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Topic: is the 2n2222 fully saturated? (Read 353 times) previous topic - next topic


Apr 17, 2017, 03:59 pm Last Edit: Apr 17, 2017, 03:59 pm by cabecinhas
Is correct to say when the voltage base>emitter is 0.7v (in case of 2n2222), the transistor is fully saturated?


Probably...   The datasheet actually says the saturation voltage can be as 2.6V (depending on CE current and part-to-part variations).

Transistors are "current operated" devices and the best way to make sure the transistor is saturated is to use the minimum-specified hFE and make sure there's enough BE current to saturate, depending on the CE saturation current for your particular circuit.


Apr 17, 2017, 05:14 pm Last Edit: Apr 17, 2017, 05:20 pm by terryking228

Saturation is usually specified as Collector-Emitter voltage being "low" typically less than 0.5V.  It depends on the collector current and base current for a particular device.

Typically the transistor current gain is relatively low at saturation, like 10mA Base current for 100mA Collector current.

SEE the "saturation region" graph for the device you are using. (Hard to find for these type devices it seems)

---------------------( COPY )---------------------
Collector-Emitter Saturation Voltage  2N2222
 (IC=150mAdc, IB=15mAdc)  0.4V
(IC=500mAdc, IB=50mAdc)   1.6V
-----------------( END COPY )----------------------
So you can see the Beta (Gain) is about 10 in saturation..
Regards, Terry King terry@yourduino.com  - Check great prices, devices and Arduino-related boards at http://YourDuino.com
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I generally figure a beta of 10 when I want to turn a bipolar transistor hard on.



Is correct to say when the voltage base>emitter is 0.7v (in case of 2n2222), the transistor is fully saturated?
No.  Vbe is always about 0.7V when a silicon transistor is "on" at all.
The other explanations of saturation being indicated by Vce are correct...
Also, in saturation Ic << Ib * beta


A really high performance modern switching transistor can have saturation voltages down in the 10 to 100mV
region, checkout this datasheet for example:http://www.redrok.com/NPN_ZTX851_60V_5.0A_1.2W_Hfe75_TO-92.pdf

Saturated behaviour is all about doping levels and diffusion, rather than transistor action where the electric
field across the base-collector junction is involved.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]


Pretty much using WORST case gain value is needed for switching saturation gaurantee... so don't go grabbing the hFE 300 VALUE from the 2N2222A and using that for the calculation.

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