# Why VCE in transistor is 0.2V?

Discussion in 'Physics' started by Labiva, Aug 7, 2012.

1. ### Labiva Thread Starter New Member

Aug 4, 2012
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in transistors, why the collector to emitter voltage VCE is 0.2V in saturation? why is it not 1.4V? because both emitter-base voltage and collector-base voltage is 0.7V.

2. ### bretm Member

Feb 6, 2012
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But the collector-base junction is forward-biased instead of the normal reverse bias, hence the voltage drops partially cancel out instead of adding together.

3. ### Wendy Moderator

Mar 24, 2008
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That is actually a very good question. My personal interpretation is the C-E is not actually a PN junction, the Base "fills" the gap between the C-E with holes or electrons. The B-E is still a junction though.

I could be wrong, but it is how I interpret it.

4. ### WBahn Moderator

Mar 31, 2012
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When the transistor is 'on' (either active or saturated), the base-emitter junction is forward biased and, hence, has a voltage of about 0.7V (but this can vary quite a bit depending on how large the areas of the junction are and how much current is actually flowing, but 0.7V is a good back-of-the-envelope figure to use an most situations).

But the collector-base is NOT forward biased. For most of the active region, it is firmly reverse biased. You can visualize that the base-emitter current allows some collector-emitter current to leak through the reverse-biased junction. To a first order approximation, the beta of the transistor describes how much collector current can leak through as a function of the base current. In a typical small signal transistor, that might be a factor of several hundred. So if the base current is, say, 1mA and the beta is 100, then 100mA of collector-emitter current would like to leak through. However, if the collector-emitter voltage is too high, the external circuit won't be able to deliver 100mA. In this case, the transistor can't sustain that high a reverse bias and the collector-emitter voltage drops, resulting in more current. But as soon as the 100mA is reached, the collector-emitter voltage stabilizes and stops dropping. Now, as it drops, the base-collector junction is becoming less reverse-biased and, as it does, the beta of the transistor starts to drop. Eventually, it starts becoming forward biased and would like to actually start letting current flow from the base to the collector, but that is still swamped by the collector current that is leaking through the junction. However, as the base-collector voltage starts getting closer to the 0.7V range, it is getting closer to being truly forward biased, but before that happens the beta of the transistor is completely trashed and as the collector-emitter voltage attempts to drop below Vcesat, the beta just drops further keeping pace.

Now, all of this is real handwavy, but hopefully it gives you a mental image that you will find useful.

5. ### Markd77 Senior Member

Sep 7, 2009
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Another thing to mention is that while VCE is 0.2V in saturation for a specific transistor at one collector current, it will be different with other transistors and currents.
This is from the ZTX1053A datasheet (the columns are typical and maximum):

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6. ### ramancini8 Active Member

Jul 18, 2012
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Invert the transistor and Vcesat reduces to about a tenth of its non-inverted value. An old time trick used before FETs became available.

7. ### Wendy Moderator

Mar 24, 2008
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With low currents most saturated transistors are considerably lower, this is a worst case.