XTR116: Request for transistor selection guide

Hello Rakyoung,

Here is what is happening internally, which I believe understanding will help you determine your transistor selection. There is an input current dictated by Vin/Rin, that will create a voltage at the IN+ of the internal amplifier A1. The output of the amplifier will try to force an output voltage to create an identical voltage at IN- of A1. With no external transistor, the output voltage of A1 will increase, which increases the voltage on and current into the base of the internal transistor until the current through the internal transistor is sufficiently large to equalize the voltage at the two inputs.

When applying an external transistor, since the output of A1 is directly connected to the base of the external transistor, and the connection to the base of the internal transistor is through a resistor divider, this will allow for some range of Vbe that is acceptable to ensure that only the external transistor is on and dissipating heat. It is not explicitly stated in the datasheet what range of Vbe that is acceptable, but we can judge the range based on the recommended transistors. I have compiled these below, and you have a fairly wide range of Vbe that is acceptable. With this, we can see a fairly wide range of acceptable Vbe that will ensure that the external transistor is on before the internal transistor.

Best,
Gerasimos