Gentlemen, I realize you're experts, but...
I didn't realize the idea was to do the OP's homework for him.
And simplified equations may get us through the homework assignment...
However I thought it would be more helpful to get the OP acquainted with real world details.
Because there's bound to be a 4.0 student in the class who goes home,
and breadboards with real components,
and checks voltage readings with a meter...
And he will come back and point out to the professor...
That if we build a real circuit as given in the OP...
The bias input will experience variations of a few tenths of a volt,
which are not covered by the simplified textbook equation,
and which cannot be regarded as noise,
and which are inherent in normal operation.
Because:
1.
Grounding one input causes a big V drop at node 1, but not necessarily to the spec we hear about. Could be one or two tenths of a volt below that, depending on where mA and fwd V cross on the real-world diode curve. As dictated by 15K resistance in the line.
And where do we find a diode curve with that fine resolution? I never saw one until I made my own.
Furthermore for each additional input you ground, it causes a further drop by maybe .1 V or so.
This is because those three diodes in parallel are really resistances in parallel (electrically speaking).
2.
The transistor bias starts admitting microamps at around .35 V. A few tenths of a volt below what we always hear as the simplified spec .65 V, or .6 or .7).
Therefore a real transistor will begin to turn on, and the output will change, before bias rises to 'simplified' spec V. (At least it will in the circuit of the OP.)
So...
Will these details change the final answer? Maybe by a few tenths of a volt. Not much to talk about.
But suppose the assignment had stated noise range as .3V? Isn't that at the same level as the details I listed under points 1 and 2? Do they matter then?
And what will the professor say to the 4.0 student who breadboarded a real circuit?
Will he say "You're that much ahead of your peers"?
Or will he say "We only need to use simplified equations"?
Or will he say "I built the circuit 20 years ago, and I simply threw in a 10V zener, and it worked fine"...
:wink: