Page 40 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Fig 3

Fig 4

Both NPN and PNP transistors are equally useful in electronic circuits. However, NPN transistors are preferred
for the reason that NPN has higher switching speed compared to PNP.
Operation of NPN transistor : During the normal operation of the transistor for amplifications the emitter base
junction must be forward-biased, and the base collector junction must be reverse-biased, as in Fig 5.

If VEB is greater than the barrier potential (0.3 V for germanium and 0.7 V for silicon), the electrons in the emitter
are repelled by the negative polarity of VEB and sent to the base. After filling a few holes in the base, these
electrons can flow in either of the two directions. A few of the electrons are attracted to the positive terminal of
VEB, producing base current IB. Many electrons in the base and collector are attracted by the high potential of
VCB, producing collector current Ic. Emitter current IE is equal to base and collector currents.
IE = IB + Ic
Working of PNP transistor: For proper operation of a PNP transistors as amplifier the base emitter junction must
be forward-biased and the collector-base junction must be reverse-biased as in Fig 6.
Holes which are the majority carries are injected from the emitter into the base region. By the reverse biasing
of the base-collection junction, the collector region is made negative with respect to the base, and hence holes,
which carry a positive charge, penetrate into to base and flow across the collector junction and flow into the
external applied voltage.

Method of identifying PNP and NPN transistors : Whether a transistor is PNP or NPN can be found with the
help of transistor data book.
In the absence of data the following procedure may be adopted to identify the type of transistor whether it is PNP
or NPN.
PNP identification : To identify the type of transistor first, make sure which is the positive lead and which is the
negative lead from the ohmmeter. If necessary, take of the back for the instrument and check the polarity of the
battery against the lead connections (positive to positive, negative to negative).
To test the transistor for its type:
1 Hook the positive lead from the ohmmeter to the base of the transistor. Fig 7
2 Connect the negative lead from the ohmmeter first to one transistor lead, then to the other.

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CITS : Power - Electrician & Wireman - Lesson 60-69