Electronics > Beginners
Help to choose the right digital pre-biased transistor
Tantalum:
Hello
I needed some help to select the right pre-biased transistor. Usually I take MOSFET, the are easier to understand, but require an external pull down and gate resistor.
I need a BJT with which support an input voltage of 3.3V (IO from an Atmega MCU), to pull down signal of 3.3V & 1.8V, only a few mA are needed.
Would a DDC123JU be ok?
R(b) = 2.2kOhm
https://eu.mouser.com/datasheet/2/115/ds30345-1512281.pdf
Thank you
Tantalum:
up :o
ogden:
--- Quote from: Tantalum on March 18, 2019, 08:16:34 pm ---Would a DDC123JU be ok?
--- End quote ---
In case transistor is just switch - then no. For logic/switch applications BJT biasing is bad idea actually. I do not see any sane reason why would anyone use transistor with built-in base resistor unless you have PCB area problem and there's no room even for SMD resistor. That would be very, very, very rare case.
Also small MOSFETs (having small gate capacitance) do not require gate resistor. You may live even without pull-down resistor (depending on application).
janoc:
--- Quote from: Tantalum on March 19, 2019, 11:06:58 am ---up :o
--- End quote ---
This doesn't really work on this forum. Also, do you expect an answer in 3 hours? On a working day? Between 7 and 10am? Seriously? |O
Tantalum:
--- Quote from: janoc on March 19, 2019, 02:37:53 pm ---
--- Quote from: Tantalum on March 19, 2019, 11:06:58 am ---up :o
--- End quote ---
This doesn't really work on this forum. Also, do you expect an answer in 3 hours? On a working day? Between 7 and 10am? Seriously? |O
--- End quote ---
3 hours?
1. Wrong math...
2. Earth is round, do you know that?
But anyway, thanks to have upped my topic again :-+
--- Quote from: ogden on March 19, 2019, 11:41:55 am ---
--- Quote from: Tantalum on March 18, 2019, 08:16:34 pm ---Would a DDC123JU be ok?
--- End quote ---
In case transistor is just switch - then no. For logic/switch applications BJT biasing is bad idea actually. I do not see any sane reason why would anyone use transistor with built-in base resistor unless you have PCB area problem and there's no room even for SMD resistor. That would be very, very, very rare case.
Also small MOSFETs (having small gate capacitance) do not require gate resistor. You may live even without pull-down resistor (depending on application).
--- End quote ---
I thought it was the purposes of digital/pre-biased transistors . That's what I read somewhere else.
Initially I wanted to reduce the number of components, that's why I found those chips interesting.
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