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Transistors
Transistors are
threeterminal devices
. Essentially, they are resistors (so, devices with two terminals, say ‘1’
and ‘2’) whose resistance can be varied over a large range of values by means of a third terminal (say ‘3’), as
shown in the Fgure below.
1
2
3
R controlled by V
3
0
2
4
6
8
10
0
2
4
6
8
10
V
3
= 1 V
V
3
= 2 V
V
3
= 3 V
V
3
= 4 V
I
V
2
– V
1
While in the right frame of this Fgure we have shown the linear characteristics of a perfect resistor, usually
transistor have
non
linear characteristics, as seen in the right frame of the Fgure below.
The usefullness of transistors lies in two possible functions they can play:
Amplifers
and
switches
. To see how this
works, lets consider the simple circuit in the Fgure below. In a Felde±ect transistor terminals ‘1’, ‘2’, and ‘3’ are
called
source
,
drain
, and
gate
, respectively. In a bipolar junction transistor they are known as
emitter
,
collector
,
and
base
contacts.
ECE344 ²all 2009
150
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ext
R
i
D
=f(v
D
)
v
D
+
–
I
D
V
D
V
ext
/R
V
ext
i
D
v
D
A twoterminal device is assumed to have nonlinear characteristics, that is, the current
i
d
fowing acorss the
device varies with the voltage
v
d
applied to the two terminals oF the device in some complicated way,
i
D
=
f
(
v
D
)
,
as we see at right above. IF we place the device in the circuit shown in the ±gure, in series with a resistor
R
and a
battery supplying a voltage
V
ext
, we can write the equation expressing the toltal voltage drop in the ‘loop’:
V
ext
=
Ri
D
+
v
D
.
(233)
This equation is not su²cient by itselF to determine the actual current
I
D
fowing in the circuit, because we have
two unknowns,
i
D
and
v
D
and only one equation. So, we must supplement Eq. (233) with the equation describing
the characteristics oF our nonlinear device,
i
D
=
f
(
v
D
)
. Now we have two equations in two unknowns. It is
instructive to solve the problem graphically: The right Frame oF the ±gure above shows the
i
D

v
D
characteristics oF
the device crossing the ‘load line’, that is, the
i
D

v
D
straight line corresponding to the circuit equation, Eq. (233).
The intersection oF the two curves clearly corresponds to the solutions,
I
D
and
V
D
, giving us the current fowing
in the circuit and the voltage drop across the nonlinear device.
Let’s now consider a transistor in the same con±guration, but with an extra battery supplying voltage to the
ECE344 ³all 2009
151
third terminal (labeled
G
for
gate
, word whose meaning we shall see later) which controlls the resistance (or, more
generally, the characteristics) of the device. This situation – shown in the Fgure below – will be similar, the main
di±erence being that now we have a full set of characteristics,
i
D
=
f
(
v
D
,v
G
)
, parameterized by the ‘gate
bias’
v
G
. ²or a given voltage
v
G
(=2.0 V in the example shown in the Fgure), we Fnd the current and voltage
across the transistor,
I
D
and
V
D
, by looking at the intersection between the load line and the device characteristics.
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This note was uploaded on 09/13/2010 for the course ECE ECE344 taught by Professor Polizinni during the Spring '10 term at University of Massachusetts Boston.
 Spring '10
 Polizinni
 Transistor

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