Master the Basics: PNP Bipolar Junction Transistors

A PNP transistor is the counterpart to the more commonly used NPN transistor. Both function similarly as current-controlled switches or amplifiers, but with reversed current and voltage polarities. Understanding how a PNP works becomes easier when you compare it side-by-side with an NPN.

Structure and Current Direction

While an NPN transistor has a P-type layer sandwiched between two N-type layers, a PNP transistor has N-type material between two P-type layers. The three terminals of a PNP transistor are:

• Emitter (E) – current flows into this terminal

• Base (B) – current flows out of the base

• Collector (C) – current flows out of this terminal

In contrast to NPN transistors, which rely on electrons as the primary carriers, PNP transistors rely on holes—the absence of electrons—which are treated as positive charge carriers.

Schematic Symbol and Mnemonics

The PNP transistor symbol looks like the NPN symbol but with the arrow pointing into the emitter, indicating current entering the transistor.

Mnemonic: Use “Not Pointing iN” to remember that an NPN transistor’s arrow points out. If that doesn’t apply, it’s a PNP.

Voltage Relationships

In a PNP transistor:

• The emitter voltage (VE) is higher than both base (VB) and collector (VC).

• The voltage from base to emitter, VBE, must be around –0.7V for the transistor to turn on.

This is the opposite of NPNs, where the base must be 0.7V higher than the emitter to conduct.

Operating Modes

Cutoff Mode (OFF):

• VBE > –0.7V

• Base-emitter junction is not forward biased

• No current flows; transistor behaves like an open circuit

Active Mode (Amplification):

• VBE ≈ –0.7V

• Base-emitter junction is forward biased

• Collector current (IC) is controlled by base current (IB):

  IC = β × IB

• Emitter current IE = IC + IB, per Kirchhoff’s Law

Saturation Mode (Fully ON):

• Base current exceeds required limit for linear amplification

• Transistor acts like a closed switch between emitter and collector

• Small voltage drop: VCE ≈ –0.2V

• VB < VC < VE

Comparing with NPN Transistors

• NPN current flows collector → emitter; PNP current flows emitter → collector

• NPN uses electrons; PNP uses holes

• NPN turns on with positive VBE; PNP turns on with negative VBE

Voltage and current conditions in a PNP transistor are simply the mirror image of those in an NPN. You can often convert an NPN equation into its PNP counterpart by reversing signs and inequality directions.

Key Takeways

PNP transistors operate much like NPN transistors but with reversed polarities. By understanding these differences and comparing the two side-by-side, you can design flexible circuits using either type. Whether switching loads or amplifying signals, PNP transistors offer complementary functionality to their NPN counterparts.