Quad transistor package, what to do with unused element - Page 1
Forum section: electronics / beginners

author: Infraviolet
author_rank: Super Contributor
timestamp: July 07, 2023, 08:49:22 pm
content: I'm designing a PCB involving the use of some MMPQ3904 quad NPN transistor packages, these packages have 4 independent transistors in them with C,B and E for each transistor broken out to a separate pin and not common between anyof the transistors in the package (very few chips like this, usually multi-transistor packages seem to have some pins common between all units within them). In most places each package uses all four of the transistors within it, but a few of them on the board only need three transistors used. Should the unused transistor in a package be left floating? have all pins grounded to shared ground? have all pins connected to each-other but floating relative to other circuits on the PCB?I know for digital logic chips one should ground unused inputs, or tie them to Vcc, so the output stays consistently in one state. And for multi-op-amppackages the rule is usually to make unused op-amps serve as buffers, or otherwise according to some datasheets for rail-to-rail ones tie the noninverting to Vcc and the inverting to Gnd.But what about for multi-transistor packages?P.S. Does anyone know if this MMPQ3904 type of quad pack is truly monolithic or not? The datasheets (I've found them form multiple manufactuers) don't say. Now if the transistors were explicitly matched I guess they'dmake a big deal of it and state it all over the datasheet, so I guess its not matched, but for thermal purposes is it monolithic?Thanks

author: Benta
author_rank: Super Contributor
timestamp: July 07, 2023, 09:13:04 pm
content: It's just four 2N3904 die in one package. Think of the spare one as lying on your workbench, don't worry about it.

author: magic
author_rank: Super Contributor
timestamp: July 07, 2023, 09:51:51 pm
content: Quote from: Infraviolet on July 07, 2023, 08:49:22 pmThose cheap jellybean (2N3904, BC547, 2N2222, etc) transistor arrays are just that - a bunch of separate transistors in one plastic blob.Sometimes a certain level of matching is guaranteed, maybe 3mV Vbe offset or so, oftentimes not even that. Any such matching is achieved by automatic testing/selection of candidate dice.Thermal coupling should be better than between two packages next to each other, but hardly perfect. I haven't seen any concrete specifications regarding that.

author: TimFox
author_rank: Super Contributor
timestamp: July 07, 2023, 09:53:15 pm
content: As Benta said, it's four independent dice in one package.Personally, I don't like to leave stuff floating, so I would connect the three unused pins to the most negative voltage on the other 9 pins.That's not necessary, but if this were a monolithic device, I would definitely make that connection.

author: Infraviolet
author_rank: Super Contributor
timestamp: July 07, 2023, 10:00:35 pm
content: As none of the pins in the used transistors ever get taken below ground, although they are usually all a volt or two above it, would grounding the unused one be easiest. Ground won't always equal to the most negative voltage on any pin of the others, but it would always be more negative than or equal to that lowest voltage. And no pin on these transistors ever gets more than 5V above ground either, so nowehere near breakdown voltages.

author: Kim Christensen
author_rank: Super Contributor
timestamp: July 07, 2023, 10:31:29 pm
content: Quote from: Infraviolet on July 07, 2023, 08:49:22 pmI don't believe they are on a single die judging by the description and the fact thatthe datasheetdoes not mention anything about breakdown voltages or leakages between transistors.Description from datasheet: "MMPQ3904 NPNMulti-ChipGeneral Purpose Amplifier"

author: magic
author_rank: Super Contributor
timestamp: July 08, 2023, 05:30:04 am
content: Just leave them floating and stop overthinking. A floating bipolar transistor sits there and does precisely nothing - exactly as required in this case.