[Gb-ccb] Project Meeting Draft agenda & schedule

[Martin Shepherd]

On Tue, 13 Jan 2004, Brian Mason wrote:
>...
> ps, It would be useful (particularly for the detector circuit
> discussion) to review Randy's latest design...:
>
> http://wwwlocal.gb.nrao.edu/~bmason/gbt-dev/KaRxDetAmpRevH.bmp

I have updated my one-page calculation of the noise contribution of
the first stage amplifier in Randy's latest design. This can be found
at:

 http://www.astro.caltech.edu/~mcs/GBT/preamp_noise.ps

As explained before, this calculation is based on the fact that in a
differential amplifier with high open-loop gain, negative feedback
ensures that the inverting and non-inverting inputs of the op-amp are
always at the same potential (unless the amplifier saturates), so all
noise voltages appearing at either input can be summed to a single
noise voltage that is common to both inputs. This sum has to be
performed in quadrature to get the RMS noise voltage.

For a signal applied to the non-inverting input of this amplifier,
this circuit has the standard non-inverting configuration, so the
noise voltage accumulated there is amplified by the non-inverting gain
of this amplifier, which is (1+Rf/Rm). Note that if you look at the
amplifier from the point of view of a signal appearing at the
inverting input of the amplifier (ie. not the input of the whole
amplifier circuit), the gain is still that of a non-inverting
amplifier, since the two inputs have the same potential. The noise
voltage would have to appear at the other side of the series resistor
to be amplified by the inverting gain of the amplifier, and this isn't
the case.

Martin