Are multiple amplifier stages better than one ???

[Bill Bowden]

Yes, but for best noise performance the first stage should have the higher gain (making
certain assumptions about source impedance and op-amps used but almost invariably the
case).

Graham

Why would a higher gain first stage improve noise performance? Seems
like identical op-amps would produce the same total noise regardless
of how the gain was distributed? Wouldn't using a low noise op-amp in
the front end with somewhat higher gain be a better idea? The next
stage (a cheaper op-amp) wouldn't care much about the noise because
the signal level would be many times higher.

-Bill

 

[Eeyore]

Why would a higher gain first stage improve noise performance? Seems
like identical op-amps would produce the same total noise regardless
of how the gain was distributed?

Let me simplifiy this for you.

Let's say the input noise of your amp stage is 1 uV ?

You set your fist stage gain to ONE. It theoretically produces 1uV of output noise. This then
adds to the second stage noise of 1uV making 1.4uV equivalent input noise (addition of random
sources).

So you just made a noisier amplifier. Now do the calculations for other gain distributions.

Graham

 

[neon]

NOISE is the noise from the first stage input and amplified from there on. it has nothing to do with more stages it has to do with the first stage input noise. Solution get a better low noise amplifier. Why we want hi gains simple the more the gain then for a specified gain the more the F/B WHICH WILL PROVIDE BETTER STABILITY OF THE LOOP. Bandwith does not comes in into the consideration execpt to mention at very low gain there is normaly more gain available at the hi freq. end approaching unity gain but not dependable. solution buy a better freq. amplifier if that is the case.

 

[Bill Bowden]

Let me simplifiy this for you.

Let's say the input noise of your amp stage is 1 uV ?

You set your fist stage gain to ONE. It theoretically produces 1uV of output noise. This then
adds to the second stage noise of 1uV making 1.4uV equivalent input noise (addition of random
sources).

So you just made a noisier amplifier. Now do the calculations for other gain distributions.

Graham

I don't follow the logic. The first stage is ideal since it adds no
noise. I assume the second stage is a also ideal and adds no noise.
Therefore two amplifiers with a gain of ONE will produce the same
output as the input.

What am I missing?

-Bill

 

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Interesting aside: if you want to make an amp out of some number of
simple RC-coupled tube stages, the plate resistor determines the gain,
where the gain-bandwidth is pretty much fixed. If you want the maximum
gain-bandwidth from a cascade of stages, the optimum gain per stage is
e, the base of the natural logs.

e keeps popping up all over the place.

John

as the op- amp generally practical one gives the less gain.


its better to use the cascading amplifiers.
ofcours it all depends on the application.
ok rinds.