multisim prints the noise figure in dB, that is: 10 log10 (F).
Consider the RF Amplifier circuit shown in Figure 1. The amplifier is configured for maximum output power at 1.5MHz. You will use Noise Figure Analysis to calculate the noise figure at 10 kHz.
Figure 1. RF amplifier circuit.
Complete the following steps to configure and run Noise Figure Analysis:
Figure 2. Frequency response.
Table 1. Parameters used in Noise Figure Analysis.
Input noise reference source
Displays nodes contained within subcircuits or hierarchical blocks. There are three options:
Figure 3. Analysis parameters.
Figure 4. Noise Figure Analysis result.
Refer to the Noise Analysis tutorial to learn how to calculate the noise spectral density curves.
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