The document discusses oscillators and feedback amplifiers. It defines positive and negative feedback, and describes their effects on gain. Oscillators generate an output signal without an external input through the use of positive feedback in an amplifier circuit. The two main types of oscillators are sinusoidal and non-sinusoidal oscillators. Common oscillator circuits discussed include the RC phase shift oscillator, Hartley oscillator, and common emitter amplifier configuration.

1. Oscillator
Feedback :
It is a process of giving back the output or part of output signal back to the input.
The amplifiers which uses mechanism of feedback are called as Feedback amplifiers
There are two types of feedback :
1. Positive Feedback
2. Negative feedback
Positive Feedback : If the feedback signal (voltage or current ) is applied in such a way that it is
in phase with the input signal is called as Positive Feedback or Regenerative Feedback or Direct
feedback .Positive feedback increases gain of the amplifier but it produces distortion in the output.
Positive feedback is used in the Oscillators .
Negative Feedback : If the feedback signal (voltage or current ) is applied in such a way that it is
out of phase with the input signal is called as Negative Feedback or Degenerative Feedback or
Inverse feedback.Negative feedback decreases gain of the amplifier.
Advantages of Negative Feedback :
1. The negative feedback reduces noise.
2. It stabilized gain.
3. It has less harmonic distortion.
4. It has less amplitude distortion.
5. It has less phase distortion.
6. Input and output impedances can be modified as desired.
7. It can increase or decrease output impedances.
8. It has less frequency distortion.
Disadvantages of Negative Feedback :
o Reduction in gain.
Oscillator:
An oscillator generates output without any ac input signal. An electronic oscillator is a circuit
which converts dc energy into ac at a very high frequency. An amplifier with a positive feedback
is called as an oscillator.
Amplifier vs. Oscillator :
An amplifier increases the signal strength of the input signal applied, whereas 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.

2. 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.
The frequency, waveform, and magnitude of a.c. power generated by an amplifier, is controlled
by the a.c. signal voltage applied at the input, whereas those for an oscillator are controlled by the
components in the circuit itself, which means no external controlling voltage is required.
Classification of Oscillators
Electronic oscillators are classified mainly into the following two categories:
Sinusoidal Oscillators ─ The oscillators that produce an output having a sine waveform are called
sinusoidal or harmonic oscillators. Such oscillators can provide output at frequencies ranging
from 20 Hz to 1 GHz.
Non-sinusoidal Oscillators ─ The oscillators that produce an output having a square, rectangular
or saw-tooth waveform are called non-sinusoidal or relaxation oscillators. Such oscillators can
provide output at frequencies ranging from 0 Hz to 20 MHz.
Nature of Sinusoidal Oscillations
The nature of oscillations in a sinusoidal wave are generally of two types. They are damped and
undamped oscillations.
Damped Oscillations :
The electrical oscillations whose amplitude goes on decreasing with time are called as Damped
Oscillations.
Undamped Oscillations
The electrical oscillations whose amplitude remains constant with time are called as Undamped
Oscillations. The frequency of the Undamped oscillations remains constant.

3. Frequency of Oscillations
The frequency of the oscillations produced by the tank circuit are determined by the components
of the tank circuit, the L and the C. The actual frequency of oscillations is the resonant
frequency (or natural frequency) of the tank circuit which is given by
The above equation, indicates the output frequency, called as natural frequency or resonance
frequency.
Barkhausen Criterion or Conditions for Oscillation
The circuit will oscillate when two conditions, called as Barkhausen’s criteria are met. These two
conditions are
1. The loop gain must be unity that is |Aβ | = 1
2. The feedback signal at the input must be phase shifted by 360 degrees (which is same as zero
degrees). In most of the circuits, an amplifier is used to produce 180 degrees phase shift and
additional 180 degrees phase shift is provided by the feedback network.

4. RC Phase Shift Oscillator :
RC phase-shift oscillators use resistor-capacitor (RC) network (Figure 1) to provide the
phase-shift required by the feedback signal. They have excellent frequency stability and
can yield a pure sine wave for a wide range of loads.
RC phase-shift oscillator is formed by cascading three RC phase-shift networks, each offering a
phase-shift of 60o
, as shown by Figure 2.
The collector resistor RC limits the collector current of the transistor, Resistors R1 and R form the
voltage divider network .The emitter resistor RE improves the stability.
The capacitors CE and Co are the emitter by-pass capacitor and the output DC decoupling capacitor,
respectively. Further, the circuit also shows three RC networks employed in the feedback path.
The transistor is used in CE configuration which provides the phase shift of 1800
and 3 RC phase
shift networks provide phase shift of 1800
(Each RC network provide phase shift of 600
).
This makes the net phase-difference to be 360o
, satisfying the phase-difference condition for
oscillator.
The frequency of oscillations produced by a RC phase-shift oscillator is given by
𝑓 =
1
2𝜋𝑅𝐶√6
R1=R2=R3=R

5. C1=C2=C3=C
The frequency of the RC phase-shift oscillators can be varied by changing either the resistors or
the capacitors. However, in general, the resistors are kept constant while values of capacitor
change together.
Advantages
 It does not require transformers or inductors.
 It can be used to produce very low frequencies.
 The circuit provides good frequency stability.
Disadvantages
 Starting the oscillations is difficult as the feedback is small.
 The output produced is small.

Hartley Oscillator:
A very popular local oscillator circuit that is mostly used in radio receivers is the Hartley
Oscillator circuit.
An NPN transistor connected in common emitter configuration .R1 and R2 are biasing resistors.
RFC is the radio frequency choke connected between collector and Vcc which permits dc current
and blocks AC .
Tank circuit consist of two coils L1 and L2 .The coil L1 is inductively coupled to coil L2 and the
combination works as a autotransformer.
CE is the emitter bypass capacitor and RE is also a biasing resistor. Capacitors CC1 and CC2 are
the coupling capacitors.

6. When the DC supply (Vcc) is given to the circuit, collector current starts increasing and starts
charging of the capacitor C.
Once C is fully charged, it starts discharging through L1 and L2 and again starts charging.
The sine wave generated by the tank circuit is coupled to the base of the transistor through the
capacitor CC2.
Since the transistor is configured as common-emitter, it takes the input from tank circuit and
provides the phase shift of 1800
.
The mutual inductance between the L1 and L2 provides the feedback of energy from collector-
emitter circuit to base-emitter circuit.
Thus the transistor provides amplification as well as inversion of signal that is phase shift of 1800
and correct the signal generated by the tank circuit. The frequency of oscillations of this circuit is
F =
1
2𝜋√ 𝐿𝐶
Where , L = L1 + L2 + 2M
For a practical circuit, if L1 = L2 = L and the mutual inductance is neglected then ;L = L1 + L2
Advantages
 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.

7.  The amplitude of the output remains constant over a fixed frequency range.
Disadvantages
 It cannot be a low frequency oscillator.
 Harmonic distortions are present.
Applications
 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.
Common Emitter Amplifier (CE) Circuit
The Amplifier is an electronic circuit that is used to increase the strength of a weak input signal
in terms of voltage, current, or power. The process of increasing the strength of a weak signal is
known as Amplification.
Common Emitter Amplifier Circuit Elements and their Functions
Biasing Circuit/ Voltage Divider
The resistances R1, R2 and RE used to form the voltage biasing and stabilisation circuit. The
biasing circuit needs to establish a proper operating Q-point otherwise, a part of the negative half
cycle of the signal may be cut-off in the output.

8. Input Capacitor (C1)
The capacitor C1 is used to couple the signal to the base terminal of the BJT. If it is not there, the
signal source resistance, Rs will come across R2 and hence, it will change the bias. C1 allows only
the AC signal to flow but isolates the signal source from R2
Emitter Bypass Capacitor (CE)
An Emitter bypass capacitor CE is used parallel with RE to provide a low reactance path to the
amplified AC signal. If it is not used, then the amplified AC signal following through RE will
cause a voltage drop across it, thereby dropping the output voltage.
Coupling Capacitor (C2)
The coupling capacitor C2 couples one stage of amplification to the next stage. This technique
used to isolate the DC bias settings of the two coupled circuits.
Operation of Common Emitter Amplifier
When a signal is applied across the emitter-base junction, the forward bias across this junction
increases during the upper half cycle. This leads to increase the flow of electrons from the emitter
to a collector through the base, hence increases the collector current. The increasing collector
current makes more voltage drops across the collector load resistor RC.
The negative half cycle decreases the forward bias voltage across the emitter-base junction. The
decreasing collector-base voltage decreases the collector current in the whole collector resistor Rc.
Thus, the amplified load resistor appears across the collector resistor
CE Amplifier Frequency Response
The curve drawn between voltage gain and the signal frequency of an amplifier is known as
frequency response. Below figure shows the frequency response of a typical CE amplifier.

9. From the above graph, we observe that the voltage gain drops off at low (< FL) and high (> FH)
frequencies, whereas it is constant over the mid-frequency range (FL to FH).
At low frequencies (< FL)
The reactance of coupling capacitor C2 is relatively high and hence very small part of the signal
will pass from amplifier stage to the load. Moreover, CE cannot shunt the RE effectively because
of its large reactance at low frequencies. These two factors cause a drops off of voltage gain at low
frequencies.
At high frequencies (> FH) The reactance of coupling capacitor C2 is very small and it behaves
as a short circuit. This increases the loading effect of the amplifier stage and serves to reduce the
voltage gain.Moreover, at high frequencies, the capacitive reactance of base-emitters junction is
low which increases the base current. This frequency reduces the current amplification factor β.
Due to these two reasons, the voltage gain drops off at high frequency.
At mid frequencies (FL to FH) The voltage gain of the amplifier is constant. The effect of the
coupling capacitor C2 in this frequency range is such as to maintain a constant voltage gain. Thus,
as the frequency increases in this range, the reactance of CC decreases, which tend to increase the
gain.However, at the same time, lower reactance means higher almost cancel each other, resulting
in a uniform fair at mid-frequency.