piclist 2008\07\28\072145a >
Thread: Low Noise, High Gain Amplifier
www.piclist.com/techref/index.htm?key=low+noise+high+gain
picon face BY : Rich email (remove spam text)



My point is not to misinform, but to shed a little clarity on the subject.
It is a good engineering practice to begin with a specification that defines
your design criteria. Newbees especially can use such advice.  And what is
presented on the list is observed also by those who are just entering
engineering and learning.  What is considered an acceptable noise level in
one application may be considered excessive in another.  If a single stage
single transistor or FET will work it is probably a better solution with
respect to noise.  I don't have anything against "rule of thumb" design, it
has a valid place in engineering.  But to say that the noise contribution
does not present itself after the first two stages is a bit misleading to a
novice or a student. You may say that the majority of the noise is in the
first stage.  When you amplify, you amplify both the signal and the noise
which is passed on to additional stages.  It would be wonderful to get a
small signal out of the noise by amplifying it up but that just can't
happen. Every stage that has active components will contribute to the
overall noise.  There is no active electronic component that I have ever
been aware of that does not contribute to noise.  If electrons move there
will be noise.  Even resistors contribute to noise.  That is apparent when
one looks at the physics of the devices. In the final analysis the formal
design begins with a design specification, and that specification includes
an error budget which also accounts for the noise; stage by stage.  I have
not been convinced that such an approach is unnecessary or an unimportant
consideration in engineering.  Perhaps at the hobby level one can dispense
with the specification but not when you are designing product for some
company.


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