hfe of MMBT2222 BJT is 35 maximum in this circuit?

Hello,
Can you confirm that the MMBT2222 NPN transistor in the schematic on page 3 of the following will have a gain (hfe) of 35 (maximum)?

AN1465 app note:
https://ww1.microchip.com/downloads/en/AppNotes/01465A.pdf

MMBT2222 transistor:
https://www.diodes.com/assets/Datasheets/ds30041.pdf

The question sounds absurd because there's no specification of maximum hfe in the MMBT2222 or any other transistor's datasheet. You also missed to tell your circuit analysis leading to this statement.

Thanks, our circuit is as in fig5 of the AN1465 app note. (linked in the top post).
Our DALI TX input is 0V to 2V5, and we need to know if we will give the MMBT2222 enough base current to allow it to sink the DALI 250mA and bring the DALI bus down to zero volts.

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Also, why does the FMMT493 datasheet not give hfe for when Vce=1V?

FMMT493 datasheet
https://www.diodes.com/assets/Datasheets/FMMT493.pdf

The application note circuit seems to lack any serious calculation of worst case component parameters. Even with 5V DALI_TX voltage, the guaranteed base current isn't higher than about 5 mA. With 2.5V input it's about 2 mA.

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I would go for a low Vce,sat, high current gain transistor like PBSS4140U.

treez said:

Hello,
Can you confirm that the MMBT2222 NPN transistor in the schematic on page 3 of the following will have a gain (hfe) of 35 (maximum)?

AN1465 app note:
https://ww1.microchip.com/downloads/en/AppNotes/01465A.pdf

MMBT2222 transistor:
https://www.diodes.com/assets/Datasheets/ds30041.pdf

Click to expand...

Beta_Forward has a dependent characteristic to Collector Current ( Ic) that's why they have given this spec. relative to Ic.
If you fear from very variable Beta_Forward and you would ensure that circuit should work in any case ( Temp, lot-to-lot variation etc.) you can use a Small Signal MOSFET which has been manufactured for these types of applications.
When Vgs reaches to defined value and beyond, MOSFET current will increase rapidly by square rate of Vgs.If you modify the circuit regarding to these specifications, you may obtain a robust and consistent result.

I would think that a logic level MOSFET might
suit you better.

For a NPN switch that wants to make a decently
low VOL (spec=?, for DALI bus?). Zero is not an
option for voltage (but logic zero is another matter
altogether). The 2N2222's Vce(sat) can't be expected
to be better than ~200-300mV under 250mA load
and to great extent improving VOL requires that
you operate at a lower "forced beta" (like maybe
10) which then demands 25mA base current, and
this in turn wants a ~20 ohm output resistance
in a sourcing I/O pad which will itself be dissipating
1.8V*25mA = ~ 50mW (either internal, or partially
in an external limiting resistor) and this will all be
pretty poorly controlled over PVT.

Of course logic level power MOSFET usually means
"5V" with a few capable at 3.3V and very few if
any, capable at 2.25V gate drive across MOSFET
product spec and product manufacturing variation.

For a NPN switch that wants to make a decently
low VOL (spec=?, for DALI bus?).

Click to expand...

Thanks, with DALI, anything below 3V is classed as zero.

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Thanks, though a FET cannot be so readily used , as the attached LTspice sim shows....as you know, it can be done, but not with so low component count as with BJT

hFE is used for a linear amplifier that has plenty of collector to emitter voltage, not for a saturated switch. Most datasheets show a base current of 1/10th the collector current for a saturated switch but higher currents need more base current that is much higher than hFE. A few European transistor datasheets that have a very high hFE of 900 show good saturation when the base current is 1/20th the collector current.