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Benefits of pre biased transistor over FET?

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fcb:

--- Quote from: Benta 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.

--- End quote ---
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.

uer166:

--- Quote from: blueskull on July 26, 2020, 02:48:43 pm ---
--- Quote from: fcb on July 26, 2020, 02:43:15 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)

--- End quote ---



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.

--- End quote ---

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.

Benta:

--- Quote from: fcb on July 27, 2020, 01:31:41 pm ---
--- Quote from: Benta 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.

--- End quote ---
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.

--- End quote ---

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.

David Hess:

--- Quote from: julian1 on July 26, 2020, 08:05:08 pm ---How does the gate capacitance of mosfets like 2N7002 compare with parasitic BE capacitance of BJTs?
--- End quote ---

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.
--- End quote ---

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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