August West
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I'll get up and fly away
Posts: 71
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Re: Input referred noise current for bipolar
Reply #1 - Jan 17th, 2004, 11:39am
Yxie, You are correct in saying that Spice only computes Vn when computing the input referred noise to a voltage source. Similarly, it only computes In when computing the input referred noise to a current source. If you want both, you can simulate your circuit twice, once with a voltage source at the input and once with a current source at the input. If you do this, you must be very careful not to disturb the bias conditions of your circuit when changing the input source. Alternatively, you can use a port for the input source and perform an S-parameter noise analysis. It computes 4 noise parameters from which you can extract Vn, In, and their complex correlation coefficient. To do so, you will need to consult a book on S-parameter noise analysis. I believe Microwave Transistor Amplifiers: Analysis and Design by Guillermo Gonzalez would have what you need. If not try Introduction to Computer Methods for Microwave Circuit Analysis and Design by Janusz A. Dobrowolski.
I must add that your initial comments are misleading. You seem to suggest that somehow Spice is giving you the wrong answer when computing the input referred noise because it does not include the shot noise components. This is wrong. The shot noise components are included, and you can see this from the output noise summary.
Spice is giving you the right answer to possibly the wrong question. When you drive your circuit with a voltage source and ask Spice to compute the input referred noise, it is telling you how much noise would have to be added to the input to compenstate for turning off all the noise sources in the circuit. What this is not telling you is how this equivalent input noise would change if the source impedance changed. If you know both Vn and In (and their correlation coefficient) you can compute this. But Spice does not give you this information, nor can you get it from Spice. Above, I did tell you how to compute Vn and In, but there is no way to compute the correlation coefficient. Instead, you must use the S-parameter noise analysis, which is available in Spectre.
-August
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