hamburger

Oscillators Study Notes Part-2

By BYJU'S Exam Prep

Updated on: September 25th, 2023

In this article, you will find the Study Notes on Oscillators-2 which will cover the topics such as Oscillators with LC Feedback Circuits, Hartley Oscillators, Armstrong Oscillators, Crystal Controlled oscillators, and Linear Wave shaping Circuits.

Download Formulas for GATE Electrical Engineering – Electrical Machines

Oscillators with LC Feedback Circuits

  • Although the RC feedback oscillators, particularly the Wien bridge, are generally suitable for frequencies up to about 1 MHz, LC feedback elements are normally used in oscillators that require higher frequencies of oscillation.
  • Also, because of the frequency limitation (lower unity-gain frequency) of most op-amps, transistors (BJT or FET) are often used as the gain element in LC oscillators.
  • This section introduces several types of resonant LC feedback oscillators like the Colpitts, Clapp, and Hartley & crystal-controlled oscillators.

The Hartley Oscillator

  • The Hartley oscillator is similar to the Colpitts except that the feedback circuit consists of two series inductors and a parallel capacitor

Oscillators Study Notes Part-2

  • In this circuit, the frequency of oscillation is

Oscillators Study Notes Part-2

  • where Oscillators Study Notes Part-2 
  • The inductors act in a role similar to C1 and C2 in the Colpitts to determine the attenuation, B, of the feedback circuit.

Oscillators Study Notes Part-2

  • To assure the start-up of oscillation, Av must be greater than 1/B.

Oscillators Study Notes Part-2

  • Loading of the tank circuit has the same effect in the Hartley as in the Colpitts; that is, the Q is decreased and thus Fr decreases.

The Armstrong Oscillator

  • This type of LC feedback oscillator uses transformer coupling to feed back a portion of the signal voltage.
  • The transformer’s secondary coil provides feedback to keep the oscillation going.
  • The Armstrong is less common than the Colpitts, Clapp, and Hartley, mainly because of the disadvantage of transformer size and cost.
  • The frequency of oscillation is set by the inductance of the primary winding Lpri in parallel with C1.

Oscillators Study Notes Part-2

Oscillators Study Notes Part-2

Crystal-Controlled Oscillators

  • The most stable and accurate type of feedback oscillator uses a piezoelectric crystal in the feedback loop to control the frequency.

The Piezoelectric Effect:

  • Quartz is one type of crystalline substance found in nature that exhibits a property called the piezoelectric effect.
  • When a changing mechanical stress is applied across the crystal to cause it to vibrate, a voltage develops at the frequency of mechanical vibration.
  • Conversely, when an AC voltage is applied across the crystal, it vibrates at the frequency of the applied voltage.
  • The greatest vibration occurs at the crystal’s natural resonant frequency, which is determined by the physical dimensions and by the way the crystal is cut.

Oscillators Study Notes Part-2

 

  • As you can see, the crystal’s equivalent circuit is a series-parallel circuit and can operate in either series resonance or parallel resonance.
  • The impedance of the crystal is minimum at the series resonant frequency, thus providing maximum feedback.
  • The crystal tuning capacitor, Cc is used to “fine tune” the oscillator frequency.

Oscillators Study Notes Part-2

  • The impedance of the crystal is maximum at parallel resonance, thus developing the maximum voltage across the capacitors.
  • The voltage across C1 is fed back to the input.

Relaxation Oscillators

  • The second major category of oscillators is the relaxation oscillator.
  • Relaxation oscillators use an RC timing circuit and a device that changes states to generate a periodic waveform.

A Triangular-Wave Oscillator:

  • The op-amp integrator can be used as the basis for a triangular-wave oscillator.
  • When the switch is in position 1, the negative voltage is applied, and the output is a positive-going ramp.
  • When the switch is thrown into position 2, a negative-going ramp is produced.
  • If the switch is thrown back and forth at fixed intervals, the output is a triangular wave consisting of alternating positive-going and negative-going ramps.

Oscillators Study Notes Part-2

A Square-Wave Oscillator:

  • The basic square-wave oscillator is a type of relaxation oscillator because its operation is based on the charging and discharging of a capacitor.
  • Notice that the op-amp’s inverting input is the capacitor voltage and the non-inverting input is a portion of the output fed back through resistors R2 and R3 to provide hysteresis.
  • When the circuit is first turned on, the capacitor is uncharged, and thus the inverting input is at 0 V.
  • This makes the output a positive maximum, and the capacitor begins to charge toward Vout through R1.
  • When the capacitor voltage Vc reaches a value equal to the feedback voltage Vf on the non-inverting input, the op-amp switches to the maximum negative state.
  • At this point, the capacitor begins to discharge from +Vf toward –Vf.
  • When the capacitor voltage reaches –Vf the op-amp switches back to the maximum positive state.
  • This action continues to repeat and a square-wave output voltage is obtained.

Oscillators Study Notes Part-2

Linear wave-shaping Circuits

  • The process where by the form of a non-sinusoidal signal is altered by transmission through a linear network is called “LINEAR WAVE SHAPING”

RC Low Pass Circuit :

Oscillators Study Notes Part-2

The circuit passes low frequencies readily but attenuates high frequencies because the reactance of the capacitor decreases with increasing frequency. At very high frequencies the
capacitor acts as a virtual short circuit and the output falls to zero. This circuit also works as an integrating circuit. A circuit in which the output voltage is proportional to the integral of the input voltage is known as an integrating circuit. The condition for integrating the circuit is RC value must be much greater than the time period of the input wave (RC>>T).

Oscillators Study Notes Part-2

RC High Pass Circuit:

Oscillators Study Notes Part-2

The higher frequency components in the input signal appear at the output with less attenuation than the lower frequency components because the reactance of the capacitor
decreases with an increase in frequency. This circuit works as a differential circuit. A circuit in which the output voltage is proportional to the derivative of the input voltage is known as
a differential circuit. The condition for the differential circuit is RC value must be much smaller than the time period of the input wave (RC<

Oscillators Study Notes Part-2

If you aiming to crack GATE & ESE, Other PSU Exams then you must try Online Classroom Program to get unlimited access to all the live structured courses and unlimited mock tests from the following links:

ESE and GATE EC Online Classroom Program (24+ LIVE Courses and 150+ Mock Tests)

ESE and GATE EE Online Classroom Program (24+ LIVE Courses and 193+ Mock Tests)

Click on the Links Below to Avail Test Series:

Click Here to Avail GATE/ESE EC Test Series !!! (150+ Mock Tests)

Click Here to Avail GATE/ESE EE Test Series !!! (193+ Mock Tests)

 

Get complete information about the GATE exam pattern, cut-off, and all those related things on the BYJU’S Exam Prep official youtube channel.

Our Apps Playstore
POPULAR EXAMS
SSC and Bank
Other Exams
GradeStack Learning Pvt. Ltd.Windsor IT Park, Tower - A, 2nd Floor, Sector 125, Noida, Uttar Pradesh 201303 help@byjusexamprep.com
Home Practice Test Series Premium