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Aim: To measure gain, frequency response, and bandwidth of an RC amplifier.
Apparatus: See your laboratory manual for the apparatus.
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THEORY:
Amplification is a process of increasing the
signal strength by increasing the amplitude of a given signal without changing
its characteristics. An RC coupled amplifier is a part of a multistage
amplifier wherein different stages of amplifiers are connected using a
combination of resistor and a capacitor. An amplifier circuit is one of basic
circuits in electronics.
An amplifier which is completely based on transistor
is basically known as transistor amplifier. The input signal may be a current
signal, voltage signal or a power signal. An amplifier will amplify the signal
without changing its characteristics and the output will be a modified version
of the input signal. Applications of amplifiers are of wide range. They are
mainly used in audio and video instruments, communications, controllers, etc.
Single Stage Common Emitter Amplifier:
The
circuit diagram of a single stage common emitter transistor amplifier is shown
below:
Single
stage common emitter RC coupled amplifier
Circuit Explanation
A single stage common emitter RC coupled
amplifier is a simple and elementary amplifier circuit. The main purpose
of this circuit is pre-amplification that is to make weak signals to be
stronger enough for further amplification. If designed properly, this RC
coupled amplifier can provide excellent signal characteristics.
The capacitor Cin at the input acts as a
filter which is used to block the DC voltage and allow only AC voltage to the
transistor. If any external DC voltage reaches the base of the transistor, it
will alter the biasing conditions and affects the performance of the amplifier.
R1 and R2 resistors are used for
providing proper biasing to the bipolar transistor. R1 and R2 form a biasing
network which provides necessary base voltage to drive the transistor in active
region.
The region between cut off and saturation
region is known as active region. The region where the bipolar transistor
operation is completely switched off is known as cut off region and the region
where the transistor is completely switched on is known as saturation region.
Resistors Rc and Re are used to drop voltage
of Vcc. Resistor Rc are a collector resistor and Re is emitter resistor. Both
are selected in such a way that both should drop Vcc voltage by 50% in the
above circuit. The emitter capacitor Ce and emitter resistor Re makes a
negative feedback for making the circuit operation more stable.
Two Stage Common Emitter Amplifier:
The circuit below represents the two stage
common emitter mode transistor amplifier where resistor R is used as a load and
the capacitor C is used as a coupling element between the two stages of the
amplifier circuit.
Two
stage common emitter RC coupled amplifier
Circuit Explanation:
When input AC. 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 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.
Transistor as Amplifiers
While knowing about different circuits for RC
coupled amplifiers, it is important to know about transistors basics as
amplifiers. The three configurations of the bipolar transistors that are
commonly used are common base transistor (CB), common emitter transistor (CE)
and common collector transistors (CE). Other than transistors,
operational amplifiers can also be used for amplification purpose.
- Common
emitter configuration is commonly used in the audio amplifier
application because common emitter has a gain that is positive and also
greater than unity. In this configuration emitter is connected to ground
and has high input impedance. Output impedance will be medium. Most of
these types of transistor amplifier applications are commonly used in RF
communication and optical fiber communications (OFC).
- Common
base configuration has a gain less than unity. In this configuration
collector is connected to ground. We have low output impedance and high
input impedance in common base configuration.
- Common
collector configuration is also known as emitter follower because
the input applied to the common emitter appears across the output of the
common collector. In this configuration collector is connected to ground.
It has low output impedance and high input impedance. It has a gain almost
equal to the unity.
Basic Parameters of a Transistor Amplifier
We
need to consider the following specifications before choosing the amplifier. A
good amplifier must have all the following specifications:
- It should have
high input impedance
- It should have
high stability
- It must have
high linearity
- It should have
high gain and bandwidth
- It must have
high efficiency
Bandwidth:
The range of frequency that an amplifier
circuit can amplify properly is known as the bandwidth of that particular
amplifier. The curve below represents the frequency response of the single
stage RC coupled amplifier.
R C
Coupled Frequency Response
The curve which represents the variation of
gain of an amplifier with frequency is called frequency response curve. The
bandwidth is measured between the lower half power and upper half power points.
P1 point is lower half power and P2 is upper half power respectively. A good
audio amplifier must have bandwidth from 20 Hz to 20 kHz because that is the
frequency range which is audible.
Gain:
Gain of an amplifier is defined as the ratio
of output power to the input power. Gain can be expressed either in decibel
(dB) or in numbers. Gain represents how much an amplifier is able to amplify a
signal given to it.
The
below equation represents gain in number:
G= Pout/Pin
Where
Pout is output power of an amplifier
Pin is
input power of an amplifier
The
equation below represents gain in decibel (DB):
Gain
in DB= 10log (Pout/Pin)
Gain can also be expressed in voltage and
current. Gain in voltage is ratio of output voltage to the input voltage and
gain in current is ratio of output current to input current. Equation for gain
in voltage and current is shown below
Gain in voltage= output voltage/ input
voltage
Gain in current= output current/ input
current
High Input Impedance:
Input impedance is the impedance which is
offered by an amplifier circuit when it is connected to the voltage source. The
transistor amplifier must have high input impedance in order to prevent it from
loading the input voltage source. So that is the reason for having high
impedance in the amplifier.
Noise:
Noise refers to unwanted fluctuation or
frequencies present in a signal. It may be due to the interaction between two
or more signals present in a system, component failures, design flaws,
external interference, or may be by virtue of certain components used in the
amplifier circuit.
Linearity:
An amplifier is said to be linear if there is
any linear relationship between the input power and the output power. Linearity
represents the flatness of the gain. Practically it not possible to get 100%
linearity as the amplifiers uses active devices like BJTs, JFETs or MOSFETs,
which tend to lose gain at high frequencies due to internal parasitic
capacitance. In addition to this the input DC decoupling capacitors sets a
lower cutoff frequency.
Efficiency:
The Efficiency of amplifier represents how an
amplifier can utilize the power supply efficiently. And also measures how much
power from the power supply is gainfully converted at output.
Efficiency is usually expressed in percentage
and the equation for efficiency is given as (Pout/ Ps) x 100. Where
Pout is the power output and Ps is the power drawn from the power
supply.
A Class A transistor amplifier has 25%
efficiency and provides excellent signal reproduction but the efficiency is
very low. Class C amplifier has efficiency up to 90%, but the signal
reproduction is bad. Class AB stands in between class A and class C amplifiers
so it is commonly used in audio amplifier applications. This amplifier has
efficiency up to 55%.
Slew Rate:
Slew rate of an amplifier is the maximum rate
of change of output per unit time. It represents how rapidly the output of an
amplifier can be changed in response to change in the input.
Stability:
Stability is the capacity of an amplifier to
resist oscillations. Usually stability problems occur during high frequency
operations, close to 20 KHz in case of audio amplifiers. The oscillations may
be of high or low amplitude.
For Procedure and Circuit Diagram, See your Electrical
Laboratory Manual on page 3.
Fill Up Tables in Your Manual and Continue
To….
PRECAUTIONS:
For
Precautions, See General
Electrical Lab Precautions
Answers to Questions:
No Questions in this experiment.
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