Avionics News May 2019 - 53

A bipolar silicon transistor consists of three
segments and could be thought of as two
diodes tied together at either the cathode or
anode end. The left side of Figure 1 illustrates
an NPN transistor. The emitter consists of
negatively doped silicon. The base, which is
very thin, consists of positively doped silicon.
The collector, like the emitter, is constructed
of negatively doped silicon. The intersection
between the emitter and base, as well as the
base to collector, are called junctions. Like
diodes, voltages applied to the transistor are
called bias. A PNP transistor is doped in the opposite manner from an NPN transistor, and current flows in the opposite
direction. Engineers will choose one type or another based on
the design of the circuit as a whole. The bulk of the discussion here will focus on NPN transistors. The only difference
when discussing PNP transistors is polarity, and current flow
is reversed in comparison to NPN devices. All other principles are the same.
In an NPN transistor, when the emitter to base junction is
forward biased (the base positive in relation to the emitter),
then current will begin to flow. Like a diode, the bias voltage
forces electrons and holes away from the terminals toward
the junction, decreasing the size of the depletion zone. For
more detail on depletion zones, forward and reverse bias, refer to the April 2019 issue of Avionics News. With the depletion zone size reduced, current jumps the barrier and flows.
Since the base is thin, electrons find it easy to jump the base
to collector junction, as well. More current will flow from
emitter to collector than from emitter to base.
Since a small amount of current in the base circuit will
cause a large amount of current in the collector circuit,
transistors are known to have gain. There are two ways to
express transistor gain. Alpha (α) is a ratio of emitter current
to collector current (Ic/Ie) and will always be less than 1. The
other type of gain is Beta (β). Beta gain is the ratio of collector current to base current (Ic/Ib) and can be as high as 200
for general purpose transistors. Regarding general purpose
transistors, I've experienced large differences between the
Beta claimed by the transistor manufacturer and the Beta
measured. This is why original equipment manufacturer parts
are important for avionics.
Like a silicon diode, the emitter to base junction in a sili-

Photos courtesy of Wikimedia Commons

Various types and
sizes of transistors.

con transistor will exhibit a 0.6 voltage drop when forward
biased. With the base junction forward biased, the transistor
is considered to be on. As a result, individual transistors can
be used as switches. Teachers will make comparisons of transistors to solenoids and relays. A solenoid or relay activation
coil draws a relatively small amount of current and can be
controlled using a small switch connected with small gauge
wires. The main terminals of a solenoid are designed to carry
heavy current loads. As a result, a small switch in an instrument panel can control a large amount of current elsewhere.
A transistor can behave in the same way with a small amount
of current through the base controlling a larger amount of
current through the collector. In the past, transmit receive
switching within avionics used relays. Today, this switching
is accomplished using transistors.
The transistor has a major advantage over a relay or solenoid in that the transistor doesn't turn on suddenly. Instead,
transistors turn on gradually, in a linear fashion, until they are
fully on. It's this characteristic that makes transistors usable
as remote-control variable resistors.
This is a good place to define a couple of terms used with
transistors. A transistor that is off (no current flow) is in cutoff
mode. When a transistor is fully on (maximum current flow),
it is considered to be in saturation mode. As described above,
the transition from cutoff to saturation is linear.
Figure 2 shows a transistor used as a light dimmer. Similar
circuits are used in several small aircraft. The potentiometer
will be mounted on the instrument panel, and the transistor
will be mounted behind the instrument panel on a heat sink.
If the potentiometer is set at the bottom of the symbol with
minimum resistance to ground, then the transistor will be in
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Avionics News May 2019

Table of Contents for the Digital Edition of Avionics News May 2019

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Avionics News May 2019 - Intro
Avionics News May 2019 - No label
Avionics News May 2019 - Cover2
Avionics News May 2019 - 1
Avionics News May 2019 - 2
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Avionics News May 2019 - Cover3
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