328 Chapter 12
The outstanding characteristics of the IGT are its
extremely high input impedance, running to 10^15 :.
IGTs have three elements but four connections—the
gate, the drain, the source, and an n-type substrate, into
which two identical p-type silicon regions have been
diffused. The source and drain terminals are taken from
these two p regions, which form a capacitance between
the n substrate and the silicon-dioxide insulator and the
metallic gate terminals. A cross-sectional view of the
internal construction appears in Fig. 12-21D, with a
basic circuit shown in Fig. 12-21E. Because of the high
input impedance, the IGT can easily be damaged by
static charges. Strict adherence to the instructions of the
manufacturer must be followed since the device can be
damaged even before putting it into use.
IGTs are used in electrometers, logic circuits, and
ultrasensitive electronic instruments. They should not
be confused with the conventional FET used in audio
equipment.
Transistor Equivalent Circuits, Current Flow, and
Polarity. Transistors may be considered to be a T
configuration active network, as shown in Fig. 12-22.
The current flow, phase, and impedances of the npn
and pnp transistors are shown in Fig. 12-23 for the three
basic configurations, common emitter, common base
and common collector. Note phase reversal only takes
place in the common-emitter configuration.
The input resistance for the common-collector and
common-base configuration increases with an increase
of the load resistance RL. For the common emitter, the
input resistance decreases as the load resistance is
increased; therefore, changes of input or output resis-
tance are reflected from one to the other.
Fig. 12-24 shows the signal-voltage polarities of a
p-channel field-effect transistor. Note the similarity to
tube characteristics.
Voltage, power, and current gains for a typical tran-
sistor using a common-emitter configuration are shown
in Fig. 12-25. The current gain decreases as the load
resistance is increased, and the voltage gain increases as
the load resistance is increased. Maximum power gain
occurs when the load resistance is approximately
40,000: , and it may exceed unity.
For the common-collector connection, the current
gain decreases as the load resistance is increased and the
voltage gain increases as the load resistance is
increased, but it never exceeds unity. Curves such as
these help the designer to select a set of conditions for a
specific result.
The power gain varies as the ratio of the input to
output impedance and may be calculated with the
equationFigure 12-22. Equivalent circuits for transistors.
rererb rerbrbrcrc rcC. Common collector.A. Common base. B. Common emitter.Figure 12-23. Current, polarity. and impedance
relationships.lClElBA. Current flow in a pnp
transistor.B. Current flow in an npn
transistor.C. Polarity and impedances in a common-base circuit.D. Polarity and impedances in a
common-collector circuit.E. Polarity and impedances in a
common-emitter circuit.lClBlEVi VoVoViViVo90 7 40 k^7
+^ +100 k 7
1 k 7++40 k 7700 7++