Question about bipolar Junction Transistors

What's the difference between the characteristics of a PNP and NPN transistor in the Common base, emitter, and collector configuration?

by lanzcabz
December 20, 2020

Most often, we use a PNP when the transistor is between the power source, VCC, and the "load" which is said to be on "the low side", since the load is between the "switch" (the PNP) and the ground.

For a NPN, the load is on the "high side": the load is between the power source, VCC, and the transistor, while the transistor is between the load and the ground.

If you consider that, for the forward active mode of a NPN, VBE >= VBE_ON, and VCE >= VBE, putting SOME load on the low side means that VE increases, and so, VB has to increase too. While we generally have VB quite low, in voltage, that is counter productive to put that load on the low side, with a NPN.

Similar considerations would lead to why with a PNP, the load is generally on the low side and not so often a (large) load is used on the high side for a PNP.

Note: for a NPN, VBE >= VBE_ON is required to have the NPN in a NOT-BLOCKING mode while VCE >= VBE is required to have the NPN in ACTIVE mode, not in saturation. If you use the NPN as a switch, you may have a benefit to drop that last condition in order to decrease VCE, leading in less power lost (to be dissipated) through the transistor.

by vanderghast
December 20, 2020

Note that the rule to use an NPN with a "high load" is not an absolute. It is mostly true for a switch where the command comes from a micro-controler, as example.

But a good example where that rule is not to be followed is given by the emiter-follower (or common collector if you prefer).

by vanderghast
December 22, 2020

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