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Title: Transistor Amplifiers
Aim: To design and construct a transistor amplifier.
Apparatus: See your laboratory manual for the apparatus.
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THEORY:
Amplifier is a circuit that is used for
amplifying a signal. The input signal to an amplifier will be a current or
voltage and the output will be an amplified version of the input signal. An
amplifier circuit which is purely based on a transistor or transistors is
called a transistor amplifier. Transistors amplifiers are commonly used in
applications like RF (radio frequency), audio, OFC (optic fibre communication)
etc. Anyway the most common application we see in our day to day life is the
usage of transistor as an audio amplifier. As you know there are three
transistor configurations that are used commonly i.e. common base (CB), common
collector (CC) and common emitter (CE). In common base configuration has a gain
less than unity and common collector configuration (emitter follower) has a
gain almost equal to unity). Common emitter follower has a gain that is
positive and greater than unity. So, common emitter configuration is most
commonly used in audio amplifier applications.
A good
transistor amplifier must have the following parameters; high input impedance,
high band width, high gain, high slew rate, high linearity, high efficiency,
high stability etc. The above given parameters are explained in the next section.
Input impedance: Input impedance is the
impedance seen by the input voltage source when it is connected to the input of
the transistor amplifier. In order to prevent the transistor amplifier circuit
from loading the input voltage source, the transistor amplifier circuit must
have high input impedance.
Bandwidth.
The range of frequency that an amplifier can
amplify properly is called the bandwidth of that particular amplifier. Usually
the bandwidth is measured based on the half power points i.e. the points where
the output power becomes half the peak output power in the frequency Vs output
graph. In simple words, bandwidth is the difference between the lower and upper
half power points. The band width of a good audio amplifier must be from 20 Hz
to 20 KHz because that is the frequency range that is audible to the human ear.
The frequency response of a single stage RC coupled transistor is shown in the
figure below (Fig 3). Points tagged P1 and P2 are the lower and upper half
power points respectively.
RC coupled amplifier frequency response
Gain.
Gain of an amplifier is the ratio of output
power to the input power. It represents how much an amplifier can amplify a
given signal. Gain can be simply expressed in numbers or in decibel (dB). Gain
in number is expressed by the equation G = Pout / Pin.
In decibel the gain is expressed by the equation Gain in dB = 10 log (Pout
/ Pin). Here Pout is the power output and Pin is the power
input. Gain can be also expressed in terms of output voltage / input voltage or
output current / input current. Voltage gain in decibel can be expressed using
the equation, Av in dB = 20 log ( Vout / Vin)
and current gain in dB can be expressed using the equation Ai = 20 log (Iout
/ Iin).
Derivation of gain.
G = 10 log (Pout / Pin)………(1)
Let Pout = Vout
/ Rout and Pin = Vin
/ Rin. Where Vout is the output voltage Vin
is the input voltage, Pout is the output power, Pin
is the input power, Rin is the input resistance and Rout
is the output resistance.
Substituting this in equation 1 we have
G = 10log ( Vout2/Rout) / (Vin2/Rin)………….(2)
G = 10log ( Vout2/Rout) / (Vin2/Rin)………….(2)
Let Rout = Rin,
then the equation 2 becomes
G = 10log ( Vout2 / Vin2
)
i.e.
G = 20 log ( Vout / Vin )
i.e.
G = 20 log ( Vout / Vin )
Efficiency.
Efficiency of an amplifier represents how
efficiently the amplifier utilizes the power supply. In simple words it is a
measure of how much power from the power supply is usefully converted to the
output. Efficiency is usually expressed in percentage and the equation is
ζ = (Pout/ Ps) x 100. Where ζ is
the efficiency, Pout is the power output and Ps
is the power drawn from the power supply.
Class A transistor amplifiers have up to 25%
efficiency, Class AB has up to 55% can class C has up to 90% efficiency. Class
A provides excellent signal reproduction but the efficiency is very
low while Class C has high efficiency but the signal reproduction is bad.
Class AB stands in between them and so it is used commonly in audio amplifier
applications.
Stability.
Stability is the capacity of an
amplifier to resist oscillations. These oscillations may be high amplitude ones
masking the useful signal or very low amplitude, high frequency oscillations in
the spectrum. Usually stability problems occur during high frequency
operations, close to 20KHz in case of audio amplifiers. Adding a Zobel network
at the output, providing negative feedback etc improves the stability.
Slew rate.
Slew rate of an amplifier is the
maximum rate of change of output per unit time. It represents how quickly the
output of an amplifier can change in response to the input. In simple words, it
represents the speed of an amplifier. Slew rate is usually represented in V/μS
and the equation is SR = dVo/dt.
Linearity.
An amplifier is said to be linear if there is
a linear relationship between the input power and the output power. It
represents the flatness of the gain. 100% linearity is not possible practically
as the amplifiers using active devices like BJTs , JFETs or MOSFETs tend
to lose gain at high frequencies due to internal parasitic capacitance. In
addition to this the input DC decoupling capacitors (seen in almost all
practical audio amplifier circuits) sets a lower cutoff frequency.
Noise.
Noise refers to unwanted and random
disturbances in a signal. In simple words, it can be said to be unwanted
fluctuation or frequencies present in a signal. It may be due to design flaws,
component failures, external interference, due to the interaction of two
or more signals present in a system, or by virtue of certain components used in
the circuit.
Output voltage swing.
Output voltage swing is the maximum range up
to which the output of an amplifier could swing. It is measured between the
positive peak and negative peak and in single supply amplifiers it is
measured from positive peak to the ground. It usually depends on the factors
like supply voltage, biasing, and component rating.
Common emitter RC coupled
amplifier.
The common emitter RC coupled amplifier is
one of the simplest and elementary transistor amplifier that can be made. Don’t
expect much boom from this little circuit, the main purpose of this circuit is
pre-amplification i.e to make weak signals strong enough for further processing
or amplification. If designed properly, this amplifier can provide excellent
signal characteristics. The circuit diagram of a single stage common emitter RC
coupled amplifier using transistor is shown in Fig1.
RC coupled amplifier
Capacitor Cin is the input DC
decoupling capacitor which blocks any DC component if present in the input
signal from reaching the Q1 base. If any external DC voltage reaches the base
of Q1, it will alter the biasing conditions and affects the performance of the
amplifier.
R1 and R2 are the biasing resistors. This
network provides the transistor Q1’s base with the necessary bias voltage to
drive it into the active region. The region of operation where the transistor
is completely switched of is called cut-off region and the region of operation
where the transistor is completely switched ON (like a closed switch) is called
saturation region. The region in between cut-off and saturation is called
active region. Refer Fig 2 for better understanding. For a transistor amplifier
to function properly, it should operate in the active region. Let us consider
this simple situation where there is no biasing for the transistor. As we all
know, a silicon transistor requires 0.7 volts for switch ON and surely this 0.7
V will be taken from the input audio signal by the transistor. So all parts of
there input wave form with amplitude ≤ 0.7V will be absent in the output
waveform. In the other hand if the transistor is given with a heavy bias at the
base ,it will enter into saturation (fully ON) and behaves like a
closed switch so that any further change in the base current due to
the input audio signal will not cause any change in the output. The voltage
across collector and emitter will be 0.2V at this condition (Vce sat = 0.2V).
That is why proper biasing is required for the proper operation of a transistor
amplifier.
BJT output characteristics
Cout is the output DC decoupling
capacitor. It prevents any DC voltage from entering into the succeeding stage
from the present stage. If this capacitor is not used the output of the
amplifier (Vout) will be clamped by the DC level present at the transistors
collector.
Rc is the collector resistor and Re is the
emitter resistor. Values of Rc and Re are so selected that 50% of Vcc gets
dropped across the collector & emitter of the transistor.This is done to
ensure that the operating point is positioned at the center of the load line.
40% of Vcc is dropped across Rc and 10% of Vcc is dropped across Re. A
higher voltage drop across Re will reduce the output voltage swing and so
it is a common practice to keep the voltage drop across Re = 10%Vcc
. Ce is the emitter by-pass capacitor. At zero signal condition (i.e, no
input) only the quiescent current (set by the biasing resistors R1 and R2
flows through the Re). This current is a direct current of magnitude few milli
amperes and Ce does nothing. When input signal is applied, the transistor
amplifies it and as a result a corresponding alternating current flows through
the Re. The job of Ce is to bypass this alternating component of
the emitter current. If Ce is not there , the entire emitter current will
flow through Re and that causes a large voltage drop across it. This voltage
drop gets added to the Vbe of the transistor and the bias settings will be
altered. It reality, it is just like giving a heavy negative feedback and
so it drastically reduces the gain.
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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