Nuts & Volts - July 2005

Q&A

Semiconductor Sex Explained

Q. Can an NPN transistor be wired as a PNP transistor? Is it a matter of reversing connections?

— Leonard Mary Thomas

A. NPN and PNP transistors are interchangeable if you remember one simple rule: A bipolar transistor is essentially two back-to-back diodes with the base being the common connection. For a transistor to work, one diode is forward biased and the other is reverse biased. Let’s take the common-emitter amplifier in Figure 2, for example. On the left is an NPN ( negative-pos-itive-negative) transistor and on the right is a PNP ( positive-negative-posi-tive) transistor. Notice that both circuits are identical — except for one thing. The polarity of the power supply is reversed.

In the NPN configuration, the emitter (the lead that looks like the arrow of a diode, labeled E) goes to negative (ground). The base (B) goes to +V via the Rb resistor. This forward-biases the base-emitter diode, which exhibits the characteristic 0.7 volts voltage drop. The collector (C), on the other hand, goes to +V — in effect, reverse biasing that diode.

The breakover voltage of this diode is the VCE parameter listed on the spec sheet, and varies from one transistor type to another. The current through the collector-emitter path is controlled by the current flowing through the base-emitter junction. The amount of influence is called the gain of the transistor, or hFE.

Substituting a PNP in the circuit reverses the current flow through the base-emitter diode and the voltage on the collector. Bottom line, most small-signal amplifiers will work equally well if you replace an NPN with a PNP and reverse the power supply polarity. And that means if you have mixed sexes, each and every transistor has to have a sex change. Please note, I said most — not all — amplifiers will work with this exchange. (Electron and hole mobility are not equal, especially at higher frequencies.)

JULY 2005

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