Make Electronics

Experiment 10: Transistor Switching

76 Chapter 2

essentIAls

All about NPN and PNP transistors

A transistor is a semiconductor, meaning that sometimes it

conducts electricity, and sometimes it doesn’t. Its internal

resistance varies, depending on the power that you apply to

its base.

NPN and PNP transistors are bipolar semiconductors. They

contain two slightly different variants of silicon, and con-

duct using both polarities of carriers—holes and electrons.

The NPN type is a sandwich with P-type silicon in the mid-

dle, and the PNP type is a sandwich with N-type silicon in

the middle. If you want to know more about this terminol-

ogy, and the behavior of electrons when they try to cross an

NP junction or a PN junction, you’ll have to read a separate

source on this subject. It’s too technical for this book. All you

need to remember is:

• All bipolar transistors have three connections: Collec-
  tor, Base, and Emitter, abbreviated as C, B, and E on the
  manufacturer’s data sheet, which will identify the pins
  for you.


• NPN transistors are activated by positive voltage on the
  base relative to the emitter.


• PNP transistors are activated by negative voltage on the
  base relative to the emitter.
  In their passive state, both types block the flow of electricity
  between the collector and emitter, just like an SPST relay in
  which the contacts are normally open. (Actually a transistor
  allows a tiny bit of current known as “leakage.”)

You can think of a bipolar transistor as if it contains a little

button inside, as shown in Figures 2-89 and 2-90. When the

button is pressed, it allows a large current to flow. To press

the button, you inject a much smaller current into the base

by applying a small voltage to the base. In an NPN transis-

tor, the control voltage is positive. In a PNP transistor, the

control voltage is negative.

NPN transistor  basics

• To start the flow of current from collector to emitter,
  apply a relatively positive voltage to the base.


• In the schematic symbol, the arrow points from base to
  emitter and shows the direction of positive current.


• The base must be at least 0.6 volts “more positive” than
  the emitter, to start the flow.


• The collector must be “more positive” than the emitter.

PNP transistor  basics

• To start the flow of current from emitter to collector,
  apply a relatively negative voltage to the base.


• In the schematic symbol, the arrow points from emitter
  to base and shows the direction of positive current.


• The base must be at least 0.6 volts “more negative” than
  the emitter, to start the flow.


• The emitter must be “more positive” than the collector.

B

C

E

Figure 2-89. You can think of a bipolar transistor as if it contains

a button that can connect the collector and the emitter. In an

NPN transistor, a small positive potential presses the button.

B

C

E

Figure 2-90. In a PNP transistor, a small negative potential has

the same effect. The arrows point in the direction of “positive

current flow.”