Author Topic: Help to choose the right digital pre-biased transistor  (Read 3072 times)

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Offline TantalumTopic starter

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Help to choose the right digital pre-biased transistor
« on: March 18, 2019, 08:16:34 pm »
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

 

Offline TantalumTopic starter

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Re: Help to choose the right digital pre-biased transistor
« Reply #1 on: March 19, 2019, 11:06:58 am »
up :o
 

Offline ogden

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Re: Help to choose the right digital pre-biased transistor
« Reply #2 on: March 19, 2019, 11:41:55 am »
Would a DDC123JU be ok?

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).
 

Offline janoc

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Re: Help to choose the right digital pre-biased transistor
« Reply #3 on: March 19, 2019, 02:37:53 pm »
up :o

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
« Last Edit: March 19, 2019, 02:40:19 pm by janoc »
 

Offline TantalumTopic starter

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Re: Help to choose the right digital pre-biased transistor
« Reply #4 on: March 19, 2019, 03:12:05 pm »
up :o

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
3 hours?

1. Wrong math...
2. Earth is round, do you know that?

But anyway, thanks to have upped my topic again  :-+

Would a DDC123JU be ok?

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).

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.
 

Offline ogden

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Re: Help to choose the right digital pre-biased transistor
« Reply #5 on: March 19, 2019, 03:19:52 pm »
I thought it was the purposes of digital/pre-biased transistors . That's what I read somewhere else.

Biasing is for amplifiers, not logic. Knowing that there are internet search engines helps same way as knowing that earth is round ;)

https://www.electronics-tutorials.ws/transistor/tran_4.html

Quote
Initially I wanted to reduce the number of components, that's why I found those chips interesting.

Nah. General purpose transistors with generic resistors are much more interesting - because you can use them any way you want, in this project and in next to come.
 

Offline janoc

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Re: Help to choose the right digital pre-biased transistor
« Reply #6 on: March 19, 2019, 05:20:41 pm »
1. Wrong math...
2. Earth is round, do you know that?

But anyway, thanks to have upped my topic again  :-+

My bad, missed that am/pm mark. Still, this is extremely annoying and a bad form. If you are in such a hurry, hire a consultant!
 

Offline bionic

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Re: Help to choose the right digital pre-biased transistor
« Reply #7 on: March 19, 2019, 08:07:03 pm »
I recommend that you fiddle around with biasing a BJT. It is a good practice when it comes to analogue calculations or discrete in this case.
 

Offline Pete66

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Re: Help to choose the right digital pre-biased transistor
« Reply #8 on: March 28, 2019, 05:46:42 pm »
I have used DTC143E for this in the past.  I don't recommend using these.  The problem is they cannot be tested in diode mode like regular transistors.  So if you have an issue good luck testing them.

Here are the ones I used:
https://www.rohm.com/products/transistors/digital-transistors/standard/dtc143ee-product
 
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Offline TantalumTopic starter

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Re: Help to choose the right digital pre-biased transistor
« Reply #9 on: March 31, 2019, 12:23:02 pm »
Thanks for your answer. I switched back to mosfet  :)
 


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