[EE] Base emitter voltage variation?

[Jason White]

Hello Everyone,

I am dealing with a design where the base-emitter saturation ("turn on")
voltage of a MMBT4403 matters quite a bit. (A preexisting design uses it
for temporary, fast-acting, over-current protection under the supervision
of a microcontroller)

Collector current = 3mA, Temperature = -40 to 70C

I would like to understand better in what ways the base emitter saturation
voltage can vary.
In addition I would like to know if there exists any "rules of thumb"

In the Diode's Inc datasheet page 4 [1] there is a graph titled "Figure 3
Typical Base-Emitter Turn-On Voltage vs. Collector Current"

From this graph I can determine the "typical" base-emitter saturation

voltage Vbe(sat) with respect to temperature.

I'd like to know what factors other than temperature and collector current
affect Vbe(sat) and if there is any way to estimate their effect's on the
voltage?

I know there should be manufacturing variations from device to device.
Perhaps there exists some upper bound to how much Vbe(sat) can vary from
device to device?

[1] https://www.diodes.com/assets/Datasheets/ds30058.pdf

Thanks,
Jason White

[Jason White]

Oh, and in addition would assuming a 10% variation from the stated typical
figures be a reasonable assumption for design purposes?
* Typical Vbe(sat) @ -40C = 730mV
* Typical Vbe(sat) @ 25C = 600mV
* Typical Vbe(sat) @ 70C = 522mV

So the max Vbe(sat) at -40C would be 10% more than 730mV and the min
Vbe(sat) at 70C would be 10% less than 522mV.

Thanks,
-Jason White

On Wed, Aug 29, 2018 at 12:45 PM Jason White <

Post by Jason White
Hello Everyone,
I am dealing with a design where the base-emitter saturation ("turn on")
voltage of a MMBT4403 matters quite a bit. (A preexisting design uses it
for temporary, fast-acting, over-current protection under the supervision
of a microcontroller)
Collector current = 3mA, Temperature = -40 to 70C
I would like to understand better in what ways the base emitter saturation
voltage can vary.
In addition I would like to know if there exists any "rules of thumb"
In the Diode's Inc datasheet page 4 [1] there is a graph titled "Figure 3
Typical Base-Emitter Turn-On Voltage vs. Collector Current"
From this graph I can determine the "typical" base-emitter saturation
voltage Vbe(sat) with respect to temperature.
I'd like to know what factors other than temperature and collector current
affect Vbe(sat) and if there is any way to estimate their effect's on the
voltage?
I know there should be manufacturing variations from device to device.
Perhaps there exists some upper bound to how much Vbe(sat) can vary from
device to device?
[1] https://www.diodes.com/assets/Datasheets/ds30058.pdf
Thanks,
Jason White

[Jean-Paul Louis]

Jason,

Your data looks right. Typical junction voltage varies just around 2mV per degree C for a given current. That’s how thermometer ICs works.
I am not sure of the variation with collector current though. But you said that your app current was 3mA, right? So 10% variation from that curve seems way out there.

Post by Jason White
Oh, and in addition would assuming a 10% variation from the stated typical
figures be a reasonable assumption for design purposes?
So the max Vbe(sat) at -40C would be 10% more than 730mV and the min
Vbe(sat) at 70C would be 10% less than 522mV.
Thanks,
-Jason White
On Wed, Aug 29, 2018 at 12:45 PM Jason White <

Post by Jason White
Hello Everyone,
I am dealing with a design where the base-emitter saturation ("turn on")
voltage of a MMBT4403 matters quite a bit. (A preexisting design uses it
for temporary, fast-acting, over-current protection under the supervision
of a microcontroller)
Collector current = 3mA, Temperature = -40 to 70C
I would like to understand better in what ways the base emitter saturation
voltage can vary.
In addition I would like to know if there exists any "rules of thumb"
In the Diode's Inc datasheet page 4 [1] there is a graph titled "Figure 3
Typical Base-Emitter Turn-On Voltage vs. Collector Current"
From this graph I can determine the "typical" base-emitter saturation
voltage Vbe(sat) with respect to temperature.
I'd like to know what factors other than temperature and collector current
affect Vbe(sat) and if there is any way to estimate their effect's on the
voltage?
I know there should be manufacturing variations from device to device.
Perhaps there exists some upper bound to how much Vbe(sat) can vary from
device to device?
[1] https://www.diodes.com/assets/Datasheets/ds30058.pdf
Thanks,
Jason White

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Jason White
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Just my $0.02,

Jean-Paul
N1JPL

[Sean Breheny]

Hi Jason,

I believe that Vbe(sat) refers to the base-emitter voltage with the
transistor in saturation (the base-collector junction is forward-biased in
addition to the base-emitter junction). This is higher than the turn-on
voltage. For example, at 25 deg C, 150mA collector current, Vbe(sat) is
given as between -0.75 and -0.95V in the Diodes, Inc. MMBT4403 datasheet.
However, if you look at figure 3, typical base-emitter turn-on voltage, and
follow the line, extrapolating off the right-hand side of the graph, it
looks like the typical turn-on voltage for 150mA would be about 0.77V, not
0.85V as suggested by the previous range. That's because figure 3 is
specifically for a non-saturated state (Vce=5V)

One thing to consider is how the rest of your circuit is connected. There
isn't just one Vbe value where a BJT turns on - it will go from conducting
nanoamps to conducting milliamps over a range of maybe 0.2V, and what
happens to the rest of your circuit could cause considerable variation in
the end result. For example, if you have a 100k resistor from the collector
to GND, the emitter to 5V, and there is a digital circuit looking at the
collector, then the variation in the digital circuit threshold could add
0.1 to 0.2V variation to your effective Vbe threshold.

So, all together, I'd say the sources of variation are:
1) Temperature (predictable and you have considered this)
2) Manufacturing variation (I would suggest +/-15% instead of +/-10%)
3) Collector resistor tolerance
4) Threshold tolerance of the next stage
5) Any effect of base current variation on the circuit which drives Vbe
(temperature and manufacturing variation, as well as whether you saturate
the transistor and if so, how hard will all affect the amount of base
current you have for a given collector current - if the source of Vbe is
not low impedance, this could affect it)

Sean