Just want to check how to choose Q1, Q2 in fig21. Thanks.
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Generally, for Q1 a integrated pair-BIJ is always recommended to achieve better matching for current mirror. Q2 can use a general BJT with enough ratings. It will be better if the base current is small.
You may use the one that recommended in datasheet directly: Q1: BCM62B. Q2: MMBT3904TT1.
To my understanding, FB/R2=(-Vbe-Vout)/R1, then if using MOS, vbe is replaced by Vgs of MOS. Since Vgs is not stable, then MOS cannot be used, right? or we can still use MOS, but only add Vgs in the equation. Thanks.
Just some supplements: from the function view, Q2 can be replaced by MOS. In the steady state, there will be a stable Vgs adjusted by the loop.
The concern is that Vgs can't be ignored in the Vout formula, and there may be more iterations needed in R1/R2 selection to achieve the wanted Vout.