Sub:Analog Electronic Circuit[EC1004]
PRESENTED BY-SATTWIK SEN (2105403)
SHANTANU SHARMA (2105404)
SHIBHAM KUMAR SINGH(2105405)
SHRESTHA GHOSHAL (2105407)
SHREYAS NAYAK (2105408)
SHRUTI MUKHERJEE (2105409)
SOHAM RAJ JAIN (2105410)
SOUMYAM SHARAN (2105411)
SPARSH CHAUDHARY (2105412)
SUDIN BEBORTA (2105414)
TOPIC NAME : Amplification process in BJT
small signal analysis
SUB TOPIC : Amplification Process in BJT
DEFINATION:
A Bipolar Junction Tranasistor ( also known as a BJT or BJT Transistor ) is a three-terminal
semiconductor device consisting of two p-n junctions which are able to amplify or magnify
a signal. It is a controlled device .A BJTis a type of transistor that uses both electons and
holes
as charge carriers
SYMBOL OF BJT:
How BJTis used as an
Amplifier
Bipolar Junction Transistor (BJT)acts as an amplifier
by raising the strength of a weak signal. The DC bias
voltage applied to the emitter base junction, makes it
remain in forward biased condition. This forward
bias is maintained regardless of the polarity of the
signal. The below figure shows how a transistor
looks like when connected as an amplifier.
The low resistance in input circuit, lets any small
change in input signal to result in an appreciable
change in the output. The emitter current caused by
the input signal contributes the collector current,
which when flows through the load resistor RL,
results in a large voltage drop across it. Thus a small
input voltage results in a large output voltage, which
shows that the transistor works as an amplifier.
❖ Voltage Gain : Voltage Gain is termed as measure of how the given amplifier can amplify the
input signal, or the factor with which the increased output is generated.Here, the voltage gain is the
ratio between the output voltage and the input voltage.
□Voltage Gain in BJT: 1) Common Emmitter Mode = High (about 100)
2) Common Collector Mode = Slightly less than unity (1)
3) Common Base Mode = Voltage Gain: Medium (about 10 to 50)
❖ Current Gain : It is the simple ratio of output current and input current of any circuit, device, etc.
The current gain of a circuit or system indicates that maximum how much current can deliver
to the load as output with respect to the particular applied input current.
□Current Gain in BJT: 1) Common Emmitter Mode = High (about 50 to 800)
2) Common Collector Mode = High (about 50 to 800)
3) Common Base Mode = Less than unity (<1)
SMALL SIGNAL MODEL OF BJT USING RE-MODEL
• There are many transistor model types; including JFETs, MOSFETS, HFTs and others. However, the first transistor,
the BJT, introduced in the mid-twentieth century, is still a widely used amplifier today. BJTscan be configured to
exhibit different signal transformation characteristics; such as low or high gain. And various circuit types can be
designed by evaluating the large signal properties of the BJT for a specific configuration.
WHAT IS SMALL SIGNAL MODELLING
signal modeling is a common analysis technique in electronics engineering used to
approximate the behavior of electronic circuits containing nonlinear devices with linear
equations. It is applicable to electronic circuits in which the AC signals (i.e., the time-varying
currents and voltages in the circuit) are small relative to the DC bias currents and voltages. A
small-signal model is an AC equivalent circuit in which the nonlinear circuit elements are
replaced by linear elements whose values are given by the first-order (linear) approximation
of their characteristic curve near the bias point.
Many of the electrical components used in simple electric circuits, such as resistors,
inductors, and capacitors are linear. Circuits made with these components, called linear
circuits, are governed by linear differential equations, and can be solved easily with powerful
mathematical frequency domain methods such as the Laplace transform.
The small signal model is dependent on the DC bias currents and voltages in the circuit (the
Q point). Changing the bias moves the operating point up or down on the curves, thus
changing the equivalent small-signal AC resistance, gain, etc. seen by the signal.
RE MODEL OF CB CONFIGURATION
CIRCUIT DIAGRAM
• The input impedance is between terminals B and E and has
a value of: re ( β + 1)
Re MODEL OF CB
CONFIGURATION
The re model employs a diode and
controlled current source to
duplicate the behavior of
a transistor in the region of
interest. In fact, in general:
BJT transistor amplifiers are
referred to as current-controlled
devices.
Re MODEL OF CE
CONFIGURATION
CE RC Coupled Amplifier Using BJT Calculation
Of Gain Input And Output
• A Resistance Capacitance (RC) Coupled Amplifier is basically a
multi-stage amplifier circuit extensively used in electronic circuits. Here
the individual stages of the amplifier are connected together using a
resistor–capacitor combination due to which it bears its name as RC
Coupled.
• a two-stage amplifier whose individual stages are nothing but the
common emitter amplifiers. Hence the design of individual stages of the
RC coupled amplifiers is similar to that in the case of common emitter
amplifiers in which the resistors R1and R2 form the biasing network
while the emitter resistor RE form the stabilization network.
• Here the CE is also called bypass capacitor which passes only AC while
restricting DC, which causes only DC voltage to drop across RE while the
entire AC voltage will be coupled to the next stage.
Circuit diagram of CE RC Coupled Amplifier
Working Principle of RC Coupled Amplifier
When an AC input signal is applied to the base of first transistor, it gets amplified and
appears at the collector load RL which is then passed through the coupling capacitor CC to
the next stage. This becomes the input of the next stage, whose amplified output again
appears across its collector load. Thus the signal is amplified in stage by stage action.
The important point that has to be noted here is that the total gain is less than the product
of the gains of individual stages. This is because when a second stage is made to follow the
first stage, the effective load resistance of the first stage is reduced due to the shunting
effect of the input resistance of the second stage. Hence, in a multistage amplifier, only the
gain of the last stage remains unchanged.
As we consider a two stage amplifier here, the output phase is same as input. Because the
phase reversal is done two times by the two stage CE configured amplifier circuit.
Circuit Diagram and
Working of RC Coupled CE
Amplifier
01
0
2
0
3
WHEN A WEAK INPUT A.C. SIGNAL IS
APPLIED TO THE BASE OF THE
TRANSISTOR, A SMALL BASE CURRENT
FLOWS. DUE TO TRANSISTOR ACTION, A
MUCH LARGERA.C. CURRENT FLOWS
THROUGH COLLECTOR LOAD RC, A
LARGE VOLTAGE APPEARSACROSS RC
AND HENCE AT THE OUTPUT.
THEREFORE, A WEAK SIGNAL APPLIED
TO THE BASEAPPEARS IN AMPLIFIED
FORM IN THE COLLECTOR CIRCUIT.
VOLTAGE GAIN (AV) OF THE AMPLIFIER
IS THE RATIO OF THE AMPLIFIED
OUTPUT VOLTAGE TO THE INPUT
VOLTAGE.
Design of Re and Ce.
Let voltage across Re; VRe = 10%Vcc ………….(1)
Voltage across Rc; VRc = 40% Vcc. ……………..(2)
The remaining 50% will drop across the collector-emitter .
From (1) and (2) Rc =0.4 (Vcc/Ic) and Re = 01(Vcc/Ic). Design of R1 and R2.
Base current Ib = Ic/hfe.
Let Ic ≈ Ie .
Let current through R1; IR1 = 10Ib.
Also voltage across R2 ; VR2 must be equal to Vbe + VRe. From this VR2 can be
found.
There fore VR1 = Vcc-VR2. Since VR1 ,VR2 and IR1 are found we can find R1 and
R2 using the following equations.R1 = VR1/IR1 and R2 = VR2/IR1.
Finding Cin.
Impedance of the input capacitor(Cin) should be one by tenth of the transistors input impedance (Rin).
i.e, XCin = 1/10 (Rin)
Rin = R1 parallel R2 parallel (1 + (hfe re))
re = 25mV/Ie. Xcin = 1/2∏FCin.
From this Cin can be found.
Finding Cout.
Impedance of the output capacitor (Cout) must be one by tenth of the circuit’s output resistance (Rout).
i.e, XCout = 1/10 (Rout).
Rout = Rc.
XCout = 1/ 2∏FCout.
From this Cout can be found.
Setting the gain.
Introducing a suitable load resistor RL across the transistor’s collector and ground will set the gain. This is
not shown in Fig1.
Expression for the voltage gain (Av) of a common emitter transistor amplifier is as follows.
Av = -(rc/re)
re = 25mV/Ie
and rc = Rc parallel RL
From this RL can be found
Diagram of double stage CE RC coupled amplifier
When input AC. the signal is applied to the base of the transistor of the 1st stage of
RC coupled amplifier, from the function generator, it is then amplified across the
output of the 1st stage. This amplified voltage is applied to the base of the next stage
of the amplifier, through the coupling capacitor Cout where it is further amplified
and reappears across the output of the second stage.
Thus the successive stages amplify the signal and the overall gain is raised
to the desired level. Much higher gain can be obtained by connecting a
number of amplifier stages in succession.
Resistance-capacitance (RC) coupling in amplifiers are most widely used to
connect the output of first stage to the input (base) of the second stage and
so on. This type of coupling is most popular because it is cheap and provides
a constant amplification over a wide range of frequencies.
DRAWBACKS OF CE RC COUPLED
AMPLIFIERS
.The voltage and power gain are low because of the
effective load resistance.
.They become noisy with age.
.Due to poor impedance matching, power transfer
will be low.
ADVANTAGES OF CE RC COUPLED
AMPLIFIERS
.The frequency response of RC amplifier provides
constant gain over a wide frequency range, hence most
suitable for audio applications.
.The circuit is simple and has lower cost because it
employs resistors and capacitors which are cheap.
.It becomes more compact with the upgrading
technology.
APPLICATIONS OF CE RC COUPLED
AMPLIFIERS
• They have excellent audio fidelity over a wide range of
frequency.
• Widely used as Voltage amplifiers
• Due to poor impedance matching, RC coupling is rarely used
in the final stages.
FREQUENCY RESPONSE OF
AMPLIFIERS
The frequency response of an amplifier refers to the frequency range in which the
amplifier will operate with negligible effects from capacitors and device internal capacitance. This range of
frequencies can be called the mid-range. At frequencies above and below the midrange, capacitance and any
inductance will affect the gain of the amplifier.
• At low frequencies the coupling and bypass capacitors lower the gain.
•At high frequencies stray capacitances associated with the active device lower
the gain.
• Also, cascading amplifiers limits the gain at high and low frequencies.
The mid-range frequency ange of an amplifier is called the bandwidth of the amplifier.
The bandwidth is defined by the lower and upper cutoff frequencies.
Cutoff – any frequency at which the gain has dropped by 3 dB.
Frequency Response
Graph of Amplifiers
We can see that the frequency response of any given
circuit is the variation in its behaviour with changes in
the input signal frequency as it shows the band of
frequencies over which the output (and the gain)
remains fairly constant. The range of frequencies either
big or small between ƒL and ƒH is called the circuits
bandwidth. So from this we are able to determine at a
glance the voltage gain (in dB) for any sinusoidal input
within a given frequency range.
As mentioned above, the Bode diagram is a logarithmic
presentation of the frequency response. Most modern
audio amplifiers have a flat frequency response as
shown above over the whole audio range of frequencies
from 20 Hz to 20 kHz. This range of frequencies, for an
audio amplifier is called its Bandwidth, (BW) and is
primarily determined by the frequency response of the
circuit.Frequency points ƒL and ƒH relate to the lower
corner or cut-off frequency and the upper corner or
cut-off frequency points respectively were the circuits
gain falls off at high and low frequencies. These points
on a frequency response curve are known commonly as
the -3dB (decibel) points.
CUT OFF FREQUENCY OF AMPLIFIER
DEFINITION
☺
Cutoff frequency (also known as corner frequency, or break frequency) is defined as a boundary in a system’s frequency response at
which energy flowing through the system begins to be attenuated (reflected or reduced) rather than passing through.
The cutoff frequency or corner frequency in electronics is the frequency either above or below which the power output of a circuit,
such as a line, amplifier, or electronic filter (e.g. a high pass filter) has fallen to a given proportion of the power in the passband.
EXPRESSION
fcl,out=1/(2πRoutC3)
Then to find,
Rout=RC//RL
BANDWIDTH OF AMPLIFIER
DEFINITION
The Bandwidth (BW) of an amplifier is defined as the difference between the frequency limits of
☺ the amplifier. Complete step by step answer: The range of frequencies within a band is known as
bandwidth.
The width of frequencies or the band of frequencies that an amplifier can amplify most
effectively is represented using a bandwidth. An amplifier has frequencies which can be
amplified, these frequencies have a lower limit (that could even be zero) and an upper limit
but the amplitude enhancement cannot be done for all these frequencies effectively.
So if an amplifier is designed to amplify the frequencies between a lower frequency f1 and a
higher frequency f2 the bandwidth will be the difference of these two frequencies. So the
difference of f2 and f1 will be the bandwidth of the amplifier.
BW=f2−f1
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