[sdiy] High input impedance -- bad ?

[cheater cheater]

How do you determine the bias current?

Thanks
D.

On Tue, Feb 16, 2010 at 19:12, ASSI <Stromeko at nexgo.de> wrote:
> On Tuesday 16 February 2010, Andre Majorel wrote:
>> Are there any drawbacks to giving inputs high (say 1 M) input
>> impedances ?
>
> More thermal noise and bias current offsets will feed through more
> easily to the output.
>
> V_n,RMS = sqrt( 4kTR (f_upper - f_lower) )
>
> Any current noise will also be much more noticeable when referred
> through a high-impedance node, although the current noise itself is
> proportional to sqrt(Ibias).
>
> There is an optimum impedance point for each bias current in an
> amplifier, so unless you can control the bias currents or select the
> amp appropriately, to high of an impedance wil just get things worse
> again.
>
>
> Achim.
> --
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