Analogue Electronic Circuits

Analogue Electronic Circuits 1
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Instructor
Analogue and digital electronics
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Analogue Electronic Circuits 2
Introduction
Electronic devices over the recent past have been extensively applied in signal amplification as well
as signal rectification applications. It is very fundamental for one to understand in-depth
characteristics of commonly used electronic semiconductor devices like transistors and diodes for
appropriate applications for the same. This report dealt with theoretical explanation of common
characteristics of NPN bipolar transistor (BJT) and a diode and their applications. Using Multisim
software, theoretical knowledge have been enhanced by simulation and analysis. Experimental data
were recorded and represented in graphical means.
Diodes
A diode is a solid state electrical device structurally made of a P-N junction that permits
unidirectional flow of current. The P-N type diode is made of two materials such that in the N-region,
electrons are the majority charge carriers while in the P-region, the majority charge carriers are holes.
Holes are regions are positively charged segments whose electrons were depleted (Eric Gregersen
2019). The two regions are separated by an interface commonly known as depletion region. The
depletion region is formed when intermediate electrons at the junction diffuse from N-region into a
portion of P-region to fill holes which thereafter stops further electrons from entering P-region. This
process occurs during doping. In doping process, the negative side of the diode also known as cathode
is made of the materials that easily displaces excess electrons while the positive side, referred to as
anode, is made of the materials that easily accepts electrons due to holes (J.M.K.C. Donev et al.
2018).
There are two modes of diode operations namely; forward and reverse bias. In forward bias, a diode
will let current flow through it. On the contrary, the current will not flow in reverse bias within
diode’s capability. An ideal diode should allow current to flow through it freely without offering any
resistance. Under an ideal condition therefore, there should be no voltage drop across the diode when
operating in forward bias mode. In the same light, an ideal diode should offer infinite resistance to
flow of current when operated in reverse bias mode. This is not the case with the practical diode
because it possess a small resistive characteristics when operated in forward bias mode, thereby
leading to a fractional voltage drop across it (All About Circuits 2018). Theoretically, voltage drop
across the diode is about 0.7V for silicon diode.
Forward biasing of diodes
Operation of the diode in forward biasing mode is effected when the anode of the battery is connected
to the anode (P-region terminal) of the diode and cathode of the battery is connected with the N-
region (cathode). For the practical case, the diode will only conduct if the biasing voltage supersedes

Analogue Electronic Circuits 3
the potential barrier, which is about 0.7V. This is the smallest threshold potential different needed to
overcome potential barrier of the depletion region. Below 0.7, the diode behaves like an open circuit.
The depletion layer shrinks by shifting majority charge carriers of either regions into the junction.
Advancing of electrons and holes into the depletion layer is caused by repulsion of charge carriers
from the anode and cathode terminals of the biasing voltage source. With enough force provided by
the voltage source, both charge carriers sooner overcomes the depletion layer, combine in an endless
process, completing the circuit for continuous current flow (J.M.K.C. Donev et al. 2018).
Reverse biasing of diodes
In reverse bias configuration, the P-region of the diode is supplied with the cathode of the biasing
voltage source and the N-region is joined with the anode of the voltage source. Mutual attraction of
holes, which are positively charged carriers, and electrons from the voltage source pulls away holes
from the depletion layer. Similarly, the attraction between electrons, majority charge carriers, in the
N-region and positive of the voltage source withdraws N-region from the depletion layer. The end
results widen the depletion layer. In addition, the potential difference between the P-region and N-
region increases in magnitude until an equilibrium is achieved with voltage magnitude of the source.
The increased depletion layer inhibits electron flow through the diode (J.M.K.C. Donev et al. 2018).
Forward and breakdown voltages of diodes.
As aforementioned, in forward bias, the diode will only conduct if the voltage source overcomes the
minimum threshold voltage across the depletion layer. For silicon diode, the threshold is 0.7V.
In reverse bias, the diode do conduct infinitesimal leakage current when reverse bias voltage is
supplied. Increasing the reverse bias voltage leads to stronger growth of electric field which pulls
more electrons into conducting. Finally, this voltage reaches a point, known as breakdown voltage,
whereby a diode undergoes a complete electronic breakdown allowing irresistible flow of reverse
current (J.M.K.C. Donev et al. 2018).
Diode rectification.
Rectification in electric context explicitly means converting an alternating current signals into a direct
current signal flow. Diodes have been expansively used semiconductors in the application of
rectification due to their ability to block current flow in reverse direction (Kip Ingram 2018).
Half wave rectification
Half wave rectification is achieved when either positive or negative half of the current sinusoidal
signals is transmitted while the opposite half is truncated. This type of rectification is only 50%
efficient since power transferred is a half of the original power. The configuration of this type of

Analogue Electronic Circuits 4
rectification is achieved by use of a single diode connected in a single phase supply line. The output, a
rectified version of the input voltage is determined by the equations below (Dmercer 2017).
V dc = 1
π ∫
0
π
V peak sin ( tdt ) = V p
π
V p=V rms √ 2
Full wave rectification
A full wave rectification converts bidirectional signals into unidirectional signals taking into account
both positive and negative magnitude of the wave. With regard to electric signals, diodes are arranged
in a bridge rectifier to convert AC signals into a DC signal of same polarity.
The average output voltage is found by;
V averout
= 1
T ∫
0
T
V msin (ωt )dt= 2 V m
π
Rectifier output smoothing.
The output of rectified signals is married with ripples making the DC output to be unsteady.
Smoothening of the DC output can be achieved by connecting a capacitor across output terminals. In
operation, the capacitor charged during rising ripple and discharges during falling ripple, therein
reducing ripples’ amplitude. The remaining ripples depends on the ability of the load to discharge
capacitor between the peaks of signal waveforms (Dmercer 2017).
Bipolar Junction Transistor input and output characteristics.
A bipolar junction transistor (BJT) consists of three terminals joined to three semiconductor regions
that are doped. There are two types of BJTs namely; NPN transistor and PNP transistor. An NPN
transistor is formed by sandwiching P-type, slightly doped, base between richly doped N-type
collector and emitter while a PNP is formed by sandwiching a thin slightly doped N-type base
between abundantly doped P-type emitter and collector (Ruye Wang 2019).
By application of small, the BJT can be modified to function as either an insulator or as a conductor.
Such abilities enables BJTs to be used as digital electronic switches or for amplification in the
analogue electronics. BJTs operates in three distinct regions namely; active region ( I c=βI b),
saturation region ( I c=I c saturation), and cut-off region ( I c=0). The BTJ in active regions can be
used as an amplifier while in saturation and cut-off regions can be used as a switch (Electronics
Tutorials).
Current relationships of a BJT are as shown by the equation below.