[sdiy] High input impedance -- bad ?

[ASSI]

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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