Hi Team,
Just want to check how to choose Q1, Q2 in fig21. Thanks.
BRs
Given
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Hi Given,
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.
Regards
Lei
Hi Lei,
can Q2 be replaced by MOSFET. To my understanding, Vbe of BJT is stable and Vgs of MOS is not stable, so Q2 cannot use MOS, right? Thanks.
BRs
Given
Hi Given,
If MOSFET is used, there won't be a simple linear formula to regulate Vo2, such as Vo2 = 1.23 x (R1/ R2).
Regards
Lei
Hi Lei,
got it.
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.
BRs
Given
Hi Given,
Yes if using MOS, vbe is replaced by Vgs. Compared with Vbe, Vgs is bigger and can't be ignored. So the Vout will be hard to adjusted to the wanted value accurately.
Regards
Lei
Hi Given,
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.
Regards
Lei