Frank:

You go it right! Paralleling transistors in the input stage gives lower
noise because the input parasitic noise resistances (base spreading
resistance, base and emitter bulk resistances and ohmic contacts for
bipolars, related parameters for FETs) are reduced. This is the now well
known, patented secret used in low noise hi-fi IC amps from Signetics
(Dolby) and others.

- Wayne Foletta
BMI - Santa Clara

> ----------
> From:         Frank A. Vorstenbosch[SMTP:fav@EIDOS.CO.UK]
> Sent:         Monday, December 01, 1997 1:49 AM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: Serial Sonar Unit (SSU) - Noise performance
>
> On Mon 01 Dec, Peter van Hoof wrote:
> >
> >> An additional tip
> >>
> >> Noise cancels out (partially) in parallel amps, this works
> especially
> >> well in preamps where you can hang several amps to an input signal
> and
> >> (with series resistors) tie all outputs together
> >
> >  Sorry, I just can't accept this statement in regard to front end
> >noise.  In fact I am sure the opposite is true, as when you parallel
> >amplifiers, you necessarily reduce the input signal level to each by
> the
> >split factor, 3dB for two, and make the noise WORSE by that amount.
>
> If you use N transistors in parallel in your frontend, then noise will
> go
> down by SQRT(N) -- same as having a transistor of N times the die
> area.
>
> Frank
> ----------------------------------------------------------------------
> --
> Frank A. Vorstenbosch                           Phone:  +44-181-636
> 3391
> Electronics & Software Engineer                    or:  +44-181-636
> 3000
> Eidos Technologies Ltd., Wimbledon, London      Mobile:  +44-976-430
> 569
>