# BJT V(ce) ON in common emitter mode

1. ## BJT V(ce) ON in common emitter mode

Hello,

For an FMMT491A BJT with 2.7mA flowing into its base and set up in common emitter mode….is there any set of circumstances (temperature, i(ce) etc etc) which might ever result in its Vce(ON) voltage ever being greater than 0.5v?
Its collector current is always less than 40mA
From the datasheet I see not, but is there any gremlin situation?

FMMT491A datasheet
https://www.diodes.com/assets/Datasheets/FMMT491.pdf

2. ## Re: BJT V(ce) ON in common emitter mode

Hi,

I don't expect higher V_CE than 5V with the given values.

If you see higher V_CE, then you should provide your test circuit..an additional photo could be helpful.

For low voltage drop I'd use a MOSFET.

Klaus

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3. ## Re: BJT V(ce) ON in common emitter mode

Not more than 10 to 20 mV Vce with the given Ic and forced B values.

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4. ## Re: BJT V(ce) ON in common emitter mode

Is there any situation , where literally any SOT23 or SOT323 NPN with VCE voltage rating of <65V may suffer a drop in Hfe down to less than 15?...surely such a situation simply does not exist?...even in the cheapest such NPN?…….(considering Ib <4mA ).
If so, please state the situation and NPN concerned

5. ## Re: BJT V(ce) ON in common emitter mode

-Ib has to be <4mA
-Vce(sat) has to be >0.5V @ Ic=40mA..

I wonder what type of application is it ?? If you would Vce(sat)>0.5V, connect simply a resistor ??

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6. ## Re: BJT V(ce) ON in common emitter mode

thanks, we have BJTs with very very little base current in them (to save energy)....and we wonder if there Hfe will always be big enough to get them to turn on fully and have a vce(on) of <0.3v.

7. ## Re: BJT V(ce) ON in common emitter mode

hFE is measured when the transistor is linear with plenty of Vce voltage and not saturated.
Your transistor is a saturated switch and like most little transistors its datasheet says it saturates well when its base current is 1/10th its collector current.

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8. ## Re: BJT V(ce) ON in common emitter mode

you can never get Vce < Vbe in common emitter config. For max Hfe at a given Ic you need to carefully select a xtor from its data sheet, Hfe falls off for very low Ic

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9. ## Re: BJT V(ce) ON in common emitter mode

Originally Posted by treez
thanks, we have BJTs with very very little base current in them (to save energy)....and we wonder if there Hfe will always be big enough to get them to turn on fully and have a vce(on) of <0.3v.
The solution comes into my mind is Darlington Transistor with Low Vce(sat) feature.. Am I wrong ??
Or using a simply a MOS ?? Zero bias current and very low sat voltage ..
I'm still not sure what your purpose is..

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10. ## Re: BJT V(ce) ON in common emitter mode

Originally Posted by Easy peasy
you can never get Vce < Vbe in common emitter config.
Of course you can have a common-emitter transistor saturate with its Vce much less than its Vbe. it is called "saturation".

- - - Updated - - -

Originally Posted by BigBoss
The solution comes into my mind is Darlington Transistor with Low Vce(sat) feature.
The input transistor of a darlington transistor prevents the output transistor from saturating with a low Vce(sat). A TIP120 power Darlington has a max allowed continuous current of 5A but its typical Vce(sat) at only 1A is 1V.
A TIP41 power transistor is not a Darlington and its max allowed continuous current is 6A, its typical Vce(sat) at 1A is 0.15V.

Wikipedia says, "Another drawback of the Darlington pair is its increased "saturation" voltage. The output transistor is not allowed to saturate."

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11. ## Re: BJT V(ce) ON in common emitter mode

I meant when the emitter is common to the load sorry, - emitter follower - which seemed what the OP was trying to do

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12. ## Re: BJT V(ce) ON in common emitter mode

Originally Posted by Easy peasy
I meant when the emitter is common to the load sorry, - emitter follower - which seemed what the OP was trying to do
No. The op is using the transistor as a common-emitter saturated switch and does not know that hFE is not used for a saturated transistor.

13. ## Re: BJT V(ce) ON in common emitter mode

thanks , i appreciate that if a NPN in CommEm has <0.3v across its Collec-Emit (what we want), then it is going to be saturated......thing is, we find ourselves having to look at hfe, to see if it will be high enough so that we do end up in saturation...even though hfe doesnt then pertain to operation in saturation...

14. ## Re: BJT V(ce) ON in common emitter mode

Don't you trust the datasheets? They guarantee that ANY transistor that has an hFE high enough to pass will saturate at the spec'd low saturation voltage. Then the base current MUST be 1/10th the emitter current. Some high hFE European transistors say 1/20th the collector current. But for you to want less base current then you must test each and every transistor to find any that can do it.

The graphs on a datasheet apply only to a "typical" transistor, not all of them, and many actually saturate fairly well with a low base current.

15. ## Re: BJT V(ce) ON in common emitter mode

for power xtors, most data sheet specify Vce ON for saturated gain, usually Ic/Ib = 10 or 5

Zetex did a range of xtors designed to give very low Vce on for quite modest base drive Ic/Ib = 20 or so

I recommend looking up these types on the new owners website - they may be just what you are looking for

Else a mosfet with 1V drive ( they do exist )

16. ## Re: BJT V(ce) ON in common emitter mode

else you have to find a xtor that is spec'd for a min gain at amin temp that will fall into your desired range, as mentioned above - if the Ice goes up - the Vce will go up - for a given base current.

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