UNIT—5
ELECTRONIC COMPONENETS AND CIRCUITS
INTRODUCTION TO SEMICONDUCTORS--A semiconductor material has an electrical conductivity value falling between that of a
conductor, such as copper, and an insulator, such as glass. Semiconductors are the foundation of
modern electronics. Semiconducting materials exist in two types - elemental materials and
compound materials. [1] The modern understanding of the properties of a semiconductor relies on
quantum physics to explain the movement of electrons and holes in a crystal lattice. [2] The
unique arrangement of the crystal lattice makes silicon and germanium the most commonly used
elements in the preparation of semiconducting materials. An increased knowledge of
semiconductor materials and fabrication processes has made possible continuing increases in the
complexity and speed of microprocessors and memory devices. Some of the information on this
page may be outdated within a year, due to the fact that new discoveries are made in the field
frequently. [2]
The electrical conductivity of a semiconductor material increases with increasing temperature,
which is behaviour opposite to that of a metal. Semiconductor devices can display a range of
useful properties such as passing current more easily in one direction than the other, showing
variable resistance, and sensitivity to light or heat. Because the electrical properties of a
semiconductor material can be modified by controlled addition of impurities, or by the
application of electrical fields or light, devices made from semiconductors can be used for
amplification, switching, and energy conversion.
Current conduction in a semiconductor occurs through the movement of free electrons and
"holes", collectively known as charge carriers. Adding impurity atoms to a semiconducting
material, known as "doping", greatly increases the number of charge carriers within it. When a
doped semiconductor contains mostly free holes it is called "p-type", and when it contains
mostly free electrons it is known as "n-type". The semiconductor materials used in electronic
devices are doped under precise conditions to control the location and concentration of p- and ntype dopants. A single semiconductor crystal can have many p- and n-type regions; the p–n
junctions between these regions are responsible for the useful electronic behaviour.
DIODES –
P-N junction diode is the most fundamental and the simplest electronics device. When one side
of an intrinsic semiconductor is doped with acceptor i.e, one side is made p-type by doping with
n-type material, a p-n junction diode is formed. This is a two terminal device. It appeared in
1950’s.
P-N junction can be step graded or linearly graded. In step graded the concentration of dopants
both, in n – side and in p – side are constant up to the junction. But in linearly graded junction,
the doping concentration varies almost linearly with the distance from the junction.
When the P-N diode is in unbiased condition that is no voltage is applied across it, electrons will
defuse through the junction to p – side and holes will defuse through the junction to n – side and
they combine with each other. Thus the acceptor atom near the p – side and donor atom near n –
side are left unutilized. An electron field is generated by these uncovered charges. This opposes
further diffusion of carriers. So, no movement of region is known as space charge or depletion
region.
If, we apply forwards bias to the p-n junction diode. That means if positive side of the battery is
connected to the p – side, then the depletion regions width decreases and carriers flow across the
junction. If the bias is reversed the depletion width increases and no charge can flow across the
junction.
P-N Junction Diode Characteristics
Let’s a voltage V is applied across a p-n junction and total current I, flows through the junction.
It is given as.
I = IS[exp(eV/ɳKBT) – 1]
Here, IS = reverse saturation current
e = charge of electron
ɳ = emission co-efficient
KB = Boltzmann constant
T = temperature
The current voltage characteristics plot is given below.
The current voltage characteristics
When V is positive the junction is forward biased and when V is negative, the junction is
reversing biased. When V is negative and less than VTH, the current is very small. But when V
exceeds VTH, the current suddenly becomes very high. The voltage VTH is known as threshold or
cut in voltage. For Silicon diode VTH = 0.6 V.
At a reverse voltage corresponding to the point P, there is abrupt increment in reverse current.
The PQ portion of the characteristics is known as breakdown region.
P-N Junction Band Diagram
For an n-type semiconductor, the Fermi level EF lies near the conduction band edge. EC but for
an p – type semiconductor, EF lies near the valance band edge EV
Now, when a p-n junction is built, the Fermi energy EF attains a constant value. In this scenario
the p-sides conduction band edge. Similarly n–side valance band edge will be at higher level than
Ecn, n-sides conduction band edge of p – side. This energy difference is known as barrier energy.
The barrier energy is
EB = Ecp – Ecn = Evp – Evn
If we apply forward bias voltage V, across junction then the barrier energy decreases by an
amount of eV and if V is reverse bias is applied the barrier energy increases by eV.
P-N Junction Diode Equation
The p-n junction diode equation for an ideal diode is given below
I = IS[exp(eV/KBT) – 1]
Here, IS = reverse saturation current
e = charge of electron
KB = Boltzmann constant
T = temperature
For a normal p-n junction diode, the equation becomes
I = IS[exp(eV/ɳKBT) – 1]
Here, ɳ = emission co-efficient, which is a number between 1 and 2, which typically increases as
the current increases.
Circuit Symbol
Every diode has two terminals – connections on each end of the component – and those
terminals are polarized, meaning the two terminals are distinctly different. It’s important not to
mix the connections on a diode up. The positive end of a diode is called the anode, and the
negative end is called the cathode. Current can flow from the anode end to the cathode, but not
the other direction. If you forget which way current flows through a diode, try to remember the
mnemonic ACID: “anode current in diode” (also anode cathode is diode).
The circuit symbol of a standard diode is a triangle butting up against a line. As we’ll cover in
the later in this tutorial, there are a variety of diode types, but usually their circuit symbol will
look something like this
Current-Voltage Relationship
The most important diode characteristic is its current-voltage (i-v) relationship. This defines what
the current running through a component is, given what voltage is measured across it. Resistors,
for example, have a simple, linear i-v relationship…Ohm’s Law. The i-v curve of a diode,
though, is entirely non-linear. It looks something like this:
The current-voltage relationship of a diode. In order to exaggerate a few important points on the
plot, the scales in both the positive and negative halves are not equal.
Depending on the voltage applied across it, a diode will operate in one of three regions:
1. Forward bias: When the voltage across the diode is positive the diode is “on” and
current can run through. The voltage should be greater than the forward voltage (VF) in
order for the current to be anything significant.
2.
3. Reverse bias: This is the “off” mode of the diode, where the voltage is less than VF but
greater than -VBR. In this mode current flow is (mostly) blocked, and the diode is off. A
very small amount of current (on the order of nA) – called reverse saturation current – is
able to flow in reverse through the diode.
4.
5. Breakdown: When the voltage applied across the diode is very large and negative, lots of
current will be able to flow in the reverse direction, from cathode to anode.
Forward Voltage
In order to “turn on” and conduct current in the forward direction, a diode requires a certain
amount of positive voltage to be applied across it. The typical voltage required to turn the diode
on is called the forward voltage (VF). It might also be called either the cut-in voltage or onvoltage.
As we know from the i-v curve, the current through and voltage across a diode are
interdependent. More current means more voltage, less voltage means less current. Once the
voltage gets to about the forward voltage rating, though, large increases in current should still
only mean a very small increase in voltage. If a diode is fully conducting, it can usually be
assumed that the voltage across it is the forward voltage rating.
A specific diode’s VF depends on what semiconductor material it’s made out of. Typically, a
silicon diode will have a VF around 0.6-1V. A germanium-based diode might be lower, around
0.3V. The type of diode also has some importance in defining the forward voltage drop; lightemitting diodes can have a much larger VF, while Schottky diodes are designed specifically to
have a much lower-than-usual forward voltage.
Breakdown Voltage
If a large enough negative voltage is applied to the diode, it will give in and allow current to flow
in the reverse direction. This large negative voltage is called the breakdown voltage. Some
diodes are actually designed to operate in the breakdown region, but for most normal diodes it’s
not very healthy for them to be subjected to large negative voltages.
BIPOLAR JUNCTION TRANSISTORS (BJT) :A Bipolar Junction Transistor (a.k.a. a BJT or Bipolar Transistor) is an active semiconductor
device formed by two P-N junctions whose function is amplification of an electric current.
Bipolar transistors are made from 3 sections of semiconductor material (alternating P-type and
N-type), with 2 resulting P-N junctions. Schematically, a bipolar transistor can be thought of in
this fashion:
One P-N junction is between the emitter and the base; the other P-N junction is between the
collector and the base. Note that the emitter and collector are usually doped somewhat
differently, so they are rarely electrically interchangeable. While the terms "collector" and
"emitter" go back to vacuum tube days, the base derives its name from the first point-contact
transistors -- here the center connection also formed the mechanical base for the structure. In
modern practice, the base region is made as thin as possible to achieve reasonable levels of
current gain; it is often only about one millionth of a meter thick.
Bipolar transistors are classified as either NPN or PNP according to the arrangement of their Ntype and P-type materials. Their basic construction and chemical treatment is implied by their
names. So an NPN transistors is formed by introducing a thin region of P-type material between
two regions of N-type material.
On the other hand, a PNP transistor is formed by introducing a thin region of N-type material
between two regions of P-type material.
Since the majority and minority current carriers are different for N-type and P-type materials, it
stands to reason that the internal operation of the NPN and PNP transistors will also be different.
These two basic types of transistors along with their circuit symbols are shown here:
NPN PNP
Note that the two symbols are subtly different. The vertical line represents the base (B), the
angular line with the arrow on it represents the emitter (E), and the other angular line represents
the collector (C). The direction of the arrow on the emitter distinguishes (graphically) the NPN
from the PNP transistor. If the arrow points in, (Points iN) the transistor is a PNP. On the other
hand if the arrow points out, the transistor is an NPN (Not Pointing iN).
Bear in mind that the arrow always points in the direction of hole flow (current), or from the Ptype to N-type sections, no matter whether the P-type section is the emitter or base. On the other
hand, electron flow is always "against" the arrow, just like in the junction diode.
As a result, a PNP transistor is "triggered" when its base is pulled low; an NPN transistor is
"triggered" when its base is brought high.
Note that the bipolar transistor is a current-amplifying device, unlike the vacuum tube and the
field-effect transistor (FET), both of which depend upon voltage changes to operate. It is the
amount of current flowing in the base circuit that controls the amount of current flowing in the
collector circuit.
Modes of operation
Applied voltages
B-E junction B-C junction
Mode (NPN)
bias (NPN) bias (NPN)
E<B<C
Forward
Reverse
Forward-active
E<B>C
Forward
Forward
Saturation
E>B<C
Reverse
Reverse
Cut-off
E>B>C
Reverse
Forward
Reverse-active
Applied voltages
B-E junction B-C junction
Mode (PNP)
bias (PNP) bias (PNP)
E<B<C
Reverse
Forward
Reverse-active
E<B>C
Reverse
Reverse
Cut-off
E>B<C
Forward
Forward
Saturation
E>B>C
Forward
Reverse
Forward-active
The relationship between
,
and
.
Bipolar transistors have five distinct regions of operation, defined by BJT junction biases.





Forward-active (or simply, active): The base–emitter junction is forward biased and the
base–collector junction is reverse biased. Most bipolar transistors are designed to afford
the greatest common-emitter current gain, βF, in forward-active mode. If this is the case,
the collector–emitter current is approximately proportional to the base current, but many
times larger, for small base current variations.
Reverse-active (or inverse-active or inverted): By reversing the biasing conditions of
the forward-active region, a bipolar transistor goes into reverse-active mode. In this
mode, the emitter and collector regions switch roles. Because most BJTs are designed to
maximize current gain in forward-active mode, the βF in inverted mode is several times
smaller (2–3 times for the ordinary germanium transistor). This transistor mode is seldom
used, usually being considered only for failsafe conditions and some types of bipolar
logic. The reverse bias breakdown voltage to the base may be an order of magnitude
lower in this region.
Saturation: With both junctions forward-biased, a BJT is in saturation mode and
facilitates high current conduction from the emitter to the collector (or the other direction
in the case of NPN, with negatively charged carriers flowing from emitter to collector).
This mode corresponds to a logical "on", or a closed switch.
Cutoff: In cutoff, biasing conditions opposite of saturation (both junctions reverse
biased) are present. There is very little current, which corresponds to a logical "off", or an
open switch.
Avalanche breakdown region
The modes of operation can be described in terms of the applied voltages (this description
applies to NPN transistors; polarities are reversed for PNP transistors):




Forward-active: base higher than emitter, collector higher than base (in this mode the
collector current is proportional to base current by
).
Saturation: base higher than emitter, but collector is not higher than base.
Cut-Off: base lower than emitter, but collector is higher than base. It means the transistor
is not letting conventional current go through from collector to emitter.
Reverse-active: base lower than emitter, collector lower than base: reverse conventional
current goes through transistor.
In terms of junction biasing: ('reverse biased base–collector junction' means Vbc < 0 for NPN,
opposite for PNP)
Although these regions are well defined for sufficiently large applied voltage, they overlap
somewhat for small (less than a few hundred millivolts) biases. For example, in the typical
grounded-emitter configuration of an NPN BJT used as a pulldown switch in digital logic, the
"off" state never involves a reverse-biased junction because the base voltage never goes below
ground; nevertheless the forward bias is close enough to zero that essentially no current flows, so
this end of the forward active region can be regarded as the cutoff region.
Active-mode NPN transistors in circuits
Structure and use of NPN transistor. Arrow according to schematic.
The diagram shows a schematic representation of an NPN transistor connected to two voltage
sources. To make the transistor conduct appreciable current (on the order of 1 mA) from C to E,
VBE must be above a minimum value sometimes referred to as the cut-in voltage. The cut-in
voltage is usually about 650 mV for silicon BJTs at room temperature but can be different
depending on the type of transistor and its biasing. This applied voltage causes the lower P-N
junction to 'turn on', allowing a flow of electrons from the emitter into the base. In active mode,
the electric field existing between base and collector (caused by VCE) will cause the majority of
these electrons to cross the upper P-N junction into the collector to form the collector current IC.
The remainder of the electrons recombine with holes, the majority carriers in the base, making a
current through the base connection to form the base current, IB. As shown in the diagram, the
emitter current, IE, is the total transistor current, which is the sum of the other terminal currents,
(i.e., IE = IB + IC).
In the diagram, the arrows representing current point in the direction of conventional current –
the flow of electrons is in the opposite direction of the arrows because electrons carry negative
electric charge. In active mode, the ratio of the collector current to the base current is called the
DC current gain. This gain is usually 100 or more, but robust circuit designs do not depend on
the exact value (for example see op-amp). The value of this gain for DC signals is referred to as
, and the value of this gain for small signals is referred to as
. That is, when a small
change in the currents occurs, and sufficient time has passed for the new condition to reach a
steady state
is the ratio of the change in collector current to the change in base current. The
symbol is used for both
and
.[9]
The emitter current is related to
exponentially. At room temperature, an increase in
by
approximately 60 mV increases the emitter current by a factor of 10. Because the base current is
approximately proportional to the collector and emitter currents, they vary in the same way.
Active-mode PNP transistors in circuits
Structure and use of PNP transistor.
The diagram shows a schematic representation of a PNP transistor connected to two voltage
sources. To make the transistor conduct appreciable current (on the order of 1 mA) from E to C,
must be above a minimum value sometimes referred to as the cut-in voltage. The cut-in
voltage is usually about 650 mV for silicon BJTs at room temperature but can be different
depending on the type of transistor and its biasing. This applied voltage causes the upper P-N
junction to 'turn-on' allowing a flow of holes from the emitter into the base. In active mode, the
electric field existing between the emitter and the collector (caused by
) causes the majority
of these holes to cross the lower p-n junction into the collector to form the collector current
.
The remainder of the holes recombine with electrons, the majority carriers in the base, making a
current through the base connection to form the base current,
. As shown in the diagram, the
emitter current,
, is the total transistor current, which is the sum of the other terminal currents
(i.e., IE = IB + IC).
In the diagram, the arrows representing current point in the direction of conventional current –
the flow of holes is in the same direction of the arrows because holes carry positive electric
charge. In active mode, the ratio of the collector current to the base current is called the DC
current gain. This gain is usually 100 or more, but robust circuit designs do not depend on the
exact value. The value of this gain for DC signals is referred to as
, and the value of this gain
for AC signals is referred to as
. However, when there is no particular frequency range of
.
interest, the symbol is used
DC Biasing of BJT :- A bipolar junction transistor, (BJT) is very versatile. It can be used in
many ways, as an amplifier, a switch or an oscillator and many other uses too. Before an input
signal is applied its operating conditions need to be set. This is achieved with a suitable bias
circuit, some of which I will describe. A bias circuit allows the operating conditions of a
transistor to be defined, so that it will operate over a pre-determined range. This is normally
achieved by applying a small fixed dc voltage to the input terminals of a transistor.
Bias design can take a mathematical approach or can be simplified using transistor characteristic
curves. The characteristic curves predict the performance of a BJT. There are three curves, an
input characteristic curve, a transfer characteristic curve and an output characteristic curve. Of
these curves, the most useful for amplifier design is the output characteristics curve. The output
characteristic curves for a BJT are a graph displaying the output voltages and currents for
different input currents. The linear (straight) part of the curve needs is utilized for an amplifier or
oscillator. For use as a switch,a transistor is biased at the extremities of the graph, these
conditions are known as "cut-off" and "saturation".
Output Characteristic Curves
For each transistor configuration, common emitter, common base and emitter follower the output
curves are slightly different. A typical output characteristic for a BJT in common emitter mode
are shown below :-
After the initial bend, the curves approximate a straight line. The slope or gradient of each line
represents the output impedance, for a particular input base current. So what has all this got to do
with biasing ? Take, for example the middle curve. The collector emitter voltage is displayed up
to 20 volts. Let's assume that we have a single stage amplifier, working in common emitter
mode, and the supply voltage is 10 volts. The output terminal is the collector, the input is the
base, where do you set the bias conditions? The answer is anywhere on the flat part of the graph.
However, imagine the bias is set so that the collector voltage is 2 volts. What happens if the
output signal is 4 volts peak to peak ? Depending on whether the transistor used is a PNP or
NPN, then one half cycle will be amplified cleanly, the other cycle will approach the limits of the
power supply and will "clip". This is shown below :
The above diagram shows a 4 volt peak to peak waveform with clipping on the positive half
cycle. This is caused by setting the bias at a value other than half the supply voltage.
The lower diagram shows the same amplifier, but here the bias is set so that collector voltage is
half the value of the supply voltage. Hence, it is a good idea to set the bias for a single stage
amplifier to half the supply voltage, as this allows maximum output voltage swing in both
directions of an output waveform.
Input Characteristic Curves
Before describing the bias circuits, it is worthwhile looking at a typical input characteristic curve
for a small signal BJT, shown left. The input characteristics for a transistor in common emitter
mode is a plot of input base emitter voltage (x-axis) verses base current (y-axis). The graph is
drawn with both x and y axis slightly zoomed.
The base emitter voltage, Vbe, for a small signal transistor is typically quoted in many text books
as either 0.6 V or 0.7 V Both values are an approximation,and as can be seen from the graph the
value of Vbe varies with collector current, device type and temperature. With low base currents
of 50uA or less, taking Vbe as 0.6 volts is a reasonable approximation. For higher base currents,
and in switching circuits using Vbe as 0.7 V is a better approximation. In large power transistors,
Vbe can be even and often be as high as 0.8 or 0.9V.
Simple Bias Circuit
The simplest bias circuit is shown below. It consists only of a fixed bias resistor and load
resistor. The BJT is operating in common emitter mode. The dc current gain or beta, hFE is the
ratio of dc collector current divided by dc base current. The BJT is a BC107A. The values of Rb
and Rc can be determined by either mathematical approach or by using the output characteristic
curves for the BC107A.
Quiescent Point (Q-Point)
The point Vo in the diagram above is where the output signal would be taken. For simplicity,the
input signal and coupling capacitors have been omitted. For minimum distortion and clipping it
is desirable to bias this point to half the supply voltage, 10 volts dc in this case. This is also
known as the quiescent point. The ac output signal would then be superimposed on the dc bias
voltage.The Q-point is sometimes indicated on the output characteristics curves for a transistor
amplifier. The quiescent point also refers to the dc conditions (bias conditions) of a circuit
without an input signal.
Q-Point Value
I have mentioned that setting the Q-point to half the supply voltage is a good idea. It gives a
circuit the highest margin for overload. However, any amplifier will clip if the input amplitude
exceeds the limit for which the circuit was designed. However, there are certain cases when it is
not necessary to bias a stage to half the supply voltage. Examples would be an RF amplifier
design where the input signal is in microvolts or millivolts. If the stage had a gain of 200 then the
output (assuming a 2mV peak input) would only need to swing up and down 400mV about the
Q-point. Hence a stage with a supply voltage of 12 volts could have its Q-point set at 10 volts or
even 2 volts without problems. Another example would be a microphone stage where similar low
level input signals are involved.
Output Characteristic Curve for a BC107A
Click on the graph to zoom in (full screen display)
Bias Design:
The collector voltage Vc for the simple bias design is 10 volt. The dc current gain, hFE for the
BC107A is obtained from the manufacturers data sheets and varies between devices. A typical
beta is around 290. Taking a base current of 20uA and reading values direct from the output
curves, the collector current, for a collector emitter voltage of 10 volts is around 3.9mA. As hFE=
Ic / Ib then a BC107A must have a beta of at least 3.9mA / 20uA = 195 to work with this circuit.
Also, the base emitter voltage, Vbe is typically 0.6v. Knowing the above data and using ohm's
law , values for Rb and Rc can be determined:
Rb = Vcc - Vbe / Ib = (20-0.6) / 20u = 970k use (1M)
Rc = Vc / Ic = 10 / 3.9m = 2.56K use (2.7K)
Mathematical Approach:
Without using the output characteristic curve, values for Rb and Rc can still be calculated. A
value for hFE must be estimated first and a desired collector current. As hFE varies in each
transistor the value chosen should be the lowest value from the manufacturers data sheets. he
equations to use are:
Rc = Vc / Ic
Ib = Ic / hFE
Rb = Vcc - 0.6 / Ib.
Using the example above with Vcc=20 and hFE =195 yields the same values.
Temperature Stability
The above circuit is not good for the following reasons. It relies heavily on a transistor with a
current gain very close to 195. Other samples will give different results. Variations in the supply
voltage produce changes in the quiescent values, and also a change in temperature will alter the
current gain of the transistor and hence quiescent point. For use as an amplifier this could mean
distortion of the output signal above a certain temperature. The graph below displays the
collector voltage and current for the simple bias circuit over a temperature range of -50 to +50
degrees Celsius.
As can be seen both Vq and Iq will vary over a wide range. This is the reason that this circuit is
seldom used. It is clear that a different circuit arrangement is needed.
Self Stabilizing Bias
Coupling capacitors have been omitted for clarity, the output is taken from the transistor
collector :
This is similar to the self bias circuit with one difference: the base resistor Rb is returned to the
transistor collector instead of the supply voltage. The reason for this is simple; if the transistor
used had a high current gain, then the collector voltage would fall. As Rb is connected to the
collector then the base current would be reduced to counter the effect. If the transistor had a low
value of beta, then the collector voltage would rise. This in turn provides more base current for
the transistor to conduct harder and stabilize the q-point. The equations to calculate Rc and Rb
follow:Rc = Vc / Ic
Rb = Vc - Vbe / Ib
as Ib = Ic / hFE then
Rb = (Vc -Vbe) * hFE/ Ic
Self stabilizing bias example:
A bias circuit is required to bias a transistor to half the supply voltage. A BC107A transistor with
hfe of 200 is used and supply voltage, Vcc is 20 volts. The collector current is to be 1mA. The
resistor values are:
Rc = Vc / Ic = 10 / 1mA = 10K
Rb = (Vc-Vbe)*hFE / Ic = (10-0.6)*200 /1mA = 1880k a 1.8M resistor is fine here.
Temperature Stability of Self Stabilizing Bias Circuit
This method of biasing is more resilient to changes of temperature as shown in the graph below.
It is unlikely that anything you make will be tested under this extreme range of temperatures,
however some parts of the world, for example Mongolia have Winters where temperatures
plumit to -40° C and Summers that can reach +40° C ! If you live in an extreme climate then the
effects of temperature must be taken into consideration. The results below show quiescent
collector voltage and currents and can be compared to the simple bias circuit above.
Potential Divider Bias
This is the most widely used biasing scheme in general electronics. For a single stage amplifier
this circuit offers the best resilience to temperature changes and variation in device
characteristics. The disadvantage is that a couple of extra resistors are required, but this is
outweighed by the advantage of excellent stability. The circuit is shown below:
Here R1 and R2 form a potential divider, which will fix the base potential of the transistor. The
current through this bias chain is usually set at 10 times greater than the base current required by
the transistor. The base emitter voltage drop of the transistor is approximated as 0.6 volt. There
will also be a voltage drop across the emitter resistor, Re, this is generally set to about 10% of
the supply voltage. The inclusion of this resistor also helps to stabilize the bias: If the
temperature increases, then extra collector current will flow. If Ic increases, then so will Ie as Ie=
Ib + Ic. The extra current flow through Re increases the voltage drop across this resistor reducing
the effective base emitter voltage and therefore stabilizing the collector current. The equations
follow:
Rc = Vc / Ic
Ie = Ib + Ic as Ic >> Ib then Ie ~ Ic
Ve = 10% * Vcc
Re= Ve / Ie
Vb = Ve + 0.6
R2 = Vb / 10 * Ib
R1 = Vcc-Vb / 10 * Ib
An Example:
Using the values of the previous examples a direct comparison of stability can be demonstrated.
The values are;
Vcc=20V, Vc=10V, Ic = 1mA, transistor is BC107A with hFE=195
Rc= Vc /Ic = 10 / 1m = 10k
Ve = 10% * 20 = 2V
Re = Ve / Ie= 2 / 1= 2k
Vb = 2+ 0.6 = 2.6V
Ib = Ic / hFE = 1 / 195 =0.005128mA
R2 = Vb / 10* Ib = 2.6 / 0.05128 = 50.7k use 47k
R1 = Vcc-Vb / 10 * Ib = (20-2.6) / 0.05128 = 339.3k use 330k
Using these values and plotting the change in quiescent conditions for Vc and Ic over a
temperature range of -50 to +50 celcius is displayed below:
As shown above, this bias circuit offers the best stability against variations in Vc and Ic over a
very wide temperature range. As the resistor values used were preferred values, then the
quiescent point will be slightly different from the calculated value.