Q1: Why do most input buffers have unity gain ? Q2: Noise

[GibsonGM]

I would bet there is much you could learn from him, the same way one could learn from a radio enthusiast...the signals are so SMALL!!   They need to use good practices and tricks to avoid noise and so on.  But don't let him/them sell you on too much of the 'huge bandwidth' stuff, he he he! 

[ElectricDruid]

Please teach me what does adding hard clipped diodes do after rail saturated Opamp ?

Not much when the op-amp is clipping too, but the diodes will clip first, and then the op-amp will clip next, since it needs a bigger signal to saturate.

[antonis]

Not much when the op-amp is clipping too,

It depends on how "much" is defined..

An op-amp saturated signal exhibits much steeper shape than a simply amplified (undistorted) one resulting into more "squared" waveform after diode(s) hard clipping..

[Steben]

Please teach me what does adding hard clipped diodes do after rail saturated Opamp ?

Not much when the op-amp is clipping too, but the diodes will clip first, and then the op-amp will clip next, since it needs a bigger signal to saturate.

I thought about answering this as well, but it is somewhat a very sterile declaration. Just as the slew rate stuff has effects beyond a normal dynamic range and becomes relatively fixed issue, this ones does too. The gain is so damn high, you'ld need to back it down so much to enjoy any gradual clipping effects.

[antonis]

Just as the slew rate stuff has effects

Plz Steven, not again..!!! 

[ElectricDruid]

I thought about answering this as well, but it is somewhat a very sterile declaration.

I agree, it's a big oversimplification. But I didn't want to go off into a whole discussion about stacked distortion in a thread about input buffers, so I was deliberately keeping it very short. The Hard Bargain design was only brought up as an example of gain-at-the-input.

[MrStab]

It's a simple short on the feedback loop of a non-inverting amplifier to get back down to unity, to accomodate the hotter signals of egomaniacs, effectively letting them provide S/N advantage themselves.

Within the pre-gain or pre-amphasis approaches lies a philosophical dilemma: if you do make it optional, would you label the switch as +0db and +6dB, or "Cut", and not mention the internal gain of this input stage? These are the things that keep me up at night. I guess it depends on your conscience.