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Lecture 1: Introduction to electronic analog circuits 36113661
1
THE AIM OF THE COURSE
Lumped linear timeinvariant (LLTI) electric circuits do not
provide any solution to the following five tasks (see Fig. 1),
which are very important in Electrical Engineering:
1) It is impossible to control the circuit transfer function
(
gain
) by an electrical signal, either voltage or current.
2) It is impossible to implement a circuit with a dc gain
greater than one.
3) It is impossible to implement a circuit with a power
gain greater than one.
4) It is impossible to implement a current source.
5) It is impossible to implement an oscillator (circuit
generating a periodic signal), for example, a sinewave
oscillator.
The aim of the course is to solve all the above tasks by using
electronic devices: diodes and transistors. To develop and
study electronic circuits, we start from elementary circuits,
analyze them, and then improve if there is a need.
1. ELEMENTARY ELECTRONIC CIRCUITS WITH A DIODE
Our main aim here is to build a circuit with a gain (not
necessarily greater than one) that can be controlled by an
electrical signal, either voltage or current. Namely we would
like to build a voltagecontrolled voltage divider and a current
controlled current divider (homework).
To reach this goal, we first develop physical, mathematical,
and finally a graphical model of the diode. Based on the
graphical model, we find equivalent
electric
circuits to replace
a diode in an
electronic
circuit. This will allow us to analyze
singlediode electronic circuit by applying electric circuit
theory.
1.1.
Diode: symbol, physical structure, analytical model
and graphical characteristic
The symbol of the diode and its physical structure are given
in Fig. 2. To develop a mathematical model of the diode we
have to describe the dependence of the diode current,
i
D
, on
the diode voltage,
v
D
.
Assuming that the
n
region is much more heavily doped that
the
p
region,
n
po
>>
p
no
, we neglect the diode current due to the
holes and consider only that due to the electrons. Neglecting
the small effect of the weak electric field within the
p
region
on the electrons, which are the minor charge carriers in this
region, we conclude that the diode current is exclusively due to
the diffusion current of electrons:
A = f
(
I,V
)
A
DC
>
1
A
P
>
1
I
v
s
Fig. 1. Circuits that
cannot
be implemented by using LLTI components only:
resistors, capacitors, and inductances.
T
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 Fall '11
 Prof.EugenePaperno

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