Benefits of pre biased transistor over FET? - Page 1
Forum section: electronics / projects

author: jrs45
author_rank: Regular Contributor
timestamp: July 26, 2020, 01:10:21 pm
content: 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.

author: Zero999
author_rank: Super Contributor
timestamp: July 26, 2020, 01:49:14 pm
content: 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.

author: JoeyG
author_rank: Regular Contributor
timestamp: July 26, 2020, 02:30:20 pm
content: 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)

author: fcb
author_rank: Super Contributor
timestamp: July 26, 2020, 02:43:15 pm
content: 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)

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 26, 2020, 03:05:58 pm
content: 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!

author: jrs45
author_rank: Regular Contributor
timestamp: July 26, 2020, 04:15:48 pm
content: Thanks! For me the defined condition during poweron is definitely useful, though all it saves is a resistor. But still, worthwhile.

author: Benta
author_rank: Super Contributor
timestamp: July 26, 2020, 04:41:37 pm
content: A couple of other points:- MOSFETs require a relatively high gate voltage to turn on (even logic-level types). BRTs don't.- MOSFETs have a relatively high drain-source leakage current. BRTs don't.

author: julian1
author_rank: Frequent Contributor
timestamp: July 26, 2020, 08:05:08 pm
content: How does the gate capacitance of mosfets like 2N7002 compare with parasitic BE capacitance of BJTs?Power mosfet driver circuits made from discrete components appear to be mostly BJT. I assumed this was due to lower parasitic capacitance making them easier to switch harder and faster.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 27, 2020, 12:25:31 am
content: BJT capacitance is smaller, and gm is much higher.2N3904 is rated 200mA, and what, 8pF E-B? 2N7000 is rated 200mA, and is more like 30pF (at low voltages). BJT needs about 0.3V swing to go from reasonably-on to reasonably-off, while the MOSFET needs a volt or so. Or in practical switching circuits, about 1V vs. >3V.I guess what's interesting is MOSFETs are a better self-match: even given that, in Si, PMOS performs ~2.5x worse than NMOS, the drain voltage and current range is very suitable for driving more gates. Hence the dominance of CMOS.BJTs go well with themselves at low voltages (I2L is probably the best example), but aren't as easy to use at conventional logic voltages. Consider TTL's relatively high power consumption, and poor V_OH vs. I_OH. The latter problem could of course be remedied with PNPs if they were available in ICs at the time, but the voltage swing is still way more than needed for Vbe, while the current requirement is the same (set by storage charge and Ccb). So the power dissipation is higher.Does mean BJTs work very nicely in RF applications, where networks can transform collector and base circuit impedances. I suppose MOSFETs still are dominant here today; LDMOS is huge, while RF BJTs are... does anyone make them anymore? Replacements for old equipment, or are those all NOS?* But this wasn't always the case I think; more that, now that they've both been heavily optimized, BJTs hit a physical wall (recombination) while MOSFETs kept going. Advanced relatives are current tech though (SiGe HBTs).*Checking, yep, still stocked; don't know if they're classic PNs or what, but the ceramic stud/flange microstrip package anyway. And still as expensive as ever.Tim

author: fcb
author_rank: Super Contributor
timestamp: July 27, 2020, 01:24:26 pm
content: Quote from: blueskull on July 26, 2020, 02:48:43 pmThanks for the tip - I've got some on order now to 'play with'.

author: fcb
author_rank: Super Contributor
timestamp: July 27, 2020, 01:31:41 pm
content: Quote from: Benta on July 26, 2020, 04:41:37 pmPre-biased transistors tend to have a potential divider between the base input and emitter, so require more than the usual 0.6V to guarantee switch on. So probably similar to to a 2N7002.Drain-source leakage with the 2N7002 (diodes inc) is rated at 1uA max (@25C), in reality it is a few nA. So most of cases I've used them it makes no difference.

author: uer166
author_rank: Super Contributor
timestamp: July 27, 2020, 04:26:33 pm
content: Quote from: blueskull on July 26, 2020, 02:48:43 pmHow does a 20V part "rule them all" when some of the other parts are 3X the Vds rating? That's barely usable except in the lowest voltage applications.

author: Benta
author_rank: Super Contributor
timestamp: July 27, 2020, 04:36:34 pm
content: Quote from: fcb on July 27, 2020, 01:31:41 pmThe series vs base-ground resistor can be almost any combination, eg, 4.7 kohm series, 100 kohm base-ground. Not a lot of voltage division going on there...EDIT: I mean base-emitter, not base-ground of course.

author: David Hess
author_rank: Super Contributor
timestamp: July 27, 2020, 07:12:21 pm
content: Quote from: julian1 on July 26, 2020, 08:05:08 pmBipolar capacitance is lower and their higher transconductance makes it effectively lower yet. Bipolar threshold voltage is lower and better defined. Die size is smaller making them less expensive.QuoteBesides economics, I think their big advantage is low threshold voltage allowing an easy to use emitter follower output stage. To do the same thing, MOSFETs would require higher supply voltages.