Help to choose the right digital pre-biased transistor - Page 1
Forum section: electronics / beginners

author: Tantalum
author_rank: Regular Contributor
timestamp: March 18, 2019, 08:16:34 pm
content: HelloI 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 aDDC123JUbe ok?R(b) = 2.2kOhmhttps://eu.mouser.com/datasheet/2/115/ds30345-1512281.pdfThank you

author: Tantalum
author_rank: Regular Contributor
timestamp: March 19, 2019, 11:06:58 am
content: up

author: ogden
author_rank: Super Contributor
timestamp: March 19, 2019, 11:41:55 am
content: Quote from: Tantalum on March 18, 2019, 08:16:34 pmIn 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).

author: janoc
author_rank: Super Contributor
timestamp: March 19, 2019, 02:37:53 pm
content: Quote from: Tantalum on March 19, 2019, 11:06:58 amThis 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?

author: Tantalum
author_rank: Regular Contributor
timestamp: March 19, 2019, 03:12:05 pm
content: Quote from: janoc on March 19, 2019, 02:37:53 pm3 hours?1. Wrong math...2. Earth is round, do you know that?But anyway, thanks to have upped my topic againQuote from: ogden on March 19, 2019, 11:41:55 amI thought it was the purposes ofdigital/pre-biasedtransistors . That's what I read somewhere else.Initially I wanted to reduce the number of components, that's why I found those chips interesting.

author: ogden
author_rank: Super Contributor
timestamp: March 19, 2019, 03:19:52 pm
content: Quote from: Tantalum on March 19, 2019, 03:12:05 pmBiasing is for amplifiers, not logic. Knowing that there are internet search engines helps same way as knowing that earth is roundhttps://www.electronics-tutorials.ws/transistor/tran_4.htmlQuoteNah. 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.

author: janoc
author_rank: Super Contributor
timestamp: March 19, 2019, 05:20:41 pm
content: Quote from: Tantalum on March 19, 2019, 03:12:05 pmMy 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!

author: bionic
author_rank: Newbie
timestamp: March 19, 2019, 08:07:03 pm
content: 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.

author: Pete66
author_rank: Regular Contributor
timestamp: March 28, 2019, 05:46:42 pm
content: 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

author: Tantalum
author_rank: Regular Contributor
timestamp: March 31, 2019, 12:23:02 pm
content: Thanks for your answer. I switched back to mosfet