BJT transistor voltage drop - Page 1
Forum section: electronics / beginners

author: Fuldup
author_rank: Contributor
timestamp: April 04, 2024, 02:49:16 pm
content: hi i made a simple And-gate circuit, but i got a problem, because every time the current goes through the transistor, the voltage drops of almost 1V, I use MPSA 42 NPN BJT transistors, but i read online that MOSFETs transistors usually have a lower voltage drop, should i change my transistor?

author: Benta
author_rank: Super Contributor
timestamp: April 04, 2024, 03:36:42 pm
content: Show a schematic of what you've done first.

author: Fuldup
author_rank: Contributor
timestamp: April 04, 2024, 05:30:31 pm
content: this is the schematics of the AND-gate

author: MarkT
author_rank: Frequent Contributor
timestamp: April 04, 2024, 06:42:53 pm
content: If your inputs are switches you don't need gates. Switches are gates! Series for AND, parallel for OR.If you inputs are voltages there are many circuits, but the good one's are NAND or NOR using common-emitter or common-source topologies, add an inverter on the output for AND or OR.

author: Zero999
author_rank: Super Contributor
timestamp: April 04, 2024, 07:27:23 pm
content: Those transistors aren't doing anything, other than acting as diodes which drop voltage. All of the current is going through the switches.Your circuit is also drawn upside down. Here's how to do it without the transistors.EDIT: I thought the LED had a built-in resistor. Obviously not.https://optoelectronics.liteon.com/upload/download/DS-20-92-0161/LTL-307EE.pdf

author: Ian.M
author_rank: Super Contributor
timestamp: April 04, 2024, 07:38:30 pm
content: ..... errrrr .... how *NOT* to do it!ALTL-307EEis an ordinary 5mm high efficiency red LED, rated for abs. max. 30mA If and a max. Vf of only 2.6V @If=20mA, so without the two diode drops*, and with no current limiting resistor in circuit, unless the 4V battery has very high internal resistance, the odds are it will blow the LED first time both switches are closed.Add a 100 ohm resistor in series with the LED to limit the current to typ. 20mA so the LED survives.* it was marginal even with the two diode drops, so the LED may already be blown.

author: Zero999
author_rank: Super Contributor
timestamp: April 04, 2024, 08:31:28 pm
content: Quote from: Ian.M on April 04, 2024, 07:38:30 pmIndeed. I would go even higher. 220R is probably enough.

author: Ian.M
author_rank: Super Contributor
timestamp: April 04, 2024, 08:47:55 pm
content: Yes, for just messing around with switches and LEDs on the bench, with a 4V or 5V supply, as its high efficiency so <expletive> bright, I might even go all the way up to 1K

author: MrAl
author_rank: Super Contributor
timestamp: April 04, 2024, 10:01:40 pm
content: Quote from: Fuldup on April 04, 2024, 02:49:16 pmIt depends on the quality of the transistor. Run of the mill transistors like the 2N2222 and similar will not have as low a CE voltage drop as one of the newer Zetex bipolar transistors. They are very high quality and more expensive. Zetex may not be called that anymore though.MOSFETs can have a lower voltage drop it depends how much current you have to run through them and how well you drive the gate, and the rating of the transistor RdsOn for example.

author: donlisms
author_rank: Frequent Contributor
timestamp: April 05, 2024, 03:20:51 am
content: Just some ways to think about this:Ultimately for this gate, you would want the input signals going into the bases of the transistors, and use the collectors and/or emitter branches in some way as the outputs.An output can, of course, become an input to the next stage.The switches can be an easy way to control the input signals. It would be unusual to find them in the middle of the circuit. In that case, they become part of the gate itself, as has been said.