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If VCE is negative in a PNP transistor does that mean that it is in saturation? Note: not given the saturation value.

This is related to a homework question.

Thanks

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  • \$\begingroup\$ Do you actually mean \$V_{CE}\$? Or maybe \$V_{CB}\$?? \$\endgroup\$ – Spehro Pefhany Apr 7 '14 at 5:28
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VCE = -VEC

For PNP transistors, some datasheets actually use VEC (or similar identifiers), most datasheets use negative values.

A negative VCE, meaning VC - VE < 0 is common for a PNP transistor in normal use. Take a look at a common emitter for both NPN and PNP:

schematic

simulate this circuit – Schematic created using CircuitLab

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Take a result from NPN transistor analysis, e.g.: for a NPN transistor to be in forward-active mode, we must have:

$$V_{BE} > 0$$

$$V_{CB} > 0$$

To get the correct result for PNP transistors, simply (1) reverse the polarity of all voltage variables, i.e., \$V_{CE} \rightarrow V_{EC}\$ and (2) reverse the direction of all current variables.

Thus, for a PNP transistor to be in forward-active mode, we must have

$$V_{EB} > 0$$

$$V_{BC} > 0$$

Since \$V_{EC} = V_{EB} + V_{BC}\$, we have that, for a PNP transistor, \$V_{EC} > 0\$ when the transistor is active.

Or, in other words, \$V_{CE} < 0\$ for a PNP transistor does not imply it is in saturation.

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