Re: Standard non-inverting opamp with a cap across + and - terminals...... Why?



John Larkin wrote:

On Mon, 13 Apr 2009 19:10:55 -0500, Vladimir Vassilevsky
<antispam_bogus@xxxxxxxxxxx> wrote:

John Larkin wrote:


You get 1 mV RMS in a 50 ohm environment from 20 nanowatts of RF. In a
higher-impedance, maybe accidentally resonant circuit, one nanowatt
can make a millivolt.

Yet getting 1mV from -60dBm input using a biased conventional silicon junction is a bit of exaggeration, isn't it?



I'm not sure what you're saying. What I meant was that 20 nW of RF
could put 1 mV RMS of RF on an input pin of an opamp, and the input
b-e junction can rectify that to produce a DC offset. The resulting DC
offset will be in the ballpark of 10 microvolts,

Agree entirely. Sorry, I thought you were talking about the rectified DC of 1mV, which looks way too much for 20nW RF input and biased silicon PN junction at any impedance.

In the chambers of the San Francisco Board of Supervisors (the city
council) one hears a blip-blip-blipblip sound over the PA system
fairly often, and quite loud. You can hear it in person or on TV. It's
apparently cell phone RF getting into microphones, and it's been like
that for years.

Oh, this effect is very typical in the audio, as they have to use cheap single layer PCBs. The audio cable works as antenna, creating hell of RF potential across the board.

Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com



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