piclist 2008\07\26\023255a >
Thread: Low Noise, High Gain Amplifier
www.piclist.com/techref/index.htm?key=low+noise+high+gain
face picon face BY : Sean Breheny email (remove spam text)



Hi Fred,

First off, what is this for? That may help to answer your question.

Your bandwidth is fairly small - I think that your rough noise "goal"
could be met with most decent op-amps. The best low-noise op-amp I
have used is the LMV772. It is about a factor of 2 better than most
decent op-amps (like the LMC6482). You would have to add some
filtering before the comparator to eliminate noise outside of your
desired bandwidth.

Taking a step back, you want to turn a 2kHz sinewave into a square
wave. How much do you care if the edges move around? Do you care if
the output is random rising and falling edges when there is no signal
input?

Since you are going to end up clipping the sine wave to a fixed
amplitude, you do not need to worry about amplitude noise (except in
terms of what happens when no signal is present) but you do have to
worry about phase noise. This will show up as jitter in your
squarewave edges. If you Google for jitter and phase noise, you will
find some info. I think there is a simple formula relating average
phase noise power and rms jitter.

As long as you design your amplifier chain properly, and as long as
the first stage has sufficient gain to boost your signal out of the
noise of the next stage, then the first stage is the only one which
really matters for noise level. A simple discrete transistor
amplifier, if carefully designed, could easily yield noise in the 1uV
range in your bandwidth. Selecting low-noise transistors and using
good low-noise design methods, something like 0.05uV is attainable in
your bandwidth. That would not be easy, though.

However, with your inexperience, I think that a low-noise op-amp would
be a better choice. You'll still have to pay attention to the values
of resistors used in the circuit to prevent the addition of extra
noise. Also, 60Hz hum could really be a plague if you aren't careful
to prevent places where it can leak into your signal path.

What is the largest signal you need to be able to handle?

Finally, you will have to place filtering at the appropriate places to
make sure that the signal and noise stay in the linear region of your
amplifiers until AFTER the filter, otherwise noise could mix with
stray signals and produce a spurious signal within your bandwidth.

Sean


On Sat, Jul 26, 2008 at 1:31 AM, Rich <RemoveMErgrazia1spamrochester.rr.com> wrote:
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> {Original Message removed}
<e726f69f0807252332y6792175aie7e37a297958da4@mail.gmail.com> 7bit

In reply to: <BB9A703CE7384C49A8CF84299CA84BD8@RichPC>
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