Page 285 - Mechatronics with Experiments

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JWST499-c05
JWST499-Cetinkunt
ELECTRONIC COMPONENTS FOR MECHATRONIC SYSTEMS 271
if (u >= x)
xdot = (1/(R*C))*(u - (1+(R/R_L)) * x) ;
else
xdot = -(1/(R_L*C))*x ;
end

5.5.3 Transistors

A transistor can function in two ways:
1. as an ON/OFF switch,
2. as a proportional amplifier.
It is an electronic switch (solid-state switch) that can be opened or closed completely or
partially. Another way of looking at a transistor is that it is a variable resistor where the
resistor value is controlled by the gate current. It is an active element which is used to
modulate (control) the flow of power from source to load. A transistor is a three-terminal
device made of two junctions of p-type and n-type semiconductors. If the junctions are
made of n-p-n order, it is called npn-type transistor. If the junctions are made of p-n-p
order, it is called pnp-type transistor. The difference between them in a circuit application
is that the supply connection polarity is opposite, hence the direction of current is opposite.
The three terminals are collector (C), emitter (E), and base (B) (Figures 5.13 and 5.15).
One of the important features of input and output relations of a transistor is that the input
strongly affects the output. However, the output variations (i.e., due to load changes) do
not affect the input. Therefore, the base of a transistor is always part of the input circuit
which controls the output circuit. Unlike passive components such as resistors, capacitors,
and inductors, the transistor is an active device. It requires a power supply to operate and
can increase the power between input and output. The input circuit acts as a control circuit
which controls a much larger power level in the output circuit using a much smaller power
level at the input circuit. The transistor is the electronic analogy of a hydraulic valve.
The main types of transistors are discussed below. Bipolar junction transistors
(BJTs) are the most common transistors. Metal oxide semiconductor field effect transistors
(MOSFETs) require smaller gate current, have better efficiency, and have higher switching
frequency. Insulated gate bipolar transistors (IGBTs) are a more recent transistor type which
attempts to combine the advantages of BJTs and MOSFETs. A transistor performance is
characterized by the maximum base current, current between the two main terminals
(collector-emitter, drain-source), forward bias voltage, reverse breakdown voltage, max-
imum switching frequency, forward current gain, reverse voltage gain, input impedance,
and output impedance.

Bipolar Junction Transistors (BJTs) There are two types of BJTs: npn-type
and pnp-type, each type being a three-terminal device (Figure 5.13a). The input–output
relationship between collector (C) and emmitor (E) terminals is controlled by the base (B)
signal. A BJT can be used as an electronic switch (ON/OFF) or a proportional amplifier
(output current i is proportional to input current i ). The voltage and current relationship
C B
across the C and E terminals of the transistor (V CE CE ) is a function of the base current.
, i
There are three main design parameters of interest: current gain (beta), voltage drop between
base and emitter when the transistor is conducting (V BE = V ), and the minimum voltage
FB
drop across the collector and emitter when the transistor is saturated or fully ON (V CE =
V SAT ). Note that i = i + i always.
E
B
C