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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