US03CPHY22 UNIT - 3 FEED BACK IN APMLIFIER AND OSCILLATOR

CONCEPTS OF FEEDBACK IN AMPLIFIERS:

The important characteristics of an amplifier are i) voltage gain, ii) input
impedance, iii) output impedance, iv) bandwidth and are almost constants for an amplifier and
cannot be changed. These parameters can be changed in number of ways. If we want to change
voltage gain, input impedance or output impedance of an amplifier a resistive network is used
either in the input or output. The feedback technique is also used to change these parameters.

Feedback is the process of taking a part of the output signal and feeding it back to
the input circuit. The voltage gain of an amplifier A is given by
𝑣𝑜
𝐴=
𝑣𝑖

In fig. a output is not connected with the input, so for any reason if output changes net
input remains unaffected. Such a system is called open – loop or non – feedback system.

In fig. b the output of the amplifier is fed back to the input through a network called
feedback network or β network. A fraction of the output voltage βvo is going back to the input.
This changes the net input voltage to the amplifier and thus the output modifies the input signal.
Such a system is called closed – loop or feedback system.

TYPES OF FEED BACK:

When feedback voltage vf is out of phase with input voltage vi, then the net input
voltage 𝑣𝑖′ of the amplifier is

𝒗′𝒊 = 𝒗𝒊 − 𝒗𝒇

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The net input vi′ of the amplifier reduced, the output of the amplifier also decreases from vo
to𝑣𝑜′ . The gain of the amplifier reduces because of the feedback. This type of the feedback is
called negative feedback, inverse feedback or degenerative feedback.

The feedback voltage can be also in phase with the external input voltage. In such a
case the net input voltage 𝑣𝑖′ to the amplifier is increased. That increases the output voltage to 𝑣𝑜′
from vo. This type of the feedback is called positive feedback, direct feedback or regenerative
feedback.

The positive feedback in amplifier increases distortion and decreases stability of the
gain, so in amplifier always negative feedback is used.

The feedback can also classify as voltage feedback or current feedback. In voltage
feedback, the signal fed back is proportional to the output voltage. In current feedback, the signal
fed back is proportional to the output current. A combination of voltage and current feedback may
be present in the circuit. Voltage and current can be fed back to the input either in the series or
parallel. Thus, there are four basic ways of connecting the feedback signal.

Series – feedback connections increases the input impedance of the amplifier, while
shunt – feedback connections tends to decrease the input impedance. Voltage – feedback
decreases the output impedance, while current – feedback increases the output impedance.

SERIES – VOLTAGE FEEDBACK:

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SERIES – CURRENT FEEDBACK

SHUNT – VOLTAGE FEEDBACK

SHUNT – CURRENT FEEDBACK:

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VOLTAGE GAIN OF FEEDBAC AMPLIFIER:

The voltage across the RL (=R1 + R2) 𝑣𝑜′ is fed to a β – network consisting of voltage
divider circuit consists of resistors R1 and R2. The feedback voltage vf is developed across resistor
R1 is fed back to the input side in series with the input voltage vi. Hence this circuit is example of
series – voltage feedback.

In single stage common source amplifier the amplified output signal 𝑣𝑜′ is 1800 out of
phase with the input signal vi so feedback signal vf is also out of phase with the input signal vi.
The effective input signal 𝑣𝑖′ = vi – vf therefore reduced. The type of the feedback is therefore
negative.

Block diagram of series voltage feedback of Fig.1 is shown in Fig. 2. The input of
this feedback amplifier is vi and output is 𝑣𝑜′ . The voltage gain with feedback is given by
𝑣𝑜′
𝐴𝑓 = 1
𝑣𝑖

The effective input of the basic amplifier is 𝑣𝑖′ and is

𝑣𝑖′ = 𝑣𝑖 − 𝑣𝑓 2

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The feedback voltage vf is due to 𝑣𝑜′ through the β feedback network. And so

𝑣𝑓 = 𝛽𝑣𝑜′ 3

The constant is known as feedback factor and form Fig. 1the value of β is
𝑅1
𝛽= 4
𝑅1 + 𝑅2

The voltage gain of the basic amplifier is A and is the ratio of the input voltage of the basic
amplifier 𝑣𝑖′ with the output voltage of the basic amplifier 𝑣𝑜′ . A is called internal gain of the
amplifier.
𝑣𝑜′ 𝑣𝑜′
𝐴= ′ 𝑜𝑟 𝑣𝑖′ = 5
𝑣𝑖 𝐴

Substituting equation 3 and 5 into equation 2 we have

𝑣𝑖′ = 𝑣𝑖 − 𝑣𝑓

𝑣𝑜′
= 𝑣𝑖 − 𝛽𝑣𝑜′
𝐴
𝑣𝑜′ = 𝐴𝑣𝑖 − 𝐴𝛽𝑣𝑜′

𝑣𝑜′ (1 + 𝐴𝛽) = 𝐴𝑣𝑖

𝑣𝑜′ 𝐴
= 6
𝑣𝑖 (1 + 𝐴𝛽)
𝑨
Comparing equation 6 with equation 1 we have 𝑨𝒇 = 7
(𝟏+𝑨𝜷)

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For the positive feedback, the feedback voltage vf is in phase with the input voltage vi, and
both are added to give net input to the amplifier 𝑣𝑖′ and we have

𝑣𝑖′ = 𝑣𝑖 + 𝑣𝑓

𝑣𝑜′
= 𝑣𝑖 + 𝛽𝑣𝑜′
𝐴
𝑣0′ 𝐴
= 𝐴𝑓 = 8
𝑣𝑖 (1 − 𝐴𝛽)

Equation 8 shows that when positive feedback is applied, it increases the voltage gain of the
amplifier. For the good performance of the amplifier always negative feedback is used. Positive
feedback is used in oscillator.

ADVANTAGES OF NEGATIVE FEED BACK:

The gain of the amplifier is reduced when negative feedback is introduced in the
amplifier. But negative feedback improves the performance of the amplifier. The advantages of
the negative are

Stabilization of gain.

Reduction in distortion and noise.

Increase in input impedance.

Decrease in output impedance.

Increase in bandwidth.

STABILIZATION OF GAIN

Voltage gain of the negative feedback amplifier with feedback is

𝐴
𝐴𝑓 = (1+𝐴𝛽)
; Where 𝐴𝛽 ≫ 1

Then we can consider 1 + 𝐴𝛽 = 𝐴𝛽 and voltage gain with feedback

𝐴 1
𝐴𝑓 = =
𝐴𝛽 𝛽

Thus, the gain of the amplifier Af of the feedback amplifier becomes independent of the
internal gain of the amplifier A. The gain Af depends only on β, depends on passive component
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like resistors. The value of the resistors remains constant, and hence the gain is stabilized if𝐴𝛽 ≫
1.

If some change in the internal voltage gain of the amplifier takes place, the
corresponding percentage change in the overall voltage gain of the feedback amplifier is given by
𝑑
𝑑𝐴
(𝐴𝑓 )

𝑑 𝑑 𝐴
(𝐴𝑓 ) = ( )
𝑑𝐴 𝑑𝐴 (1 + 𝐴𝛽)
(1 + 𝐴𝛽) − (𝐴𝛽)
=
(1 + 𝐴𝛽)2
𝑑𝐴𝑓 1
=
𝑑𝐴 (1 + 𝐴𝛽)2
𝑑𝐴
𝑑𝐴𝑓 =
(1 + 𝐴𝛽)2
𝑑𝐴𝑓 𝑑𝐴 (1 + 𝐴𝛽)
= 𝑋
𝐴𝑓 (1 + 𝐴𝛽)2 𝐴

𝑑𝐴𝑓 𝑑𝐴 1
= 𝑋
𝐴𝑓 𝐴 (1 + 𝐴𝛽)

Since(1 + Aβ) ≫ 1, the percentage change in Af is much less than the percentage changes
in A.

REDUCTION IN DISTORTION AND NOIS:

Negative feedback reduces harmonic distortion. When sinusoidal voltage v i is given to the
input of a basic amplifier gives distorted output by flattening the peaks. The feedback signal v f
has the same waveform as the output voltage. The feedback voltage v f gets subtracted from input
voltage vi and gives net input voltage to 𝑣𝑖′ the amplifier. Since the pick of the voltages vf are
flattened when subtracted from the vi, the peak of the resulting 𝑣𝑜′ will become more picked. Thus
net input 𝑣𝑖′ is predistorted in such a way so as to partially compensate for the flattening caused by
the amplifier. Now when peaked input voltage 𝑣𝑖′ gets amplified, the output will tend to be
sinusoidal because the amplifier tries to flatten the peaks.

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The amount of the reduction in the distortion

caused by negative feedback can be found by
following figure. The amplifier with a gain A
produces distortion D without feedback. This
distortion appears at the output. After
feedback is applied, the gain becomes Af and
the distortion in the output becomes Df. A part
βDf of the distortion Df is fed back to the input.
This gets amplified by basic amplifier by A
times and becomes AβDf. This gets added in
reverse polarity to the original distortion D to
make the net distortion Df, and is

𝐷𝑓 = 𝐷 − 𝐴𝛽𝐷𝑓

𝐷
𝐷𝑓 =
(1 + 𝐴𝛽)

The distortion is reduced by the factor 1 + Aβ. If negative feedback is employed, the net
noise in the output reduces, and the performance of the amplifier is much improved.

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INCREASE IN INPUT IMPEDANCE:

For an amplifier it is desired to
have high input impedance, so that it will
not load the preceding stage of the input
voltage source. The increase input
impedance characteristics can be achieved
with the help of negative series – voltage
or series – current feedback.

Input impedance with feedback is given by

𝑣𝑖
𝑍𝑖𝑓 = 1
𝐼𝑖

𝑣𝑖 = 𝑣𝑖′ − 𝑣𝑓 ; 𝑣𝑓 = 𝛽𝑣𝑜′

Hence, 𝑣𝑖 = 𝑣𝑖′ − 𝛽𝑣𝑜′ ; 𝑣𝑜′ = 𝐴𝑣𝑖′

𝑣𝑖 = 𝑣𝑖′ − 𝛽𝐴𝑣𝑖′ ; 𝑣𝑖 = 𝑣𝑖′ (1 + 𝐴𝛽) 2

Input impedance without feedback is given by

𝑣𝑖′
𝑍𝑖 = ; 𝑣𝑖′ = 𝑍𝑖 𝐼𝑖 3
𝐼𝑖

On substituting equation 2 into equation 1 we get

𝑣𝑖′ (1 + 𝐴𝛽)
𝑍𝑖𝑓 =
𝐼𝑖

Substituting equation 3 in above equation gives

𝒁𝒊𝒇 = 𝒁𝒊 (𝟏 + 𝑨𝜷 ) 4

From equation 4 we can say that when negative series – feedback is introduced in an
amplifier, the input impedance increases by the factor 1 + 𝐴𝛽. The negative shunt – voltage and
shunt – current feedback decreases the input impedance by the factor 1/ (1 + 𝐴𝛽)

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DECREASE IN OUTPUT IMPEDANCE:

Amplifier with low output

impedance is capable of delivering
power to the load without much loss.
The negative series – voltage feedback
or shunt – voltage feedback, when
employed in the amplifier it reduces
output impedance.

Above figure is the block diagram of a voltage – series feedback, in which output circuit has
been replaced by an equivalent voltage source Aβv o in series with impedance zo. The input
terminals have been shorted. Apply a voltage source having voltage v o at the output terminals. If
input impedance of the feedback network is assumed to be very high then on applying KVL at the
output loop we get

𝑉𝑜 = 𝐼𝑜 𝑍𝑜 − 𝐴𝛽𝑉𝑜

𝑉𝑜 + 𝐴𝛽𝑉𝑜 = 𝐼𝑜 𝑍𝑜 ; 𝑉𝑜 ( 1 + 𝐴𝛽) = 𝐼𝑜 𝑍𝑜 2

Output impedance with feedback Zof is written as

𝑉𝑜
𝑍𝑜𝑓 =
𝐼𝑜

Substituting equation 2 in above equation we have,

𝐼𝑜 𝑍𝑜 𝑍𝑜
𝑍𝑜𝑓 = ; 𝑍𝑜𝑓 = 3
𝐼𝑜 (1 + 𝐴𝛽) (1 + 𝐴𝛽)

From above equations we can see that when negative series – voltage or shunt – voltage
feedback introduced in the amplifier the output impedance decreases by a factor (1 + Aβ). The
inclusion of negative series – current or shunt – current feedback in amplifier increases the output
impedance.

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INCREASE IN BANDWIDTH:

When negative feedback introduced in amplifier, the gain of the amplifier reduces by the
factor (1+Aβ). Where A is the internal gain of the amplifier and β is the feedback factor of the
feedback network. The negative feedback reduces the lower cutoff frequency f1f by a factor
(1+Aβ) and increases upper cutoff frequency f2f by a factor (1+Aβ). The bandwidth of the
amplifier is the difference between upper cutoff frequency and lower cutoff frequency. So, when
negative feedback employed in an amplifier it increases the bandwidth by a factor (1+Aβ) and
decreases the gain of the amplifier by the same factor (1+Aβ). The gain bandwidth product (GBP)
of the amplifier is always constant.

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AMPLIFIWE CIRCUIT WITH NEGAVTI FEEDBACK:

RC COUPLED AMPLIFIER WITHCOUT BYPASS CAPACITOR:

The common emitter RC coupled amplifier circuit is shown in Fig 1. The effective
input voltage of this amplifier is the ac signal between the base and emitter same as the voltage vs
supplied from the signal source.

When the bypass capacitor CE is removed as shown in Fig.2, the effective input
voltage is changed. During the increase in positive half cycle of the input source voltage vs, the
emitter – base junction becomes more forward biased. The collector current ic increases, which
increases emitter current ie in the same direction. This develops ac voltage ve (ieRE )across the
resistor RE. The effective input voltage between base and emitter is given by applying KVL at the
input loop we get

𝑣𝑠 = 𝑣𝑏𝑒 + 𝑣𝑒

𝑣𝑠 − 𝑣𝑒 = 𝑣𝑏𝑒

From above equation we can say that the effective input voltage between base and emitter
decreases. Hence it is the case of the negative feedback. The feedback voltage ieRE is
proportional to the output current ie=ic, and it appears in series with the source voltage. Hence this
is the case of the series – voltage feedback. This circuit is widely used in public address systems,

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tape recorders etc... Sometimes the emitter resistor is partly bypassed, so that the gain is not
reduced excessively with the advantages of the negative feedback.

EMITTER FOLLOWER:
To construct emitter follower circuit the collector biasing resistor RC is reduced to zero and
output voltage is taken from the emitter is called emitter follower circuit or common collector
amplifier. The collector is common between input and output. From the ac point of view, the
supply voltage VCC is short circuited and hence the collector is grounded. The input is given
between the base and collector and the output appears between emitter and collector as shown in
Fig.b.

The effective input voltage for this circuit is vs – vo, because the whole output vo is fed back
to the input side. The gain of the amplifier is drastically reduced. The voltage of this amplifier is
less than unity. The output of this amplifier is less than the input. The input impedance of this
circuit is very high and output impedance is very low. The emitter amplifier is used for impedance
matching in the multistage amplifier, the last stage of the signal generator. Because of high input
impedance and low output impedance properties, emitter amplifier is capable of giving power to
the load connected to its output without requiring much power at its input. So emitter amplifier is
used as a buffer amplifier.

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When input signal vs becomes positive, the output voltage is also becomes positive. Thus
output and input are in same phase with the output is almost same as the input (vo is slightly less
than vs). The emitter closely follows the input. Hence it is called emitter follower.

NEED OF AN OSCILLATOR:

An oscillator is a circuit which produces a continuous, repeated, alternating waveform

without any input. Oscillators basically convert unidirectional current flow from a DC source into
an alternating waveform which is of the desired frequency, as decided by its circuit components

CLASSIFICATION OF AN OSCILLATOR:
Mainly there are two types of the oscillator, Sinusoidal & non– sinusoidal. Sinusoidal
oscillator produces sine wave, while non – sinusoidal oscillator generates square wave, triangle
wave, saw– tooth wave or pulses. The circuit which generate pulses or square wave is called
multivibrators.

The sinusoidal oscillators are classified by the type of the feedback network used in an
amplifier.

Tuned circuit oscillators: inductor L and capacitor C are used to generate high frequency
signals. Hartley and colpitts are the example of the tuned circuit oscillator.
RC oscillators: resistors R and capacitors are used to generate low or audio frequency
signal. Phase shift and wein bridge are the example of rc oscillator
Crystal oscillators: quartz crystals are used to generate high frequencies up to 10MHz
signals. It generates a highly stabilized output signal.

TUNNED CIRCUIT FOR GENERATION OF OSCILLATOR:

An inductor and capacitor connected in parallel form a tank or tuned circuit.
The energy is introduced into tank circuit by
connecting the capacitor to a D.C. voltage source as
shown in figure. The negative terminal of the battery
supplies electrons to the lower plate of the capacitor.
Because of the accumulation of electrons, the capacitor
gets charged and there is a voltage across it. The energy
is stored in the capacitor in the form of electric potential
energy.

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When the switch is thrown to position 2, current

starts flowing in the circuit. The capacitor now starts
discharging through the inductor. Since the inductor has
the property of opposing any change in current, the
current builds up slowly. Maximum currents flows in the
circuit when the capacitor is fully discharged at this
instant, the potential energy of the system is zero but the
electron motion being greatest, the magnetic field energy
around the coil is maximum.

Once the capacitor is fully discharged, the magnetic field begins to collapse. The back emf
in the inductor keeps current flowing in the same direction.
The capacitor starts charging, but with opposite polarity as
shown in figure. As the charge builds up across the capacitor,
the current decreases and the magnetic field decrease. The
magnetic field drops to zero when the capacitor charges to the
value it had in previous condition. Again all the energy is in
the form of potential energy. The capacitor now begins to
discharge again. This time current flows in opposite direction.

As shown in figure the capacitor is fully

discharged and shows the maximum current
flowing in the circuit. Again, all the energy is
in the magnetic field. The interchange or
oscillation of energy between L and C is
repeated again and again.

The inductor coil will have some resistance and dielectric material of the capacitor will have
some leakage. Because of these some energy loss takes place during each cycle of the oscillation.
As a result of this loss, the amplitude of oscillation decreases continuously and ultimately
oscillations die down. Thus, the tank circuit is capable of producing oscillations but they are
damped as shown in below figure.

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FREQUENCY OF OSCILLATIONS IN LC CIRCUIT:

In the electrical LC circuit, the constants of the systems are the inductance of an inductor L
and the capacitance of the capacitor C. The frequency of oscillation for the tank circuit is given by
equation
𝟏
𝒇𝒐𝒔𝒄 =
𝟐𝝅√𝑳𝑪

SUSTAINED OSCILLATION:
The oscillations of LC circuit can be maintained at a constant level by supplying a pulse of
energy at the right time in each cycle, the resulting undammed oscillations are called sustained
oscillations. Such oscillations are generated by the electronic oscillator circuits.

There are many verities of LC – oscillator circuits. All of them have following three features in
common

i) They must contain an active device (BJT or FET) that work as an amplifier.
ii) There must be positive feedback in the amplifier.
iii) The amount of feedback must be sufficient to overcome the losses.
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POSITIVE FEEDBACK AS AN OSCILLATOR:

Main application of positive feedback is in oscillator. Oscillator generates ac output signal
without any ac input signal. A part of the output is fed back to the input; and this feedback signal
is the only input to the internal amplifier.
The voltage source vi drives the input
terminals of the internal amplifier. The
amplified signal Avi drives the feedback
network to produce feedback voltage Aβvi.
This voltage returns to the input point X. if
the phase shift due to the amplifier and
feedback network is correct. The signal at
point X will exactly in phase with the signal
driving the input terminals of internal
amplifier.

Now we connect the point X and Y and

remove voltage source vi. The feedback
signal now drives the input terminals of
internal amplifier. If Aβ is less then unity,
Aβvi is less than vi and the output signal will
die out as shown in fig. a, because enough
voltage is not returned to the input of the
amplifier.

If Aβ > 1, Aβvi is greater than vi and

output voltage builds up as shown in fig b.
Such oscillations are called growing
oscillations.

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If Aβ =1, no change occurs in the output, the amplitude of the output signal remains
constant as shown fig. c.

To find the necessary condition for the sustained oscillations, we have to derive the
expression for the overall voltage gain of an amplifier with positive feedback and are written as
𝐴
𝐴𝑓 =
1 − 𝐴𝛽

If Aβ = 1 then Af =∞. The gain becomes infinity that means there is output without any
input. The amplifier becomes oscillator. The condition Aβ = 1 is called Barkhausen criterion of
oscillation.

THE STARTING VOLTAGE:

The staring voltage of an oscillator is provided by noise voltage. Noise voltages are
produced due to the random motion of electrons in resistors used in the circuit and the active
device.

This noise voltage contains almost all the sinusoidal frequencies of very small amplitude. It
gets amplified and appears at the output terminals. The amplified noise drives the feedback
network, which is either LC network or RC network. Because of this feedback voltage Aβ, is
maximum at particular frequency fosc, the frequency of oscillation. The phase shift required for
positive feedback is produced only for this frequency signal only. Thus, from the noise voltage
which contains almost all frequencies, the output of the oscillator will contain only a single
sinusoidal frequency.

When the oscillator circuit is switched on, the loop gain Aβ >1, then becomes Aβ= 1. The
oscillations build up. Once a suitable level is reached, the gain of the amplifier decreases, and the
value of the loop gain decreases to unity. So the constant level oscillations are maintained.
The requirements of an oscillator circuit are

i) There must be a positive feedback.

ii) Initially loop gain Aβ > 1.

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iii) After, the desired level is reached, the loop gain Aβ must be decreased to unity.

HARTLEY OSCILLATOR:
The RFC radio frequency chock permits an easy flow of DC current and also offers very
high impedance for the high frequency currents. RFC is short for DC signal and open for ac
signal. Hartley oscillator uses split – tank inductor in the feedback circuit.

The coupling capacitor CC in the output circuit does not permit the DC currents to go to the
tank circuit. The radio – frequency energy developed across the RFC is capacitively coupled to
the tan circuit through the coupling capacitor CC. The circuit oscillates at a frequency
𝟏
𝒇𝒐𝒔𝒄 =
𝟐𝝅√𝑳𝒆𝒒 𝑪

𝐿𝑒𝑞 = 𝐿1 + 𝐿2 + 2𝑀

And M is the mutual inductance of an inductor.

COLPITT’S OSCILLATOR:
Colpitts oscillator is widely used in commercial signal generators above 1MHz. The
colpitts oscillator uses a split – tank capacitor. The load across the C2 capacitor provides
regenerative feedback required for the sustained oscillations. The frequency of oscillation is given
by

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𝟏
𝒇𝒐𝒔𝒄 =
𝟐𝝅√𝑳𝑪𝒆𝒒

𝐶1 𝐶2
𝐶𝑒𝑞 =
𝐶1 + 𝐶2

BASIC PRINCIPLES OF RC OSCILLATORS:

The amplifier gives amplification of the input signal and also shifts the phase of the input
signal by 1800the part of the output is feedback to the input, a negative feedback takes place. The
net output decreases. For producing oscillations, positive feedback is required. Positive feedback
occurs only when the fed back signal is in phase with the original input signal. This condition can
be achieved by two ways. The part of the output of the single stage amplifier when passed
through the feedback network, the feedback network provides additional 1800 phase shift. Thus, a
total phase shift of 3600 occurs. This principle is used in phase – shift network.

Another way of getting 3600 is to use two stage of an amplifier each giving 1800. A part of
the output is fed back to the input through a feedback network without producing further phase
shift. This principle is used in wein bridge oscillator.

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PHASE SHIFT OSCILLATOR:

In phase shift oscillator resistor RS and capacitor CS in the source circuit provides self –
biasing for the amplifier. Output of this amplifier is 1800 out of phase with input signal. The
output of the amplifier is applied to the feedback network. The feedback network is consisting of
three RC networks. Each RC network provides a phase shift of 600. The total of 1800 of phase
shift is provided by the feedback network. The output of the feedback network is in the same
phase as the original input signal of the amplifier.

When the condition Aβ = 1 is satisfied the circuit sustain sinusoidal oscillation at the
frequency
𝟏
𝒇𝒐𝒔𝒄 =
𝟐𝝅𝑹𝑪√𝟔
At this frequency, the feedback factor if the feedback network is β = 1/29. For self starting
of oscillations, Aβ > 1. Hence the gain of the amplifier must be greater than 29, only then
oscillation can start.

Dr. M.H.Patel Page 21

 US03CPHY22 UNIT - 3 FEED BACK IN APMLIFIER AND OSCILLATOR

WIEN BRIDGE OSCILLATOR:

Wien bridge oscillator is used to generate frequencies in the range of 10 Hz to 1MHz. it is
used in audio signal generators. It consists of two stage RC coupled amplifier and a feedback
network.

A1 and A2 represent the two stages of amplifier. The output of the A2 goes to the feedback
network. The voltage across the parallel combination of R2C2is fed to the input of amplifier A1.
The net phase shift through two amplifiers is zero. Therefore, it is the case of positive feedback.
The phase shift through the feedback network must be zero, and this condition occurs at a
frequency
𝟏
𝒇𝒐𝒔𝒄 =
𝟐𝝅√𝑹𝟏 𝑹𝟐 𝑪𝟏 𝑪𝟐

When this condition is satisfied the feedback factor β = 1/3. This means that the amplifier
must have gain of at least 3 to satisfy the barkhausen criteria Aβ = 1. To have a gain of as low as
3 a negative feedback is added by introducing resistor divider network R 3 and R4 as shown in
below figure.

Dr. M.H.Patel Page 22

 US03CPHY22 UNIT - 3 FEED BACK IN APMLIFIER AND OSCILLATOR

The resistors R3 and R4 provide negative feedback and are connected to the lower input
terminal. Frequency of the oscillator can be changed by varying two capacitors C 1 and C2
simultaneously. We can change frequency range by switching various resistors R1 and R2.

Reading materials for students

Reference:

Basic Electronics and Linear Circuits

D. C. Kulshreshtha, N. N. Bhargava, and S.C. Gupta

Dr. M.H.Patel Page 23