# Very simple question about transistors

Tags: simple, transistors
 P: 9 About the stages of a transistor.. Active,Saturation and Cutoff. Active its obviously on and cutoff off. May be a dumb question but is the transistor ON on Saturation mode? Also my textbooks says that when the BE junction is forward bias, Vbe= 0.7V. But at the same time in another problem it says that the cutoff voltage in the BE junction of an NPN transistor is Vbe=0.5V. Which leads to my question, What is the cutoff voltage of the BE junction in an NPN transistor, 0.7V or 0.5V? Is the 0.7 value given as cushion flor the transitor to be fully ON? I'm kind of confused about this whole thing so any help would be appreciated. And lastly, what is the difference of current behavior in Active with Saturation mode? Where is the current the largest?
P: 1,782
I've attached the datasheet for a popular transistor, the 2N3904. Note the values for VCE(sat) for Ic of 10mA and 50mA and VBE(sat) for Ib of 1mA and 5mA.

Frankly an Ib of even 1mA is quite high for this transistor. Normally Ib runs in the tens of uA. I've measured VBE with normal Ib at room temperature and typically got around 0.65 V and it varies with temperature about -2mV/C. As you can see, VCE(sat) is lower than VBE(sat).

The VBE(cutoff) can be anything down to the reverse breakdown voltage. As you will note it's not specified.

The difference between the active region and the saturated region is that when a transistor is in the active region, a small change in Ib causes a much larger but proportional change in Ic. When a transistor is in saturation, a small change in Ib will not cause a perceptible change in Ic.
Attached Files
 2N3904.pdf (317.6 KB, 5 views)
 P: 9 I see, thanks a lot, I'll take a look at the file.
P: 177

## Very simple question about transistors

To add to skeptic's answer, saturation mode simply means that the transistor is fully on.

A transistor is somewhat like a relay if you only use cutoff mode and saturation mode.

In the active region (triode mode) it's more like an amplifier because small changes in the input become large changes in the output.

Vbe can vary due to manufacturing, temperature, and base current. 0.7V is sort of like the golden standard for doing calculations because it's mostly right. Unless you're told otherwise, assume that Vbe is 0.7V.
 P: 9 thanks
P: 1,782
 Quote by Okefenokee Vbe can vary due to manufacturing, temperature, and base current. 0.7V is sort of like the golden standard for doing calculations because it's mostly right. Unless you're told otherwise, assume that Vbe is 0.7V.
Better yet, once you get a circuit working in the active region, measure it for yourself. Most engineers assume that Vbe is 0.7V and never measure it.
 P: 9 I have a last question guys, and thanks for the input so far. Can the BJT transistor act as a voltage amplifier? Or does it just amplify current and power?
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