Input-current noise by DC blocking capacitor?

[kvd]

Hi,

I am trying to improve the noise figure of my FM preamp. I am trying to quantify the noise sources.

In Art of Electronics 3rd edition there is an instructive example, in figure 8.36E which is being calculated on page 499. What surprises me is that to calculate the input-current noise for Q1, they do not take the DC blocking capacitor C1 into account. I do not see any reason to omit this cap from the calculations. Only the capacitance of the photo-diode, collector base capacitance of Q1 and wiring are mentioned. But C1 is about 1000 times as large as any of these.

What is the reason C1 falls out of the equation?

Koen

[mjs]

C1 is in series with the 10pF, so it has insignificant contribution. 10nF in series with 10pF is still almost exactly 10pF.

[kvd]

Ah sure, thank you!