two questions about OP

[zarric]

1. when I use bicmos process to design a simple two stage amp,
if bipolar as different input pair ,then small noise, large gm/Gain, any other..?
if bipoalr as active load and second stage input , then what benefit for it???

2. How to decided a OP Gain, GBW.
For a OP used in BandGap, LDO, DC_DC ,..... how to choose the better GBW,

 

[eladla]

I have not used bipolar transistors in a long time, since I work with CMOS.
I do have a few ideas...

I would not use BJTs as input devices to the first stage, since they have a finite input resistance, and that is generally a bad thing for amps.
You don`t generally want to resistivly load the stage before the amp.
What I would use them for is the active load, since they have a good output resistance, which raises gain, and as the second gain stage.
That way you have an infinate input resistance, OK first stage gain, good first stage current to "push" second stage and finally a gain boost from the BJT at the second (output) stage.
Hope this all makes sense.