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Resistor in bioolar current mirror with beta helper

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redapplepie

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Hello all:
A bipolar current mirror with beta helper is aiways used.
Sometimes I can see that a resistor is connected between the common base node and the ground. But in classic books ,such as gray and meyer's book,there is no resistor fron the common base node and the ground.
I want to know what is the resistor used for?

My second question is : in BICOMS process,can the beta hepler bjt be replaced by a mos transistor?
 

beta-helper

1.To make the beta of the beta-helper-transistor larger.

2.I don't think can use MOS. (if I am not wrong)
 

helper transistor

The beta and Ft of a bjt is dependent on collector current, thus the resistor is placed to get the beta-helper transistor biasing.

I think it is possible to use a mos instead of bjt beta-helper transistor. The function of beta-helper transistor is to provide base current for right hand transistor and to follow voltage on collector of the left hand transistor. But voltage follower bjt based has closer to 1 gain. So bjt is benefit here.
 

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