Electronics > Projects, Designs, and Technical Stuff
Benefits of pre biased transistor over FET?
jrs45:
One of my colleagues recommended using devices like MMUN2211 instead of FETs (like 2N7002) for simple logic switching, small LED driving with PWM, and the like.
These are small packages of a BJT with biasing resistors built in. They look like a direct drop-in replacement for the FET. But what are the benefits? It looks like it's marginally cheaper, but are there other differences I should be aware of?
The other benefit might be that it removes the need for a pulldown resistor, so that the state of the switch is well defined during startup (for example, from a microcontroller pin before it's assigned as an output). Maybe also better for ESD withstanding?
Just wondering, they seem useful but not often used.
Zero999:
A FET is a transistor.
I can't see any benefit of used a BJT with built-in resistors, over a MOSFET, other than cost.
ESD is only an issue for MOSFETs, when they're not connected to anything. Once the MOSFET is in the circuit it's not much more susceptible to ESD, than any other component.
JoeyG:
We found in a logic switch application the FET had very low RDSon drove the voltage below Vss , where as a RET (resistor transistor did not)
fcb:
Obviously 'it depends'. I use both in the different designs, mainly I use an MMUN when I need to guarantee the start up condition (due to the base-emitter pull-down effect from the potential divider).
MMUN parts tend to be even lower cost than a 2N7002, probably only matters if you have alot of them. There is also a low-cost matching PNP MMUN which saves finding a P-type FET.
My goto parts for these and a little bit of power switching that have permanent feeders on the P&P are:
2N7002KT1G (N-type)
DMG2305UX-7 (P-type)
FDN337N (N-type)
MMUN2211LT1G (pbt NPN)
MMUN2111LT1G (pbt PNP)
schmitt trigger:
Everyone will have, of course, his/her own favorite devices depending on his particular experience.
I remember seeing the very first BRT devices when I was troubleshooting a Japanese VHS camcorder. Testing the device's junctions with a DMM gave very oddball readings. I finally could procure the unit's schematic and saw a couple of tiny resistors attached to the BJT.
I thought at the time this was the greatest invention since sliced bread!
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