Author Topic: Benefits of pre biased transistor over FET?  (Read 2819 times)

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Offline jrs45Topic starter

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Benefits of pre biased transistor over FET?
« on: July 26, 2020, 01:10:21 pm »
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.

 

Online Zero999

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Re: Benefits of pre biased transistor over FET?
« Reply #1 on: July 26, 2020, 01:49:14 pm »
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.
 

Offline JoeyG

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Re: Benefits of pre biased transistor over FET?
« Reply #2 on: July 26, 2020, 02:30:20 pm »
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)
 

Offline fcb

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Re: Benefits of pre biased transistor over FET?
« Reply #3 on: July 26, 2020, 02:43:15 pm »
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)



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Offline schmitt trigger

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Re: Benefits of pre biased transistor over FET?
« Reply #4 on: July 26, 2020, 03:05:58 pm »
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!
 

Offline jrs45Topic starter

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Re: Benefits of pre biased transistor over FET?
« Reply #5 on: July 26, 2020, 04:15:48 pm »
Thanks!  For me the defined condition during poweron is definitely useful, though all it saves is a resistor.  But still, worthwhile.
 

Offline Benta

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Re: Benefits of pre biased transistor over FET?
« Reply #6 on: July 26, 2020, 04:41:37 pm »
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.

 

Offline julian1

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Re: Benefits of pre biased transistor over FET?
« Reply #7 on: July 26, 2020, 08:05:08 pm »
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.
 

Offline T3sl4co1l

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Re: Benefits of pre biased transistor over FET?
« Reply #8 on: July 27, 2020, 12:25:31 am »
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. :P

Tim
« Last Edit: July 27, 2020, 12:30:17 am by T3sl4co1l »
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Offline fcb

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Re: Benefits of pre biased transistor over FET?
« Reply #9 on: July 27, 2020, 01:24:26 pm »
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)

I would recommend a part that rules them all, BSD235C form Infineon.

A tiny (SC70-6) dual P/N MOSFET with good current capability, low gate charge and fast switching speed.

It serves very well in low power load switching applications, double MHz switching applications (driven by no more than 5*74LVC14, which is driven by the rest), and charge pumps (my favorite power electronics topology of all time, I use them mostly for gate driving and for bias voltage rails).

It is considerably more expensive then the rest, but it saves quite some PnP slots as it replaces basically all non-RF, non-emitter-follower small active parts I can ever use.
Thanks for the tip - I've got some on order now to 'play with'.
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Offline fcb

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Re: Benefits of pre biased transistor over FET?
« Reply #10 on: July 27, 2020, 01:31:41 pm »
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.
Pre-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.

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Offline uer166

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Re: Benefits of pre biased transistor over FET?
« Reply #11 on: July 27, 2020, 04:26:33 pm »
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)



I would recommend a part that rules them all, BSD235C form Infineon.

A tiny (SC70-6) dual P/N MOSFET with good current capability, low gate charge and fast switching speed.

It serves very well in low power load switching applications, double MHz switching applications (driven by no more than 5*74LVC14, which is driven by the rest), and charge pumps (my favorite power electronics topology of all time, I use them mostly for gate driving and for bias voltage rails).

It is considerably more expensive then the rest, but it saves quite some PnP slots as it replaces basically all non-RF, non-emitter-follower small active parts I can ever use.

How 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.
 

Offline Benta

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Re: Benefits of pre biased transistor over FET?
« Reply #12 on: July 27, 2020, 04:36:34 pm »
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.
Pre-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.

The 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.

« Last Edit: July 27, 2020, 07:51:30 pm by Benta »
 

Offline David Hess

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Re: Benefits of pre biased transistor over FET?
« Reply #13 on: July 27, 2020, 07:12:21 pm »
How does the gate capacitance of mosfets like 2N7002 compare with parasitic BE capacitance of BJTs?

Bipolar 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.

Quote
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.

Besides 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.
 


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