why source followers are avoided in low-noise amplification?

[wholx]

Since source followers add noise to the input signal while providing a voltage gain less than unity, they are usually avoided in low-noise amplification.

Anyone can explain a little more about these words by Razavi (p232, section 7.4.3)? I don't quite get the consequency here.

 

[flatulent]

Re: why source followers are avoided in low-noise amplificat

Think of the concept of signal to noise ratio. The source follower increases the noise but not the signal.


There is one restriction to the statement in the book. You can have power gain without any voltage gain if the impedance levels are different.

 

[wholx]

very helpful flatulent.

still, any example for your comment on the case where we have power gain without voltage gain?

 

[flatulent]

Re: why source followers are avoided in low-noise amplificat

One example is for a high impedance sensor such as a piezoelectric microphone. Putting this to a source follower which is driving a 100 ohm load causes more signal power in the load than is out of the microphone. (p=v*v/r)