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

On Apr 14, 2:39 am, John Larkin
<jjlar...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
On Mon, 13 Apr 2009 19:10:55 -0500, Vladimir Vassilevsky

<antispam_bo...@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, plenty enough to
cause trouble. It doesn't take 26 or 52 millivolts of RF to rectify.

It takes 26/52mV before a substantial proportion of the input RF is
rectified, and the proportion maxes out at higher signal levels

As a square-law detector, the dc offset is linear on RF power input.

But not above the 26/52mV where the proportion of the input signal
being rectified stops rising, and the DC offset rises with signal
level, which is to say with the square root of signal power.

So 1 microwatt of RF rectifies to about a 500 uV DC offset. Then
multiply that DC offset by the noise gain of the opamp circuit.

20nW to 1uW is a step up of 50 in power, about 7 in amplitude - 7mV
rms at the input, and still a little below the point of diminishing
returns.

--
Bill Sloman, Nijmegen


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