Finding the Noise Figure for Transistor Level – Example

Find the NF for 1 stage transistor

Assume this to be a simple amplifier circuit. At the input, we have source Vs and source resistance Rs. We have a MOS transistor in this circuit. Read more about the MOS transistor in this section. Vo is the output.

As we know from the previous examples that NF is:

First, we are going to find the gain for this circuit:

Next is finding the noise sources:

1. Noise sources coming from Rs, so shorting the other sources, we get Vs=0. We are finding the effect of this noise Rs at the output:

2. Finding RD and shown is the equivalent circuit. Vs= 0 (eliminating the effect of source), the circuit would be as shown. There is no source in the circuit; therefore, Vgs would be 0, and gmVgs is also 0. We only have r_o. The circuit would look like as shown below, with the voltage division.

3. Channel noise for the transistor. Here also we are finding the effect of the noise at the output. As there is no source Vgs = 0, the circuit is open.

The Noise Figure can be calculated now using the equation:

Having high g_m and R_D the NF can be reduced.

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