Electronics Circuits Module 3

MODULE 3: FEEDBACK IN AMPLIFIERS AND OSCILLATORS

FEEDBACK IN AMPLIFIERS
❖ The process of injecting a fraction of output energy of a device back to the input is known as
feedback.
❖ Depending on whether feedback energy aids or opposes the input signal, feedback is classified into negative
feedback and positive feedback

COMPARISON

POSITIVE FEEDBACK

❖ When feedback energy(voltage/current) is in phase with the input signal and thus aids it,it is called positive
feedback
❖ Both amplifier and feedback circuit introduce 180-degree phase shift. Thus a 360-degree phase shift around
the loop is introduced.
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❖ Feedback voltage Vf is in phase with input signal Vin
❖ Used in oscillators (device that converts dc power into ac power of any desired frequency)
NEGATIVE FEEDBACK
❖ When feedback energy(voltage/current) is out of phase with the input signal and thus opposes it, it is
negative feedback.
❖ Amplifier circuit introduces 180-degree phase shift. Feedback network introduces no phase shift.
❖ Feedback voltage Vf is out of phase with input signal Vin

GAIN

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TYPES OF NEGATIVE FEEDBACK

Depending on the quantity chosen there are four basic types of feedback connections
1. Voltage series feedback
2. Voltage shunt feedback
3. Current series feedback
4. Current shunt feedback

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Types of Feedback
Characteristics
Voltage-Series Voltage-Shunt Current-Series Current-Shunt

Voltage Gain Decreases Decreases Decreases Decreases

Bandwidth Increases Increases Increases Increases

Input resistance Increases Decreases Increases Decreases

Output resistance Decreases Decreases Increases Increases

Harmonic distortion Decreases Decreases Decreases Decreases

Noise Decreases Decreases Decreases Decreases

EFFECTS OF NEGATIVE FEEDBACK

Following are the effects of negative feedback on amplifier

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Electronics Circuits Module 3

OSCILLATORS
Amplifier vs. Oscillator
❖ An amplifier increases the signal strength of the input signal applied, where as
an oscillator generates a signal without that input signal, but it requires dc for its operation.
❖ This is the main difference between an amplifier and an oscillator.
❖ It clearly shows how an amplifier takes energy from d.c. power source and converts it into a.c.
energy at signal frequency.
❖ An oscillator produces an oscillating a.c. signal on its own.

Oscillators: A electronic circuit that generates an alternating voltage of desired frequency without
requiring any externally applied input signal is called an oscillator
PRACTICAL OSCILLATOR CIRCUIT
❖ A Practical Oscillator circuit consists of a tank circuit, a transistor amplifier, and a feedback
circuit. The following circuit diagram shows the arrangement of a practical oscillator.

Tank Circuit
❖ The tank circuit consists of an inductance L connected in parallel with capacitor C.
❖ The values of these two components determine the frequency of the oscillator circuit and hence
this is called as Frequency determining circuit.
Transistor Amplifier
❖ The output of the tank circuit is connected to the amplifier circuit so that the oscillations
produced by the tank circuit are amplified here.
❖ Hence the output of these oscillations are increased by the amplifier.
Feedback Circuit
❖ The function of feedback circuit is to transfer a part of the output energy to LC circuit in proper
phase.
❖ This feedback is positive in oscillators while negative in amplifiers.

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Barkhausen’s criterion for oscillations

1. The loop gain is equal to unity in absolute magnitude, that is,
|Aβ|=1
Where
A is the voltage gain of amplifier without feedback
β is the feedback factor
2. Total phase shift around the closed loop should be 0 degree or 360 degrees

RC PHASE SHIFT OSCILLATOR

❖ The oscillator consists of a common emitter amplifier with a three stage RC network as
feedback.
❖ The value of resistors and capacitors of RC network and amplifiers are so chosen such that it
satisfies Barkhausen Criterion.
❖ The CE amplifier provides 180-degree phase shift.
❖ Each RC section provides a phase shift of 60o
❖ Thus, a total 60o x 3 =180o phase shift is provided by RC phase shift feedback network
❖ The circuit when switched ON the amplifier picks-up noises present in the circuit
and gives to the RC network.
❖ Feedback factor β =1/29
❖ For self-starting the oscillations, the loop gain should be slightly greater than 1so as to meet the
losses A 𝛽 > 1
❖ Amplifier gain A must be slightly greater than 29 for sustained oscillations
❖ When the circuit is switched ON, it produces oscillations of frequency fo

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WEIN-BRIDGE OSCILLATOR
❖ Wien Bridge Oscillator is an oscillator which uses RC network to produce a sine wave.
❖ These are basically the low-frequency oscillator that generates audio and sub audio frequency
that ranges between 20 Hz to 20 KHz.
❖ The Wien bridge circuit that we use is a lead-lag network as with the rise in frequency phase
shift lags and with the reduction in frequency, it leads.
❖ It gives highly stable oscillation frequency and does not vary much with supply or temperature
variation.
❖ The two transistors produce a total phase shift of 360o so that proper positive feedback is
ensured. The negative feedback in the circuit ensures constant output
❖ Feedback factor β= 1/3.
❖ Thus, in this case, voltage gain A, must be equal to or greater than 3, to sustain oscillations
❖ The frequency of oscillations is determined by the series element R1C1 and parallel element R2C2 of
the bridge.

❖ If R1 = R2 and C1 = C2 = C
Then,

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Electronics Circuits Module 3

HARTLEY OSCILLATOR
❖ A Hartley Oscillator (or RF oscillator) is a type of harmonic oscillator.
❖ The oscillation frequency for a Hartley Oscillator is determined by an LC tank circuit (i.e. a
circuit consisting of capacitors and inductors).
❖ Hartley oscillators are typically tuned to produce waves in the radiofrequency band (which is
why they are also known as RF oscillators).
Construction
❖ Resistors R1 and R2 form the voltage divider bias network for the transistor in common-emitter CE
configuration.
❖ Emitter resistor RE forms the stabilizing network
❖ Capacitors Ci and Co are the input and output decoupling capacitors
❖ Emitter capacitor CE is the bypass capacitor used to bypass the amplified AC signals.
❖ All these components are identical to those present in a common-emitter amplifier which is biased
using a voltage divider network.
❖ The radio frequency choke (R.F.C) offers very high impedance to high frequency currents which
means it shorts for d.c. and opens for a.c. Hence it provides d.c. Load for collector and keeps a.c.
currents out of d.c. supply source
Tank Circuit
❖ The frequency determining network is a parallel resonant circuit which consists of the inductors
L1 and L2 along with a variable capacitor C.
❖ The junction of L1 and L2 are earthed.

Circuit Diagram

Working
❖ On switching ON the power supply, the transistor starts to conduct, leading to an increase in the
collector current, IC which charges the capacitor C.
❖ On acquiring the maximum charge feasible, C starts to discharge via the inductors L1 and L2.
❖ These charging and discharging cycles in the tank circuit result in the damped oscillations.
❖ The oscillation current in the tank circuit produces an AC voltage across the inductors L1 and L2
which are out of phase by 180o as their point of contact are grounded.

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❖ Further from the figure, it is evident that the output of the amplifier is applied across the inductor
L1 while the feedback voltage drawn across L2 is applied to the base of the transistor.
❖ We can conclude that the amplifier’s output is in-phase with the tank circuit’s voltage, while the
energy fed back to the amplifier circuit will be out-of-phase by 180o.
❖ The feedback voltage which is already 180o out-of-phase with the transistor, is provided by an
additional 180o phase-shift due to the transistor action.
❖ Hence the signal which appears at the transistor’s output will be amplified and will have a net
phase-shift of 360o.
❖ Feedback ratio given by

❖ The frequency of such an oscillator is given as

Where,

Advantages
The advantages of Hartley oscillator are
• Instead of using a large transformer, a single coil can be used as an auto-transformer.
• Frequency can be varied by employing either a variable capacitor or a variable inductor.
• Less number of components are sufficient.
• The amplitude of the output remains constant over a fixed frequency range.
Disadvantages
The disadvantages of Hartley oscillator are
• It cannot be a low frequency oscillator.
• Harmonic distortions are present.
Applications
The applications of Hartley oscillator are
• It is used to produce a sinewave of desired frequency.
• Mostly used as a local oscillator in radio receivers.
• It is also used as R.F. Oscillator.
COLPITTS OSCILLATOR
❖ A Colpitts Oscillator is a type of LC oscillator.
❖ The oscillation frequency for a Colpitts Oscillator is determined by an LC tank circuit (i.e. a
circuit consisting of capacitors and inductors).
Construction
❖ Resistors R1 and R2 form the voltage divider bias network for the transistor in common-emitter CE
configuration.
❖ Emitter resistor RE forms the stabilizing network
❖ Capacitors Ci and Co are the input and output decoupling capacitors
❖ Emitter capacitor CE is the bypass capacitor used to bypass the amplified AC signals.

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❖ All these components are identical to those present in a common-emitter amplifier which is biased
using a voltage divider network.
❖ The radio frequency choke (R.F.C) offers very high impedance to high frequency currents which
means it shorts for d.c. and opens for a.c. Hence it provides d.c. load for collector and keeps a.c.
currents out of d.c. supply source
Tank Circuit
❖ The frequency determining network is a parallel resonant circuit which consists of variable
capacitors C1 and C2 along with an inductor L.
❖ The junction of C1 and C2 are earthed.
Circuit Diagram & Working

❖ As the power supply is switched ON, the transistor starts to conduct, increasing the collector current
IC due to which the capacitors C1 and C2 get charged.
❖ On acquiring the maximum charge feasible, they start to discharge via the inductor L.
❖ During this process, the electrostatic energy stored in the capacitor gets converted into magnetic flux,
which is stored within the inductor in the form of electromagnetic energy.
❖ Next, the inductor starts to discharge, which charges the capacitors once again. Likewise, the cycle
continues, which gives rise to the oscillations in the tank circuit.
❖ Further the figure shows that the output of the amplifier appears across C 1 and thus is in-phase with
the tank circuit’s voltage.
❖ On the other hand, the voltage feedback to the transistor is obtained across the capacitor C2, which
means the feedback signal is out-of-phase with the voltage at the transistor by 180o.
❖ The voltages developed across the capacitors C1 and C2 are opposite in polarity as the point where
they join is grounded.
❖ Further, this signal is provided with an additional phase-shift of 180o by the transistor which results
in a net phase-shift of 360o around the loop, satisfying the phase-shift criterion of Barkhausen
principle.
❖ At this stage, the circuit can effectively act as an oscillator producing sustained oscillations by carefully
monitoring the feedback factor given by

β= (C1 / C2).

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❖ The frequency of such a Colpitts Oscillator depends on the components in its tank circuit and is given
by

Where the Ceff is the effective capacitance of the capacitors expressed as

Advantages
The advantages of Colpitts oscillator are as follows −
• Colpitts oscillator can generate sinusoidal signals of very high frequencies.
• It can withstand high and low temperatures.
• The frequency stability is high.
• Frequency can be varied by using both the variable capacitors.
• Less number of components are sufficient.
• The amplitude of the output remains constant over a fixed frequency range.
The Colpitts oscillator is designed to eliminate the disadvantages of Hartley oscillator and is known to have no
specific disadvantages. Hence there are many applications of a colpitts oscillator.
Applications
The applications of Colpitts oscillator are as follows −
• Colpitts oscillator can be used as High frequency sinewave generator.
• This can be used as a temperature sensor with some associated circuitry.
• Mostly used as a local oscillator in radio receivers.
• It is also used as R.F. Oscillator.
• It is also used in Mobile applications.
CRYSTAL OSCILLATOR
❖ Whenever an oscillator is under continuous operation, its frequency stability gets affected. There
occur changes in its frequency.
❖ The main factors that affect the frequency of an oscillator are
Power supply variations
Changes in temperature
Changes in load or output resistance
❖ In RC and LC oscillators the values of resistance, capacitance and inductance vary with temperature
and hence the frequency gets affected.
❖ In order to avoid this problem, the piezo electric crystals are being used in oscillators.
❖ The use of piezo electric crystals in parallel resonant circuits provide high frequency stability in
oscillators. Such oscillators are called as Crystal Oscillators.
❖ The principle of crystal oscillators depends upon the Piezo electric effect.
Piezo Electric Effect
❖ The crystal exhibits the property that when a mechanical stress is applied across one of the faces of
the crystal, a potential difference is developed across the opposite faces of the crystal.
❖ Conversely, when a potential difference is applied across one of the faces, a mechanical stress
is produced along the other faces. This is known as Piezo electric effect.

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Working of a Quartz Crystal

❖ In order to make a crystal work in an electronic circuit, the crystal is placed between two metal
plates in the form of a capacitor.
❖ Quartz is the mostly used type of crystal because of its availability and strong nature while being
inexpensive.
❖ The ac voltage is applied in parallel to the crystal.The circuit arrangement of a Quartz Crystal will
be as shown below −

❖ If an AC voltage is applied, the crystal starts vibrating at the frequency of the applied voltage.
❖ However, if the frequency of the applied voltage is made equal to the natural frequency of the
crystal, resonance takes place and crystal vibrations reach a maximum value. This natural
frequency is almost constant.
Equivalent circuit of a Crystal
❖ If we try to represent the crystal with an equivalent electric circuit, we have to consider two cases,
i.e., when it vibrates and when it doesn’t.
❖ The figures below represent the symbol and electrical equivalent circuit of a crystal respectively.

❖ The above equivalent circuit consists of a series R-L-C circuit in parallel with a capacitance Cm.
❖ When the crystal mounted across the AC source is not vibrating, it is equivalent to the capacitance
Cm.
❖ When the crystal vibrates, it acts like a tuned R-L-C circuit.
❖ crystal has two closely spaced resonant frequencies.
❖ The first one is the series resonant frequency (fs), which occurs when reactance of the inductance
(L) is equal to the reactance of the capacitance C.
❖ In that case, the impedance of the equivalent circuit is equal to the resistance R and the frequency
of oscillation is given by the relation,

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❖ The second one is the parallel resonant frequency (fp), which occurs when the reactance of R-L-
C branch is equal to the reactance of capacitor Cm. A
❖ t this frequency, the crystal offers a very high impedance to the external circuit and the frequency
of oscillation is given by the relation.

Where

❖ The value of Cm is usually very large as compared to C.

❖ Therefore, the value of CT is approximately equal to C and hence the series resonant frequency
is approximately equal to the parallel resonant frequency (i.e., fs = fp).
Crystal Oscillator Circuit

❖ In this circuit, the crystal is connected as a series element in the feedback path from
collector to the base.
❖ The resistors R1, R2 and RE provide a voltage-divider stabilized d.c. bias circuit.
❖ The capacitor CE provides a.c. bypass of the emitter resistor and RFC (radio frequency choke)
coil provides for d.c. bias while decoupling any a.c. signal on the power lines from affecting the
output signal.
❖ The coupling capacitor C has negligible impedance at the circuit operating frequency. But it
blocks any d.c. between collector and base.
❖ The circuit frequency of oscillation is set by the series resonant frequency of the crystal and its
value is given by the relation,
❖ It may be noted that the changes in supply voltage, transistor device parameters etc. have no
effect on the circuit operating frequency, which is held stabilized by the crystal.

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Advantages
The advantages of crystal oscillator are as follows −
•They have a high order of frequency stability.
• The quality factor (Q) of the crystal is very high.
Disadvantages
The disadvantages of crystal oscillator are as follows −
• They are fragile and can be used in low power circuits.
• The frequency of oscillations cannot be changed appreciably.

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